V O LU M E
T E N
HANDBOOK OF EXPLORATION AND ENVIRONMENTAL GEOCHEMISTRY
The Indian Ocean Nodule Field: Geology and Resource Potential
HANDBOOK OF EXPLORATION AND ENVIRONMENTAL GEOCHEMISTRY Series Editor
MARTIN HALE 1.
Analytical methods in geochemical prospecting
2.
Stastistics and data analysis in geochemical prospecting
3. Rock geochemistry in mineral exploration 4.
Regolith exploration geochemistry in tropical and sub-tropical terrains
5.
Regolith exploration geochemistry in arctic and temperate terrains
6.
Drainage geochemistry
7.
Geochemical remote sensing of the sub-surface
8. Life cycle of the phosphoria formation: From deposition to the post-mining environment 9. 10.
Biogeochemistry in mineral exploration The Indian Ocean nodule field: Geology and resource potential
V O LU M E
T E N
HANDBOOK OF EXPLORATION AND ENVIRONMENTAL GEOCHEMISTRY
The Indian Ocean Nodule Field: Geology and Resource Potential RANADHIR MUKHOPADHYAY National Institute of Oceanography, Goa, India
ANIL K. GHOSH University of Calcutta, Kolkata, India
SRIDHAR D. IYER National Institute of Oceanography, Goa, India
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PRINTED AND BOUND IN THE NETHERLANDS 08 09 10 11 12 10 9 8 7 6 5 4 3 2 1
CONTENTS
Preface Series Editor’s Foreword About the Authors
1. The Indian Ocean Nodule Field 1. Physiography and Geology 2. Physics, Chemistry and Biology 3. Evolution of the Indian Ocean Nodule Field
2. Tectonics and Geomorphology 1. Ridge-Normal Lineaments 2. Ridge Parallel Lineations and Anomalies 3. Seamounts
3. Volcanics 1. 2. 3. 4.
Major Volcanics Minor Volcanics Alteration of Volcanics Tectonics and Volcanics: Interrelations
4. Sediments 1. Distribution and Source 2. Sedimentary Processes
5. Ferromanganese Deposits 1. 2. 3. 4. 5.
Nodule Characteristics Factors Influencing Nodule Formation Dynamics of Nodule Formation Ferromanganese Encrustation The World Oceans Scenario
vii xi xiii
1 5 25 31
37 40 45 54
67 69 86 103 112
115 117 135
155 157 184 194 205 211
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Contents
6. Resource Management 1. 2. 3. 4. 5. 6.
Resource Identification Mining Technology Environmental Impact Assessment Metal Extraction and Processing Law of the Sea Global Perspectives
References Author Index Subject Index Colour Insert between pages 18 and 19
225 225 231 233 249 254 255 261 283 289
PREFACE
Oceans, with their enormity and mystery covering 259 million km2 and some 72% of the earth’s surface, have always been a part of human consciousness. Climate and weather, and even the quality of the air people breathe, depend in great measure on the interplay between the ocean and the atmosphere in ways still not fully understood. Besides being a prime source of nourishment for life it helped to generate, the ocean is considered as a storehouse for several living and non-living resources. Making use of such resources, in general, and minerals, in particular, essentially needs intelligent and sustainable treatment. The 1982 United Nations Convention on the Law of the Sea, since its entry into force on 16 November 1994, provided for the first time a universal legal framework for the rational management of marine resources and their conservation. The Central Indian Ocean Basin (CIOB), bordered by the Indian Subcontinent, the Ninetyeast Ridge and the Indian Ocean Ridge system, is the largest and most unique of all the basins in the Indian Ocean. Because of its complex tectonic fabric, vastness and ferromanganese deposits, this basin has attracted the attention of oceanographers since long. However, in comparison to the quantum of investigations carried out in the Pacific and the Atlantic oceans, the study in the CIOB has left much to be desired. Even the international drilling programmes (Deep Sea Drilling Project, Ocean Drilling Programme) have not extensively covered this basin. As a result, a paucity of data has limited a proper understanding of the basin and the evaluation of its resources. During the last two decades, the CIOB was explored in some detail for its mineral resource potential, with India taking the lead. The broad objectives were to explore and delineate economically feasible manganese nodulebearing areas, and ultimately concentrate on resource exploration from the Self-allocated area. Extensive exploration for the resources saw more than 50 oceanographic voyages to this basin. The amount and type of underway data collected are immense, ranging from single-beam to multi-beam bathymetry, to seafloor magnetism and gravity and to underwater photography. A large number of manganese nodules, rocks, sediments and water column samples were recovered from an area more than 700,000 km2, bordered between 9 S and 16 300 S and 72 E and 80 E. This helped delineate a nodule-rich area to be later known as the Indian Ocean Nodule Field (IONF). Subsequently, close-grid exploration, followed by the critical resource assessment of the IONF, identified the ‘first generation mine site’. Globally, the IONF represents the second largest and second richest deep-sea manganese nodule resources after those of the Equatorial North Pacific. vii
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Preface
A conservative estimate places the total manganese nodules availability in the IONF as more than 1400 million tons, with an average abundance of little more than 4.5 kg of nodules per square metre. Considering the recovery capacity of available mining technology and metal extraction processes, the nodule resources are expected to last for many hundreds of years, and resource mining would be economically viable too. The publications resulting from the field and laboratory studies, and covering diverse aspects (such as evolution of the CIOB, structure and tectonics, volcanism, sedimentation, and resource characterisation and potential), are indeed numerous. Several workers from different institutions in various countries have published their important findings, but these are scattered in different journals, all of which are not easily accessible to researchers interested in the nodule resources of the Indian Ocean. In this scenario, the basic purpose of the present book is to collate the available information in a concise and systematic manner, and carry out a critical evaluation of such information. The necessity of this book is all the more relevant because the currently available books on manganese nodules largely pertain to the Pacific and the Atlantic oceans, making inadequate coverage of the Indian Ocean, in general, and the IONF, in particular. Hence, this book is targeted at students and researchers at the master’s and doctoral levels, and for all those interested in the marine geology and resource potentiality of the IONF. We hope that this book will serve as a ready reference to those involved in the development and management of geo-resources, and to economists and policy planners. After introducing the IONF in the first chapter, we discuss the geomorphology and tectonics of this field in the second and volcanics in the third chapter. The bottom sedimentary regimes—sediment source and its characteristics, dynamics of sedimentation—are discussed in Chapter 4. The distribution, grade and processes of formation of ferromanganese deposits (both nodules and crusts) during the last 5–10 million years are discussed in Chapter 5. And finally in Chapter 6, we describe the equipment used for exploration, sketch on probable mining methods, assess environmental implications concerning mining, discuss metallurgical techniques and converse on international legal constraints on exploration and exploitation of the nodules. While writing this book, we received help from many quarters. The Council of Scientific & Industrial Research (CSIR) and the Ministry of Earth Sciences, both in New Delhi, India, are thanked for all the logistic and financial support—in the laboratory and at sea—during the last two decades through grants to the National Institute of Oceanography (NIO), Goa. The director, National Institute of Oceanography, and the vice-chancellor, University of Calcutta, are specially thanked for their support and encouragement. We place on record with thanks the generous support of all the members of the project ‘Surveys for Polymetallic Nodules’ in helping one way or the other during collection and analysis of samples, and data interpretation. Many of them readily provided their reprints, preprints and other
Preface
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unpublished material. The cooperation of the crew of the various ships, sometimes under excruciating conditions, is acknowledged. India’s deep-sea mineral campaign owes a great deal to the vision and untiring efforts of the late Dr Hassan Nasiem Siddiquie, the former director of the National Institute of Oceanography. We respectfully remember his initiation of the manganese nodule programme for India. Dr. S. Z. Qasim, as former Secretary of the then Department of Ocean Development and ex- member, Planning Commission, Government of India, has always been a source of inspiration to the project. The earlier versions of the chapters received critical reviews and comments from A. L. Paropkari, G. V. Rajamanickam, J. N. Pattan, K. S. Krishna, M. Shyam Prasad, M. V. Ramana, N. H. Khadge, R. K. Drolia, R. P. Das, R. V. Karanth, and V. Ramaswamy. We thank them all. Besides, we had rewarding discussions with many of our colleagues at NIO and at the University of Calcutta. Technical help from A. K. Saran, A. Y. Mahale, R. Uchil and S. Akerkar is acknowledged. We thank Elsevier for publishing this book and also Martin Hale, Series Editor, for patiently going through the manuscript and for writing the Foreword. We acknowledge the support (and patience) of Particia Massar, Pauline Riebeek and Tirza Van Daalen during preproduction stage, to Conny Krainz for supervising the production of the Title and Prasenjit Bakshi and his dedicated team for printing the book. Most of the materials presented here are in public domain. Many of the figures were reprinted from various books and journals. We thank the authors and publishers for kindly permitting us to reproduce these figures. Last but not the least, we express a deep sense of appreciation to the sacrifice of our wives Sumita, Jayanti and Kamakshi, respectively, and to our children. It is a pleasure to place on record our gratitude to our colleagues both from within and outside India who helped with literature support and liberal comments to help characterise the resources and to better understand the formational regimes of ferromanganese deposit in the IONF. However, any discrepancy regarding the views and opinions expressed in this book is solely the responsibility of the authors. 31 January 2007 Ranadhir Mukhopadhyay National Institute of Oceanography, Goa
[email protected] Anil K. Ghosh University of Calcutta, Kolkata
[email protected] Sridhar D. Iyer National Institute of Oceanography, Goa
[email protected]
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SERIES EDITOR’S FOREWORD
On 13 March 1874, in a part of the Pacific Ocean between Hawaii and Tahiti, the crew of HMS Challenger raised a bucket of ocean-floor sediment from a depth of 4727 m and discovered that it contained potato-shaped nodules. This event marks the starting point of the ocean-floor research that has yielded a wealth of scientific information, including that presented in this latest volume of the Handbook of Exploration and Environmental Geochemistry. During the period 1872–1876, HMS Challenger made a 127,500-km circumnavigation of the globe, taking deep-sea soundings, bottom dredges, open water trawls and serial water temperature observations in the Atlantic, Indian, Pacific and Antarctic oceans. It was conducting the world’s first oceanographic expedition which, in the words of the supervising scientist of the expedition report, led to ‘the greatest advance in the knowledge of our planet since the celebrated discoveries of the fifteenth and sixteenth centuries.’ Although HMS Challenger, formerly a frigate of the British Royal Navy, had a team of scientists on board and had been fitted out with laboratories, the potato-shaped nodules remained little more than a scientific curiosity for almost 20 years. Then, in 1891, they were analysed in Britain and found to comprise mainly oxides of manganese and iron and to contain unusually high concentrations of nickel, copper and cobalt. Further, oceanographic observations over the next century gradually established that such nodules were, to various extents, abundant in the low-latitude zones of the deep oceans. The Indian Ocean floor began to receive particular attention from the early 1970s, when the French research ship Marion Dufresne recovered 450 ocean-floor sediment cores. At about the same time, ocean-floor nodules were recognised as mineral resources of potential economic value. Commercial interest soared and entrepreneurial ocean-floor mining consortia made substantial investments in innovative technologies for nodule exploration, mining and mineral processing—even though a legal framework for the exploitation of the ocean floors was lacking until the United Nations finally concluded its Law of the Sea Treaty in 1982. Subsequently, the UN International Seabed Authority allotted a 1.5 million km2 site of the floor of the central Indian Ocean to India. The country’s National Institute of Oceanography then set out to map, characterise and evaluate the nodule resources of this region, the best-endowed portion of which has become known as the Indian Ocean Nodule Field (IONF). xi
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Series Editor’s Foreword
In this volume, Ranadhir Mukhopadhyay and Sridhar D. Iyer, senior scientists at the National Institute of Oceanography, and Anil K. Ghosh, an eminent academician in the field of mineral resources at University of Calcutta, bring together the extensive but hitherto scattered scientific results of the exploration of the IONF since the 1980s. Their first chapter explains the general physiography, geology, biology, physics and chemistry of the seafloor occupied by the IONF and some characteristics of the overlying water column. The subjects of their next two chapters are the structural, tectonic and volcanic features of the IONF. In Chapter 4, they describe the bottom sediment that hosts the ferromanganese nodules and crusts and go on, in Chapter 5, to explain the processes of the formation of nodules and crusts in the light of variable source material, tectonic activity and mid-plate volcanism. The mining, environment, metallurgy, legal and economic aspects of the IONF resources are described in Chapter 6. Throughout, recent concepts, hypotheses and critical appreciation of the state-of-the-art knowledge on nodule formation and resource management are incorporated. The wealth of information that the authors have brought together here marks out this volume as the definitive work on the IONF. Martin Hale The Netherlands March 2007
ABOUT THE AUTHORS
Ranadhir Mukhopadhyay, 49, was the Director of the Mauritius Oceanography Institute (2002–2004) and currently is a senior scientist with the National Institute of Oceanography, Goa. He is the recipient of the Asiatic Society Medal, Raman Research Fellowship and M. S. Krishnan Gold Medal. He has co-authored a book and has about 40 research papers and articles to his credit. He has worked on plate boundary geomorphology, seamount-tectonics and marine minerals. Anil K. Ghosh, 67, is an UGC Emeritus Fellow with the University of Calcutta and received several awards including the Universtiy Gold Medal, Asiatic Society Medal and Coggin Brown Gold Medal. He specialises in mineral resources and has been teaching at the postgraduate level for more than four decades in two faculties (Geology and Marine Sciences). He has written about 40 research papers, co-authored a book on marine mineral resources and edited another on land-based resources. Sridhar D. Iyer, 48, is a keen petrologist, and is currently a senior scientist with the National Institute of Oceanography, Goa. He is the recipient of Young Scientist award and Raman Research Fellowship. He has worked extensively on volcanics, occurring both on the younger and on older oceanic crusts of the Indian Ocean and has authored about 60 research papers and articles.
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C H A P T E R
O N E
The Indian Ocean Nodule Field
Contents 5 6 8 11 13 17 22 24 25 25 27 28 31 32 35
1. Physiography and Geology 1.1. India–Australia plate boundary 1.2. Australia–Capricorn plate boundary 1.3. The Chagos–Laccadive Ridge 1.4. The Ninetyeast Ridge 1.5. The Carlsberg Ridge 1.6. The Central Indian Ridge 1.7. The Southeast Indian Ridge 2. Physics, Chemistry and Biology 2.1. Physical characteristics 2.2. Chemical characteristics 2.3. Biological characteristics 3. Evolution of the Indian Ocean Nodule Field 3.1. Break-Up of Gondwanaland 3.2. Formation of the Indian Ocean Ridge system
The Indian Ocean constitutes about one-seventh of the earth’s surface and is the world’s third largest water body. This ocean covers an area of 73.6 million km2, and is separated from the Atlantic and the Pacific oceans by roughly 20 E and 147 E, respectively (Fig. 1.1). This ocean has no extension towards the North Pole unlike the Atlantic and the Pacific oceans. The detachment from the Arctic polar water system and the presence of mountainous Asian landmass are responsible for several interesting phenomena in this ocean, such as atypical circulation of water mass, extensive upwelling of nutrients along the east African coast and the formation of summer and winter monsoon in Asia, among others. Scientific exploration in the Indian Ocean started long after the same was initiated in the Pacific and the Atlantic oceans. HMS Challenger from Great Britain was the first ship to start research in this ocean in a big way during its famous transworld voyages between 1872 and 1876. The John Murray Expedition, onboard Mabahiss in 1933–1934, was the next major one. A few other oceanographic expeditions, namely, Dana (1928–1930), Snellius (1929–1930), Albatross (1950– 1952) and Ob (1955–1957), were also made with specific objectives. Later, two major events paved way for a full-fledged scientific research in this ocean. These were (1) International Geophysical Year (IGY, 1957–1958) and (2) International Indian Ocean Expedition (IIOE, 1963–1966). Handbook of Exploration and Environmental Geochemistry, Volume 10 ISSN 1874-2734, DOI: 10.1016/S1874-2734(07)10001-2
#
2008 Elsevier B.V. All rights reserved.
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20⬚E
40⬚
60⬚
80⬚
100⬚ 20⬚ N
0⬚
20⬚ S
40⬚
Figure 1.1 General physiography, essential tectonic elements and bathymetry of the Indian Ocean and neighbouring seas (base map from NGDC). The Indian Ocean Nodule Field (IONF) within the Central Indian Ocean Basin (CIOB) is shown in square box. Note inverted Y-shaped profusely fractured mid-ocean ridge system, the long north-south-trending Chagos^Laccadive Ridge and Ninetyeast Ridge on either side of the IONF, and the Andaman^Sumatra subduction zone to the east.
The magnitude of IIOE, in terms of involvement of several countries, made it the greatest oceanographic endeavour to date. There were in all 36 ships from 13 countries, which took part in the expedition lasting for 4 years. After IIOE, the next major work was the drilling of the oceanic crust by Glomar Challenger, under the Deep Sea Drilling Project (DSDP, later known as the Ocean Drilling Programme, ODP). The drilling programme was further complemented by the Joint Oceanographic Institutions for Deep Earth Sampling ( JOIDES) and, in recent times, by Integrated Ocean Drilling Programme (IODP). In the 1980s, the studies of the Carlsberg and Central Indian ridges started in an organised manner. The German vessel FS Sonne deciphered two sites of significant hydrothermal activity along this ridge under an Indo–German collaborative programme, Gemino. The French, American, British and Indian efforts are presently concentrated along these ridges in search of hydrothermal sulphide mineralisation and to understand the interactions among asthenosphere, lithosphere, hydrosphere, biosphere and atmosphere. Some of the important characteristics of oceanic crust and mantle are furnished in Table 1.1. Among the various basins in the Indian Ocean (Fig. 1.1), the Central Indian Ocean Basin (CIOB) was particularly studied for its mineral potential. During the last two decades, the scientific community explored the CIOB both extensively and intensively. Since 1982, more than 50 cruises have been undertaken in this basin, which ultimately led to the identification of a manganese nodule-bearing area in the CIOB. Such resource is also known as polymetallic nodules and ferromanganese
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The Indian Ocean Nodule Field
Table 1.1 Physical characteristics of oceanic crust and mantle
Layer
Lithology
1 2A
Sediments Basaltic sheet and pillow lavas Basaltic dykes Gabbros Layered gabbros Peridotites
2B 3A 3B 4
P-wave velocity
Thickness (km)
Avg. density (g/cm3)
1.7–2.0 2.0–4.1 (3.6)
<1 0.0–1.5 (0.5)
2.3 2.7
4.0–5.6 (5.2) 6.5–6.9 (6.9) 7.0–7.5
0.6–1.3 (0.9) 2.0–3.0 (2.5) 2.0–5.0
2.7 3.0 3.0
8.1
–
3.4
Sources: Iyer and Ray (2003) and references therein.
deposits (see Chapter 5) as they comprise more than two metals, of which iron and manganese remain predominant. The commercially exploitable portion of the nodule-bearing area within the CIOB was demarcated and termed as the Indian Ocean Nodule Field (IONF; Mukhopadhyay et al., 2002). The IONF, characterised by high nodule abundance, rich metal grade and seemingly unobstructed topography, contains the world’s second largest and the second richest manganese nodule resources, after the equatorial North Pacific nodule belt. The field extends roughly from 9 S to 16 300 S and 72 E to 80 E (square block in Fig. 1.1), covering an area of 739,260 km2. Studies carried out so far in the IONF (average water depth of 5000 m) have successfully identified two sites of nodule resources, each of 150,000 km2, which could be exploited economically in the future. The IONF is bordered on the north and southeast by plate boundaries between India and Australia and Australia and Capricorn, respectively, and on the west and east by aseismic ridges such as the Chagos–Laccadive Ridge (CLR) and the Ninetyeast Ridge (NER), respectively. The floor of the IONF was generated from the Indian Ocean Ridge system (IORS) during the Paleocene (between 60 and 49 Ma) and can be approximately divided into four sectors, having variable rates of formation of their underlying crust, as deduced from magnetic anomaly data (Royer et al., 1989; Mukhopadhyay et al., 1997; Fig. 1.2). The northern part (sector A), which appears to have generated at a faster rate (90 mm/year, half rate), extends between 9 S and 10.26 S. This part was formed prior to chron 26 (i.e. earlier than magnetic anomaly 26, corresponding to an age earlier to 57.9 Ma). The crust to its south in the north-central region, occurring between 10.26 S and 10.96 S (sector B), was formed at an intermediate rate of generation (55 mm/year, half rate) between chron 26 and 25 (i.e. between 57.9 and 55.9 Ma). The south-central region (sector C) extends between 10.96 S and 13.76 S and was formed at a superfast rate of 95 mm/year during the period 55.9–50.8 Ma (between chron 25 and 23).
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Mukhopadhyay, Ghosh and Iyer
10⬚ N
20 20
5 5 5
Indrani FZ
CLR
0⬚
Vishnu FZ
CIR
IAPB
5 A B
10⬚
24 24
C
5
D
24
20 20
5 5
20 T-
In
5
5
TJ
20⬚ S
5
5 5
5
5 SEIR
SWIR 5
5 5
5 5 70⬚E
80⬚
Figure 1.2 The four sectors in the Indian Ocean Nodule Field (IONF)çA to Dçare shown in the box. The other notable features are Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), Southeast Indian Ridge (SEIR), Chagos^Laccadive Ridge (CLR), Indian^Australian Plate Boundary (IAPB) and fractures zones (Indrani and Vishnu). The crust of the IONF was formed between 60 and 45 Ma, with sectors A and C formed at a fast rate, while sectors B and D were generated at a slow rate (sector D, in fact, formed after the India^Eurasia collision). Dashed long largely north-south lines are fracture zones, and smaller dashes with numbers are magnetic anomalies. Long curved line connecting black dots is the trace of the triple junction on the Indian Plate (TJT-In) (base map from Royer et al.1989).
The remaining southernmost part of the IONF (sector D, 13 760 S–16 300 S) was formed at a slow rate (26 mm/year) after chron 23 (i.e. younger than 50.8 Ma). The structural details of the IONF are given in Chapter 2.
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The Indian Ocean Nodule Field
The formation, evolution and management of the mineral resource in the IONF and related aspects are taken up in the subsequent chapters. However, before the resources are discussed, the salient geological, physical, chemical and biological characteristics of the IONF, which have influenced the origin, distribution and enrichment of manganese nodule deposits, are described here.
1. Physiography and Geology Plate boundaries of various types in the world oceans, such as divergent, convergent, transitional and diffused, are shown in Fig. 1.3. Interestingly, all these four types of plate boundaries can be found in the India Ocean. The most spectacular geological features within and in the neighbourhood of the IONF (including its flanks), which have a direct or indirect bearing on the formation, distribution and economics of its mineral resources, are briefly described.
Mid-plate boundaries: The IONF is sandwiched between two plate boundaries— India–Australia Plate boundary (IAPB) in the north and Australia–Capricorn Plate boundary (ACPB) to the southeast. Aseismic ridges: The influence of hotspots on the volcano-tectonic framework of any basin in transferring energy fluxes from warmer regions to the colder regions has always been outstanding. Two such prominent aseismic ridges run along the western and eastern boundaries of the IONF. These ridges are Chagos–Laccadive and Ninetyeast that were formed by the eruption of Reunion and Kerguelen hotspots, respectively. Mid-Oceanic Ridge (MOR) system: The IONF floor is disturbed by several robust, long fracture zones and transform faults, which appear very significant in
60⬚
30⬚
0⬚
30⬚
60⬚ 0⬚
30⬚
60⬚
90⬚ 120⬚ 150⬚ 180⬚ 210⬚ 240⬚ 270⬚ 300⬚ 330⬚
0⬚
Figure 1.3 Plate boundaries of various types. Green ¼ subduction zones, light blue ¼ midoceanic ridge, deep blue ¼ deformed boundary zone, red ¼ continental deformation zone (Kreemer etal.,2002).
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Mukhopadhyay, Ghosh and Iyer
creating the delicate ambience for the formation of ferromanganese deposits in the field. The origin of all these ancient structural lineaments, generated from the IORS, is of great interest. Volcanic environment: The IONF hosts a good number of seamounts of various heights and dimensions. Although a majority of these seamounts are ancient in nature and formed along with the underlying crust, a good number of smaller seamounts and abyssal hills are however formed at later times during the northward journey of the underlying crust. These seamounts appear to influence the nature of substrate and metal budget of nodules. We describe this aspect in detail in Chapter 2. Ganges–Bramhaputra fan: The several kilometres thick sedimentary fan of the Ganges–Bramhaputra river complex, comprising of drained sediment load from the Himalayas and the Indian continental plains, represents an important physiographic feature in the area. The fan extends up to about 10 S and influences the paleoceanography, besides impacting on the formation and distribution of manganese nodules in the IONF. We describe this aspect in detail in Chapter 4.
1.1. India–Australia plate boundary As the collision between India and Eurasia intensified after an initial impact at about 51 Ma, compressional forces accumulated in the mid-plate region of the IndoAustralian Plate. These forces reached a stage of failure and triggered seismic activity, thus deforming the upper lithosphere over a wide region around the equator. The deformed zone, often considered as the boundary between the Australian and Indian plates (Fig. 1.4), occurs between 4 N and 10 S, and extends from the Central Indian Ridge (CIR) in the west to the Wharton Basin in the east. This IAPB is characterised by an unusually high level of seismicity, free-air gravity anomaly (30–80 mGal), high geoid anomaly of about 4-km amplitude, folding and high angle faulting of oceanic basement and overlying sediments and hydrothermal activity (Stein et al., 1989; Sykes, 1970; Weissel et al., 1980). Yet the exact nature of the deformation at the IAPB is yet to be fully understood. The questions that arise are: Is a new subduction zone being created at the IAPB? Are folding and faulting in the region developed by regional deformations? Is there any evidence of a new tectonic phenomenon beyond the comprehension of the existing plate tectonic model? The IAPB deformed zone appears to be bounded on all four sides by fracture zones: on the north and south by younger fractures, and on the east and west by older ones. The roughly north-south-trending Chagos–Laccadive aseismic ridge, running along the 73 E meridian, seems to have divided the IAPB into two parts. The crust east of this longitude seems to be older in age (60–90 Ma), is characterised by a compressive regime and contains very few north-south-oriented short fracture zones. In contrast, the crust west of 73 E is relatively younger (30 Ma), includes NESW-trending long fractures and involves extensional tectonic activities. The deformation in the IAPB appears to have occurred in two phases. The first phase is characterised by contraction of the oceanic crust during which most of the overlying sediments were folded in an east-west direction with long wavelengths
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The Indian Ocean Nodule Field
6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 N
S 10 km
TWT (s) 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 TWT (s)
Figure 1.4 Multi-channel seismic section based on two-way travel time (TWT) across the Indian^Australian deformation zone (IAPB), north of the Indian Ocean Nodule Field (IONF). Note uninterpreted (top) and interpreted (bottom) sections, with south-dipping reverse faults and basement highs on the either end with trough at the centre (Bull et al.,1992).
(100–300 km) and high amplitudes (1–3 km). Superposed on these undulations is the second phase of deformation with 5–20-km wide faulted and rotated blocks, separated by high-angle reverse faults (Fig. 1.4). As a possible consequence of that tectonic superposition and reactivation, many of the north-facing normal faults formed at the ridge crest were reversed to south-facing thrust faults. Such reversals of fault planes resulted in crustal shortening by 18 (6) km to 37 km at a rate of 4 (0.9) mm/year. This calculation using Euler vectors remotely suggests that the distance between Kolkata in India and Sydney in Australia is decreasing by 12 (3) mm/year, as determined also from space geodetic data (Chamot-Rooke et al., 1993; Gordon et al., 1990). The IAPB may have been formed as a result of isostatic inequilibrium in the lithosphere due to buckling and thickening owing to collision of the Indian Plate with Eurasia along the present Himalayan trend and to the variable spreading across the IORS. A localised high heat flow of >50 mWm2 above regional background and non-linear thermal gradient suggest circulation of hydrothermal fluid along fault planes in the deformation zone. The high heat flow (100–200 mWm2) and three large free-air negative gravity anomalies at 4 S/80 E (40 mGal), at 0 450 N/ 82 550 E (30 mGal) and at 1 N/83 450 E (70 mGal), with corresponding geoidal
8
Mukhopadhyay, Ghosh and Iyer
undulations, favour a uniform crustal folding or buckling within the Moho (cf. Neprochnov et al., 1998). The gravity model study suggests that the average thickness of the crust in the IAPB is about 5 km, increasing up to 8 km, particularly towards the east at around 80 E due to interaction with intensely folded crustal blocks (elastic thickness, Te ¼ 12–15 km). The deformation in the IAPB is a subject of debate. This deformation appears to have influenced drainage and depositional dynamics in the CIOB in conjunction with Himalayan tectonics. The large contribution of clastic materials (including heavy minerals) from the Himalayas is noted in the sediment that tops the crust in the IAPB deformed zone. Such deposition appears to have occurred in two periods— first between 11 and 7.5 Ma (Late Miocene) and the second around 0.9 Ma (Late Pleistocene). These two phases of sedimentation were interrupted by sediment contributions from the Lesser Himalayas and the Indian Subcontinent. It hints at two primary phases of Himalayan uplift during Late Miocene and Pleistocene. The intensification of the Indian monsoon at about 8 Ma resulted in a rapid denudation of clastic sediments. New seismic reflection data reveal (Fig. 1.5) that the first deformation in the IAPB may have occurred during Late Miocene (7.5–8.0 Ma, well evident in the southern part of the IAPB, south of 7 S), followed by another during Early Pliocene (4–5 Ma, well developed in the northern part, north of 7 S), and the last deformation during Late Pleistocene (0.8 Ma, overlapping the Miocene and Pliocene deformed areas). The sedimentation history also supports this seismic finding. For example, the syn-deformation sediments are separated from predeformation sediments by an unconformity dating 7 Ma (Late Miocene), probably indicating the onset of plate boundary deformation. The second unconformity dates to Upper Pleistocene (800 ka) and separates glacial from non-glacial strata.
1.2. Australia–Capricorn plate boundary The data on seismicity and plate motion beyond the southern borders of the IONF are inconsistent, keeping in view the accepted plate tectonic theory that plates are rigid, least deformed in their interior and should have narrow boundaries. However, Two-way travel time (S)
0
10
20
30
40
50
60
70
80
90
100 110 km
Upper Upper miocene u/c pleistocene u/c
6.4 6.6 6.8 7.0 7.2 7.4 7.6
6.4 6.6 6.8 7.0 7.2 7.4 7.6
Deformed sediment Basement rise 04⬚34.86⬘S 86⬚52.47⬘E
Faulted basement
AS10-05
03⬚31.38⬘S 86⬚53.85⬘E
Figure 1.5 Crumpled oceanic basement and sediment in the form of tight folds and high-angle faults located in the Indian^Australian deformation zone (IAPB), just north of the Indian Nodule Ocean Field (IONF).These were formed during various geological periods as marked by unconformities (Krishna et al.,1998).
9
The Indian Ocean Nodule Field
several discrepancies to this concept were encountered in the Australian Plate (southeast of 16 300 S) in terms of (1) uncharacteristic NW-SE shortening, (2) change in the orientation of axis of shortening from NS to NW-SE from the west to east of 86 E, (3) occurrence of an anomalous plate boundary type, for example, off-axis normal faulting earthquakes near 80 E on the Southeast Indian Ridge (SEIR) and (4) reactivation of extinct fracture zones in the Wharton Basin. The reason for such inconsistencies was investigated on an 11-Ma-old seafloor flanking the South West Indian Ridge (SWIR) and SEIR (¼ anomaly 5, Royer and Gordon, 1997), to see whether such a discrepancy could be justified by any deformation within the Australian Plate. The findings suggested that these discrepancies could be suitably rationalised if one recognises the occurrence of a new Capricorn Plate (Figs. 1.6, 1.7). The N40E-oriented gravity undulations in the ACPB(200-km wavelength and 10-mGal amplitude) differ in trend from the IAPB by about 40 and in intensity by a factor ranging between 2 and 3. Incidentally, undulations in the ACPB are oriented along the absolute motion direction of the Indian and the Australian plates in the hotspot reference frame, and occur largely on the fast spreading Australian Plate and not on the slow spreading Antarctica Plate. The plate reconstruction, based on magnetic anomaly data and fracture zone across the SEIR, suggests convergence
69⬚E
Gravity (mgal)
68⬚E
50
70⬚E
71⬚E
72⬚E
73⬚E
74⬚E
CIR 5⬚S
0 −50 6⬚S
CB −100
20⬚N
IND
7⬚S DG
0⬚
8⬚S 20⬚S
CAP
80⬚E
M = 5.0 AUS
100⬚E
Figure 1.6 Plate configuration in the Central Indian Ocean Basin (CIOB) from satellite-derived gravity field (Sandwell and Smith, 1997). IND ¼ Indian, CAP ¼ Capricorn, AUS ¼ Australian plates, CB ¼ Chagos Bank, DG ¼ Diego Garcia, CIR ¼Central Indian Ridge. Earthquake epicentre data (courtesy: ANSS) in open and black circles in the main and inset figures. Gray in inset represents plate boundary zones, while horizontal lines represent extension of India^Capricorn plates (Henstock and Minshull, 2004).
10
Mukhopadhyay, Ghosh and Iyer
60⬚
EURASIA 50⬚
40⬚ 8 30⬚ Bhuj
7
ARABIA
20⬚
6 INDIA
10⬚
5 0⬚ 4 SOMALIA
−10⬚
CAPRICORN 3
−20⬚
M
−30⬚
AUSTRALIA km 0
ANTARCTICA
−40⬚ 40⬚
50⬚
60⬚
70⬚
80⬚
90⬚
20 mm/yr
1000 100⬚
110⬚
120⬚
Figure 1.7 Seismicity recorded between the years 1900 and 2001 on the Indian, Capricorn and Australian plates shown in circles with radius proportional to intensity. Arrows show direction of spreading (Stein et al., 2002).
of 23 2.6 km since 11 Ma between the Capricorn Plate at 10.3 S, 83.5 E and the Australian Plate at 17 S, 105 E. This corresponds to a rate of 2.4 2.1 mm/year. This rate of convergence is however much slower than the global average rate of 70 mm/year, and even less than the slowest one known so far (20 mm/year, DeMets et al., 1990). The reconstruction also indicates that since 11 Ma, a point now at 17 S, 105 E on the Australian Plate has moved 27 km along N45W relative to the Capricorn Plate. Large earthquakes reflect the convergence and large-scale folding of the lithosphere, represented by undulations in northeast striking gravity, occurring on both sides of the NER (Royer and Gordon, 1997). The reconstruction also indicates a divergence of 13 2.4 km since 11 Ma between the Capricorn Plate at a point 26 S, 74 E and the Australian Plate at 41 S, 90 E. The convergence corresponds to a rate of 1.2 2.2 mm/year, which is much slower than global values (average 40 mm/year, least 10 mm/year; Chu and Gordon, 1998). The average rate of rotation during the last 11 Ma between the Indian and the Capricorn plates has been estimated as 0.24 0.02 per million years,
The Indian Ocean Nodule Field
11
while that between the Capricorn and the Australian plates has been 0.07 0.03 per million years (Royer and Gordon, 1997). A recent study suggests extension of 10–12 km of crust over a zone of about 50 km in Chagos area as evidenced by crustal thinning and concentration of seismicity. The localisation of seismic activities beneath Chagos Bank is due to a weak rheology, probably caused by thick crust. The area of deformation related to the composite Indian, Capricorn and Australian plates (IAPB þ ACPB) exceeds the size of several individual plates, and may have influenced the development of an increased number of closely spaced transform faults across the northern CIR.
1.3. The Chagos–Laccadive Ridge Aseismic ridges are a group of non-spreading, topographically elevated chained features on the seafloor formed either by volcanic eruption along a leaky transform fault, or hotspot eruption, or through uplift and rifting associated with tectonic activities. The hotspots are long lived and relatively fixed, mantle thermal manifestations that produce magma chemically distinct from that erupting at mid-ocean ridges (Table 1.2). The hotspot-generated aseismic ridges of the Indian Ocean suggest that rapid, northward migration of the Indian Plate over several hotspots during Cretaceous and Early Cenozoic times produced the majority of the chained volcanic traces (Fig. 1.8). Of these, the two aseismic ridges, which act as western and eastern borders of the IONF, are (1) the CLR (a part of Reunion Island–Mauritius Islands– Southern Mascarene Plateau–Chagos Ridge–Maldives Ridge–Laccadive Ridge– Deccan Traps chain believed to have been formed by the eruption of Reunion hotspot) and (2) the NER(a part of the Kerguelen Islands–Ninetyeast Ridge–Broken Ridge–Rajmahal Traps chain probably caused by Kerguelen mantle plume). Extending from 14 N to 9 S on the Indian and the Australian plates, the CLR is an 2550-km long and 200-km wide aseismic volcano-topographic feature created by the eruption of the Reunion hotspot approximately along 73 E meridian (Fig. 1.8). This ridge, slightly convexed towards the east, forms the western boundary of the IONF. A considerable length of the crest of the CLR is composed of shoals, banks, coral reefs and atolls at depths <1500 m (Fig. 1.9). The Laccadives, Maldives and Chagos on the Indian and the Australian plates and St. Brandon, Mauritius and Reunion on the African Plate represent the islands formed along the track of the Reunion hotspot. The eastern flank of the CLR between 8 and 10 S is bounded by a steep depression called the Chagos Trench. The free-air gravity anomalies over the CLR are negative and relatively low, but are higher than those of the surrounding regions. The origin of the CLR is attributed to various processes such as eruption through a leaky transform fault (Fisher et al., 1971), mixing (transition) between oceanic and continental crust (Narain et al., 1968) and a hotspot trace (Duncan and Hargraves, 1990; Francis and Shor, 1966). The seismic refraction measurements and high amplitude magnetic (HAM) anomalies support a volcanic origin for the CLR. The Chagos Bank (the southern part of the CLR), located close to the western boundary of the IONF, has basaltic basement of 49 Ma at a depth of 107 m below the seafloor (ODP site 713). The results of DSDP and ODP suggest that the Deccan
Table 1.2 Characteristics of major hotspots in the Indian Ocean Products Hotspot
Present latitude
Present longitude
Initiation (Ma)
Afar
12 N
42 E
30–15
Seamount chaina
Oceanic plateaua
–
–
–
– Mascarene (9.71) – Kerguelen (44.66) Conrad Rise (9.45) Del Cano Rise
Reunion
21 S
56 E
68–66
Ethiopian Flood Basalt Aden & Yemen Traps Deccan Traps
Kerguelen
45 S
65 E
136–105 128–108
Bunbury Basalts Rajmahal Traps
Crozet
53 S
50 E
115–110
–
Chagos–Laccadive Ridge (15.58) – Ninetyeast Ridge (23.74) 85E Ridge
Prince Edwards/ Marion
47 S
42 E
200–175 95–85
Karoo Basalt Dykes, intrusives
Madagascar Ridge
29–20
a
Igneous provincea
Volume in million square kilometre. Sources: Baksi et al. (1987), Courtillot et al. (1988), White and McKenzie (1989), and Muller et al. (1993).
13
The Indian Ocean Nodule Field
Deccan 117
Rajmahal
65 82
57
77
64 48 49 31 35 2 8 Reunion
62 10
58 43
27
38
Kerguelen 14
Figure 1.8 Computer-modelled hotspot tracks in the Indian Ocean with predicted trails (heavy dashes) at 10-Ma increment (Duncan and Storey,1992).
Trap province and the CLR were formed by volcanic build-up during the northward motion of the Indian Plate over the Reunion hotspot. The CLR ages towards the north with volcanics of younger age found to the south. Geochemical and geochronological data from ODP site 713 indicate a possible mixing of melts from hotspot and Mid-Ocean Ridge Basalt (MORB) sources. Such a mixing is possible if the trapped magma from the Reunion hotspot resulted in the production of excessive crustal material at about 50 Ma at site 713 of the ODP leg 115. It was suggested that lava solidified sub-aerially between the Maldives and the Laccadive islands, whereas between the Maldives and the Chagos archipelagoes the lava poured out at great depths (Duncan and Hargraves, 1990). However, a few other ideas preferred continental origin for the Laccadives–Maldives section because of lack of appreciable magnetic anomalies found over this stretch, yet maintaining the hotspot eruption theory for the Maldives–Chagos section of the CLR.
1.4. The Ninetyeast Ridge The NER, one of the longest major linear volcanic features in the world, was generated by the eruption of the Kerguelen hotspot over the northeast moving Indian Plate. The ridge extends for about 5000 km, starting from Rajmahal volcanics in India located at 17 N to about 35 S. The NER trends north-south approximately
14
Mukhopadhyay, Ghosh and Iyer
100 0 −100 5.0
1
4
2
5
6
0 −5.0 100 0 7
−100 5.0 0 −5.0
INDEX 100 mGal
100 0 −100 5.0
Gravity anomaly
−100 5.0 Km
3
0
0
0
Bathymetry
−5.0
−5.0 0
250
500
Kilometers
Figure 1.9 Projected gravity (top) and bathymetry (bottom) profiles across the Chagos^ Maldives^Laccadive Ridge. Profiles 1 to 3 are across Laccadive, profiles 4, 5 and 6 across Maldives, while profile 7 is across Chagos (Ashalata et al.,1991).
along the 90 E meridian (Figs. 1.8, 1.10). The width and height of the NER are 170 and 3.4 km, respectively, north of 15 S, and about 220 and 3.6 km south of 15 S. Thus, the NER is wider and a little shallower towards the south. The steep and deep eastern flank of the NER (relative to the western flank) is interpreted to represent a transform fault, across which the crust to the west ages towards the north, while that to the east ages to the south. Opinions vary for the origin of the NER such as (1) formed as a horst (Francis and Raitt, 1967), (2) formed because of over thrusting (Le Pichon and Heirtzler, 1968), (3) a hotspot trace (Morgan, 1972), (4) formed through localised emplacement of gabbro (Bowin, 1973) and (5) formed through eruptions at the ridge-transform fault intersection (RTI) (Sclater and Fisher, 1974). The high-resolution seismic reflection and refraction studies, aided by gravity and magnetic anomaly data, suggest that the NER was formed because of a combination of several complex tectono-magmatic processes (Fig. 1.10). For example, the portion of NER north of 2.5 S was a product of volcanism on the Indian Plate, the portion between 2.5 S and 11 S was most likely emplaced on the edge of the Indian Plate, the portion between 11 S and 17 S
15
The Indian Ocean Nodule Field
86⬚E FZ
UC 24
ZO
30
28
24
27
23
23
0⬚ 20
22 26 30
21
21
20
25
29
20
33 32
20
23
27
61my
29 27
28
26
27
23B 29? 28?
26
214 29? 28?
59m
26 26
25 23A 24B 25 22 24A 24B 23B 24B 24A 21 23A 23B 24A 22 23B 23A 20 21 22 23A 21 22 ? 21
20 22
24B
22? 20
24B
27 27
26
30 29
29
23B
31
15⬚
27 30
22
28
24? 59my 21 B 20?
29
32A 31
32
30?
20?
20⬚
31 33
29? 30?
32
18 30?
20
>44my
94⬚E FZ
13
31?
92⬚E FZ
33
253
32B 33
25
21?
29?
28 29
28
24B
?
10⬚
26
26
25
23B 23? 757 B
26 25
23
20 21? 23A
27? 27?
25
23A 22 20
25
22
96⬚E FZ
30
24
21
88⬚E FZ
215
31 29
24
5⬚
22
32
30
25
22
21
28 31
E
N
32
33
ON
>65my
31
25
29
216
TI
84⬚E FZ 84.5⬚E FZ
30
32
34
32
5⬚ N
BD
83⬚E (Indira) FZ
31
>80my
SU
34
DA
34
32
758 33
SU N
34?
SRI LANKA
25⬚ S
756 >38my
85⬚E
90⬚
95⬚
Figure 1.10 Tectonic disposition of the Ninetyeast Ridge (closed north-south contour) with a decrease in age from north to south. Location of ODP sites (756, 757, 758) and DSDP sites (214, 215, 216, 253) are shown by small solid dots. Also shown magnetic lineation, abandoned spreading centres (solid rectangles) and fracture zones (dashed lines) (Krishna et al.,1995).
16
Mukhopadhyay, Ghosh and Iyer
0 A
50 km
2
4
6 S
2.5
3.0 S B TWT
5 km
88⬚E
2
4 S
0
0
50 km
2
N
S
3.0
3.5 S
4 TWT
TWT
88⬚E
6
0
5 km
8
TWT = Two-way travel time (s)
Figure 1.11 Seismic reflection sections across the Ninetyeast Ridge. Note the thick pelagic sediment cover at the crest, more than 400 m at places (cf. Ramana et al., 2001).
resulted from hotspot volcanism over a spreading axis and the portion south of 17 S was formed again on the edge of the Indian Plate (Fig. 1.11). Emplacement of the NER, particularly between 14 S and 17 S, has been unique as during the period 60 and 54 Ma excess crust was generated because of the location of the Kerguelen hotspot below the Wharton Spreading Ridge. This resulted in thickening of the crust (20 1.3 km, Krishna and Rao, 2000; Sinha et al., 1981). The magnetic and seismic conducted studies in the vicinity of the NER reveal the presence of a chain of abandoned spreading centres (ASC) of different ages. These ASC ceased spreading at about 42 Ma and are interpreted to be the western extension of the east-west-trending Wharton Spreading Ridge. This ridge jumped southward between 65 and 42 Ma, and in the process a considerable amount of the oceanic crust formed originally on the Antarctic Plate between anomalies 30 and 32 n.2 was transferred to the Indian Plate (Fig. 1.12; Krishna and Rao, 2000). Petrologically, the NER is composed of tholeiitic basalt, ranging from low MgO and aphyric olivine in the south to strongly plagioclase-phyric, high MgO towards the north. The rocks are extensively altered (H2Oþ > 1%) in a low-temperature environment under both reducing and oxidising conditions (ODP 115, DSDP legs 22, 26; Weis et al., 1992). Major element and isotopic composition of basalts along
17
86⬚E FZ
84.5⬚E FZ
217 (85 Ma)
A30-A34
4 A3 0-
A3
Spreading center ceased at 65 Ma
A28-A34 A25-A34 A29-A34
A
30
-A
3
. 2n
2
Portion of ANT plate transferred to IND plate
A3
4
A3
756 (82 Ma)
216 (78 Ma)
A3 34 0-A
4
A3
0-
A3
Ridge jump between A32n.2-A33
Ninetyeast ridge
A3
216 (78 Ma)
0-
A3 4
756 (82 Ma)
Indian plate
Indira FZ
A3 4
217 (85 Ma)
0-
Indira FZ
84.5⬚E FZ
Indian plate
Ninetyeast ridge
86⬚E FZ
The Indian Ocean Nodule Field
4 A3 0A3
A3
0-A
34
A3
0-
215 (61 Ma) A26-A29
Kerguelen hotspot
Antarctica plate
Indian plate
65 Ma (after anomaly 30)
Present (only north of 108S)
Figure 1.12 Left panel shows a spreading ridge (slanting brick) separating the Antarctic Plate (dotted) from the Indian Plate (small squares). The spreading ridge jumped towards south between chron A32 (72 Ma) and A33 (78 Ma). In the process, it transferred (right panel) the oceanic crust that accreted originally at the Antarctic (ANT) Plate between anomalies 30 and 32 n.2 to the Indian (IND) Plate (Krishna et al.,1998).
the NER suggest the presence of two compositionally distinct parental magmas. The compositional variations in the rocks along the NER may be due to fractional crystallisation and variable extent of melting and magma mixing. It appears that the lavas for both CLR and NER erupted close to the spreading axis, consistent with the plate tectonic reconstruction model of the eastern and the western Indian Ocean basins (Bhattacharya and Chaubey, 2001; Ramana et al., 2001). As both the Reunion and Kerguelen hotspots were crossed by one spreading axis each (CIR and Wharton Ridge, respectively), the mixing of the plume with ridge-axis melt (N-MORB) depletes the melt both in trace element and in isotopic components. Early products of the Reunion and the Kerguelen hotspots have more depleted isotopic signatures than the later products, reflecting an increase in the mixing of hotspot and ridge-axis melts in the past.
1.5. The Carlsberg Ridge The MOR system in the Indian Ocean (known as the IORS) differs from that in the Pacific Ocean in terms of rate of spreading (Table 1.3), element chemistry (Table 1.4) and isotopic composition (Table 1.5). The entire MOR system forms a 70,000-km long serpentinous mountain chain and rises to about 2–3 km above the surrounding seafloor. This submarine chain encompasses about 33% of the total area of the ocean floor and covers 35.9% of the Pacific, 31.2% of the Atlantic and 30.2%
18
Mukhopadhyay, Ghosh and Iyer
Table 1.3
Characteristics of accreting mid-ocean ridges
Slow-spreading ridge
Fast-spreading ridge
Spreading rate < 40 mm/year (full rate).
Spreading rate 80–160 mm/year (full rate). Absence of accumulated stress and hence very less prone to earthquakes. Smooth seafloor morphology.
Presence of deep-seated earthquakes and major normal faults. Rough to very rough seafloor morphology. Median valley present.
A rise develops because of absence or poorly formed median valley. Symmetrical and elevated volcanic edifices are present. Magmatic activity dominates over tectonic processes.
Highly segmented and asymmetric rifted depressions are conspicuous. Nature and scale of segmentation is predominantly controlled by tectonic activity. Neo-volcanic zone is wider (2–12 km) and Neo-volcanic zone is narrow has small seamounts. (100–200 m) and seamounts are virtually absent. Near-absence of off-axis seamounts. Frequent presence of off-axis seamounts. Magmatic discontinuities and unfocussed Magmatic continuity and relatively magmatism. focussed magmatism. A low rate of magma outpouring prevails. The magma outflow rate is higher. Volcanic eruptions are larger. Volcanic eruptions are smaller. Dykes are less but if present are large in Dykes are common but are of smaller dimension. dimension. Pillow lavas abundant. Sheet flow dominates. A narrow range of relatively A wider range of generally more undifferentiated lavas is produced. differentiated lavas is produced. The compositional variations are The lavas show more complex chemical dominated by relatively simple lowtrends ascribed to polybaric fractional pressure fractional crystallisation crystallisation and/or phenocryst trends. reaction associated with widespread accumulation of calcic plagioclase. Small and/or intermittent, long-lived and Large, short-lived and steady-state non-steady-state magma chambers. magma chambers. Magma mixing is most evident. Crystal fractionation largely overrules magma mixing. Mafic and ultramafic rocks commonly Rare occurrence of mantle rocks. occur. Sources: Iyer and Ray (2003) and references therein.
of the Indian Ocean floors. Along with the Central Indian and Southeast Indian ridges, the Carlsberg Ridge (CR) forms the northwestern boundary of the IONF and appears to have considerable bearing on the evolution of the mineral deposits within the field.
19
The Indian Ocean Nodule Field
Table 1.4
Major and trace element chemistry of the three oceanic ridges IORS
MAR
EPR
Oxides SiO2 TiO2 Al2O3 FeOt MgO CaO Na2O K2O P2O5
50.93 1.19 15.15 10.32 7.69 11.84 2.32 0.14 0.10
50.68 1.49 15.60 9.85 7.69 11.44 2.66 0.17 0.12
50.19 1.77 14.86 11.33 7.10 11.44 2.66 0.16 0.14
Trace elements Rb Sr Ni Cr Co Cu Y Zr Nb V
2.54 141 106 320 42 81 35 112 4.0 243
2.24 116 121 300 49 77 25 46 1.6 289
1.4 120 100 318 47 78 39 88 2.0 318
Sources: Melson et al. (1976), Banerjee and Iyer (1991), Subbarao et al. (1977) and Sun et al. (1979). Note: IORS ¼ Indian Ocean Ridge system, MAR ¼ Mid-Atlantic Ridge, EPR ¼ East Pacific Rise. Oxides in wt%, elements in ppm, t ¼ total.
The CR runs northwest-southeast across the middle of the Arabian Sea between the Owen Fracture Zone in the north and the equator in the south. The CR accounts for the generation of new oceanic crust between Iran–Afghanistan in the north and Madagascar in the south, separating, as a result, the Indian Plate from the African Plate. Along the Owen Fracture Zone, the CR is offset right laterally by about 300 km against the northern Sheeba Ridge. Geophysical investigations have shown that the CR was possibly formed during Late Paleocene (63–61 Ma) by a WNW-trending spreading centre. It displays a rough topography, typical of slowspreading ridge, having half-spreading rate of 13–15 mm/year since 30 Ma. The ridge is displaced by major transform faults particularly in the southern part. The gravity data suggest the median valley of the CR to presently be in a nonisostatic equilibrium. Supporting magnetic, bathymetric and gravity evidences indicate that prior to the Himalayan collision, the CR was spreading at a faster rate and that during the hard collision of India with Eurasia in the Eocene, the spreading rate slowed down considerably. These phenomena, represented by two asymmetric, discordant systems of linear magnetic anomalies, attest to the fact that the structure and spreading of the CR was non-stationary. In contrast, the mantle Bouguer
20
Mukhopadhyay, Ghosh and Iyer
Table 1.5
87
Isotopic composition of basalts from the three spreading ridge axes
Sr/86Sr
206
Pb/204Pb
207
Pb/204Pb
208
Pb/204Pb
143
Nd/144Nd
IORS
MAR
EPR
0.7032–0.7035 0.70261–0.70278 0.70291–0.70329 0.70273–0.70356 0.70257–0.70289 18.43–18.75 17.307–18.019 17.898–18.839 18.000–18.157 15.43–15.49 15.505–15.563 15.437–15.604 15.443–15.472 38.01–38.19 37.214–37.838 37.820–39.077 37.772–38.001 0.51302–0.51305 0.51304–0.51309 0.51293–0.51316 0.51305–0.51309
0.70230–0.70321 0.70215–0.70287
0.70253–0.70275 0.7023–0.7028
17.84–19.28 17.829–19.437
18.43–18.56 17.721–18.726 18.0–18.8
15.46–15.59 15.439–15.586
15.44–15.49 15.309–15.514 15.3–15.6
37.33–38.83 37.418–38.913
37.64–38.07 37.029–38.805 37.0–38.8
0.51309–0.51329
0.51302–0.51313 0.51306–0.51318
Sources: Subbarao and Hedge (1973), Cohen et al. (1980), Price et al. (1986), Mahoney et al. (1989), Rehkamper and Hofmann (1997), Hamlein et al. (1984). Note: IORS ¼ Indian Ocean Ridge System, MAR ¼ Mid-Atlantic Ridge, EPR ¼ East Pacific Rise.
anomaly (MBA) values suggest a more continuous magmatic source beneath this chaotic morphology. The along-axis variations in MBA values and the swath bathymetry map of a part of the CR (Fig. 1.13) reveal differences in magmatic and tectonic processes. Tiny wiggles, that is, second-order magnetic signals, reveal two eastwardpropagating rifts between anomalies 26 and 25, and seven westward-propagating rifts between anomalies 24 and 20, leading to a regional assymetricity (Dyment, 1998). For example, about 56% of the crust formed between anomalies 26 and 25 was accreted to the African Plate, while during anomalies 24 to 20, more than 75% of the generated crust was added to the Indian Plate. The backscatter amplitude data confirm the presence of a 2200–4500-m deep rift valley filled with sediment, and of ultramafics such as lherzolite and serpentinite, indicating the shallow depth of mantle melting in variable thermo-tectonic regimes (Mudholkar et al., 2002). Broken rock fragments of neighbouring continents are also found in many places along the CR. Well-preserved pteropods in the proximity of the CR may suggest the presence of hydrothermal activities (Bhattacharya, 1996). The whole rock and mineral chemistry of the CR suggest a greater influence of tectonic setting (transform fault effect) on the formation of crust through fractional
21
The Indian Ocean Nodule Field
A
9⬚N
6⬚
3⬚
0⬚
−160
−100
−120
58⬚E
−50
−60
−20 −40
60⬚
10 0
62⬚
40 20
80 60
64⬚
110
3⬚S 66⬚
68⬚E
B 1.4 1.3
S, west flank (Cm/y)
1.2 1.4
S, east flank
2
1.3 1.2 1.4
1
3
S, mean
1.3 1.2
Figure 1.13 (A) Mantle Bouguergravity anomaly map of apart of the Carlsberg Ridge (CR) based on satellite altimetry data (Sandwell and Smith, 1997).The NW-SE-trending thick black line is the ridge axis. (B) Spreading rate distribution for anomalyA5 along the CR for the west (top), east (middle) flanks and average spreading rate (bottom).The solid dots are rates as measured, thin lines are running average, while the thick lines show theoretical rate calculated using anomaly A5 and CR pole finite rotation (Merkourievand Sotchevanova, 2003).
crystallisation and magma mixing (Iyer and Banerjee, 1993). The presence of plagioclase phenocrysts with anorthite-rich core and corroded margin, coexistence of calcic plagioclase (An86–89) and olivine (Fo83–91), presence of twinned sodic plagioclase and/or reverse zoned plagioclase, low CaO/Al2O3 ratio, absence of clinopyroxene, hyperbolic relation of the ratio–ratio plots of trace elements and variable range of TiO2 at a given FeO/MgO in the bulk rocks, when considered together, suggest short interval of crystallisation, differential extent of partial melting at shallow depth and magma mixing.
22
Mukhopadhyay, Ghosh and Iyer
1.6. The Central Indian Ridge The CIR with a trend of NNW-SSE and a length of about 2775 km separates the Indian, Australian and Capricorn plates from the African Plate. The CIR joins the CR in the north around the equator and the Indian Ocean Triple Junction (IOTJ) at 25 S. The ridge shows a linear progressive increase in spreading rate from 34 mm/year (spreading direction N142E) at the equator to 55 mm/year (spreading direction N152E) at the IOTJ. It is characterised by en echelon displacement by numerous NE-SW-trending fracture zones and transform faults. The southern part of the ridge differs in topography, structure and age from the northern part. Magnetic and multibeam bathymetric data from the intersection between the Vema Fracture Zone and the CIR reveal asymmetric spreading up to chron A5 (10 Ma; Drolia et al., 2000) and variable along-axis crustal thickness (Kamesh Raju et al., 1997). The deepest point (>6200 m) over the CIR is encountered along the Vema Fracture Zone (also known as Vema Trench). Multibeam swath bathymetry profile along and across the trench is shown in Fig. 1.14. Except for the offsets along the Alula-Fartak FZ and the fracture zone at 3 S, all other offsets of the CIR are left lateral. The length and nature of these offsets appear to have influenced the variable extent of melting of the source rock and the depth of magma generation (Mukhopadhyay and Iyer, 1993). The comparatively slow-spreading northern CIR and the intermediatespreading southern CIR meet at the Wide Boundary Zone (WBZ, i.e. the extended part of the deformation zone occurring at the boundary of the Indian and the Australian plates, Fig. 1.5) on the ridge axis between 6 S and 9 S, suggesting initiation of a new triple junction among African, Indian and Australian plates (DeMets et al., 1994; Drolia et al., 2000). Recent gravity model indicates that the northern CIR in the equatorial region has a low elastic thickness of about 5–10 km (average 7 2 km) and a crustal thickness of 18 km. The former may have been caused by the higher than average temperatures beneath this portion of the ridge. This in turn may suggest probable perturbation of the thermal structure by the effect of the nascent triple junction, which may be the contact of the CIR with the WBZ (Drolia et al., 2003). Rao et al. (1996) proposed different ages of evolution for the northern and the southern parts of the CIR at about 30 Ma (anomaly 10) and 68–66 Ma (anomaly 28), respectively. The presence of a transform fault at 7 450 S displacing the CIR axis in a right lateral sense has also been reported (Kamesh Raju et al., 1997). Recent detailed multidisciplinary studies carried out under the InRidge programme from the northern CIR revealed variable and atypical petro-tectonic characteristics at the transform fault (fracture zone), ridge axis, ridge-transform fault intersection and at near-axis seamount (Mukhopadhyay et al., 1998). The presence of pillow and columnar basalts in a neo-volcanic zone at some transform faults suggests a complex relation between magmatism and tectonics (Drolia et al., 2003). At 19 S, the interaction between mantle plumes (Reunion–Rodrigues–CIR hotspot track) and spreading centres (CIR) is manifested in terms of significant variations in axial topography, non-transform offset morphology, segmentation characteristics, crustal thickness and basalt chemistry. Rocks along the CIR are
23
The Indian Ocean Nodule Field
NW 4
Depth in m
−4500
Distance in km 6 8 10 12
SE 14
Across transform
−5000
PTDZ
−5500 −6000
−9.00⬚ −4000 −4500 −5000 −5500
Depth in m
Along transform axis
−6000
−9.50⬚ 67.00⬚ 2000
67.50⬚
3000
4000
5000
50 6000
NW
100 150 200 Distance in km SE
Depth in m −7.00⬚
−7.50⬚
−8.00⬚
−8.50⬚ 67.50⬚
68.00⬚
2000 3000
4000
68.50⬚ 5000 6000
Depth in m
Figure 1.14 Fifty-nine beam swath bathymetry mosaic across and along theVema Fracture Zone cutting across the Central Indian Ridge (Drolia and DeMets, 2005). Note the deepest point (>6200 m) along the Vema Fracture Zone (arrow) (Courtesy: InRidge, project CLP 0886, cruise SK165).
24
Mukhopadhyay, Ghosh and Iyer
mostly normal Mid-Oceanic Ridge Basalt (N-MORB) type, but transitional and enriched basalts (T/E-MORB) are also found at places. In addition, ultramafics and rocks containing hydrothermal indications also occur (Mukhopadhyay and Iyer, 1993). Rocks along the CR–CIR stretch indicate that the magma is very primitive and less differentiated than that of the Mid-Atlantic Ridge (MAR) and the East Pacific Rise (EPR). The melt beneath the IORS is isotopically distinct from those in the Atlantic (MAR) and the Pacific (EPR) oceans. For example, the IORS is characterised by a higher 87Sr/86Sr and a lower 206Pb/204Pb and 143Nd/144Nd than the EPR and MAR (Table 1.5). This suggests either mixing of Kerguelen, Marion, Reunion and Crozet mantle plumes with the ridge-axis melt (Dupal anomaly) or mixing of earlier consumed continental crust with ridge-axis melt (Iyer and Ray, 2003; Mahoney et al., 1989; Rehkamper and Hofmann, 1997; Subbarao and Hedge, 1973). Along the 4200-km long CR–CIR stretch, variations in mineralogy, texture, trace element geochemistry and isotope chemistry of rocks in response to ridge segmentations were observed, and seem to have played a significant role in facilitating polybaric fractional crystallisation.
1.7. The Southeast Indian Ridge The SEIR separates the Antarctic and the Australian plates and connects the IORS with the EPR at the Macquarie triple junction (Fig. 1.1). Except for a much younger part between Broken and Kerguelen–Gaussberg ridges (age 44 Ma), the other stretches of the SEIR hold a spreading history since 95 Ma. The SEIR has a spreading rate varying from 58 mm/year in the west near IOTJ to 76 mm/year in the east around 120 E (Fig. 1.3). The SEIR is characterised by remarkable variations in ridge-crest morphology, suggestive of excess magma supply caused by hotspots. This ridge is divided into two super-segments, one of which extends from 70 E (IOTJ) to 90 E and is highly segmented, bordered by several transform faults and influenced along part of its length by the Amsterdam/St. Paul and Kerguelen hotspots (Klein et al., 1991). Some of the prominent fracture zones extend up to the Bay of Bengal. The other super-segment extends from 90 E to the Tasman Sea in the east and includes Australia–Antarctica Discordance (AAD). The satellite-derived gravity and limited bathymetry data suggest that the SEIR exhibits transition in axial morphology, separated by two transform faults. For example, the SEIR west of the 102 450 E transform fault is characterised by axial rift volcanic ridge (EPR type) of 400-m height and 10-km width, while on the east of 114 E transform fault, the SEIR shows a rift valley of about 1-km depth and 20-km width. The SEIR between these two faults is represented by an axial valley of moderate depth and width (600 m, 10 km). The absence of any large variation in mantle temperature (25 C–50 C) is manifested by near-uniform crustal thickness below the SEIR, and this may be explained by the presence of a steady-state axial magma chamber and a nearly constant spreading rate (Shah and Sempere, 1998). However, the transition in axial morphology appears to coincide with variations in the geochemistry of axial lavas (Sempere and Klein, 1995).
The Indian Ocean Nodule Field
25
The AAD, located between 119 E and 126 E, is an anomalously deep portion and marks a boundary between the isotopic provinces of the Indian and the Pacific oceans. The rocks from AAD are highly enriched with incompatible elements as well as with Si, Fe and Na compared to N-MORB rocks on its either side. The lowest solidus pressure and least extent of melting, supported by the bathymetric, gravity and seismic evidences from beneath the AAD, suggest the presence of a cooler temperature in the mantle (Klein et al., 1991). It is well known that the Indian Ocean MORB has isotope systematics that are different from those of the Atlantic and the Pacific MORB (Table 1.5). Also, the SEIR basalts are different from those occurring at the IOTJ, and their source may involve variable proportion of a component tentatively assigned to recycled ancient hydrothermal and abyssal sediments. The Sr, Pb and Nd isotope data suggest that the SEIR basalts may also be accounted for by binary mixing of an Indian MORB with ocean-island basalt melt. Hence, the SEIR is an ideal site to study the effects of varying mantle temperatures on crustal accretion, the thermo-mechanical structure and the magmatic interaction between MORB and mantle plumes (Amsterdam/St. Paul and Kerguelen hotspots).
2. Physics, Chemistry and Biology The physical, chemical and biological characteristics of the sediment and water column in the IONF appear to have influenced the formation and growth of its mineral resources. A gist of these parameters is outlined below.
2.1. Physical characteristics Much of the physical oceanographic data from the IONF were systematically collected and analysed under the Environmental Impact Assessment (INDEX) programme of India’s deep-sea mineral venture (see Chapter 6 for details). It has been found that the physical parameters (temperature, salinity and potential density) vary down the water column in the IONF, but the variations are largely limited to the top 3500 m, and suggest a restricted basin-scale deep circulation. The topography at 5000 m in the central part of the IONF displays the abyssal circulation and is generally characterised by a southwestward weak flow around 10 S. This flow regime is flanked by cyclonic and anti-cyclonic eddies to its right and left, respectively (Ramesh Babu et al., 2001), and could be linked to the entry of Antarctic bottom water into the CIOB through a saddle across the NER at 10 S (Warren, 1982). The annual mean temperature and salinity distribution at surface, and at depths of 100 and 1000 m in the IONF, have also been studied (Fig. 1.15). Geostrophic current circulation patterns were estimated at a few locations in the IONF. At 79 E, an anticlockwise cell is clearly discernible in the upper 300 m, with moderately strong westerly flow (0.7 m/sec) in sectors A and B and in the northern half of sector C. In sector D and the southern part of sector C, the flow becomes weak (0.3 m/sec), easterly and clockwise. At 71 E, the circulation cell in the upper
26
Mukhopadhyay, Ghosh and Iyer
34.60
34.80 S (psu)
34.70
0.0 0
1.0
2.0
3.0
PO4-P (mM)
1000
Depth (m)
2000 Salinity
3000 Phosphate
DO 4000
Nitrate
pH 5000 0 0 7.6
100 10
20 7.8
300 DO (mM)
200 30 8.0
40 NO3-N (mM) 8.2 pH
Figure 1.15 Vertical profile in the water column in the Central Indian Ocean Basin (CIOB) showing annual mean salinity, pH, dissolved oxygen, nitrate and phosphate (De Sousa et al., 2001).
300 m shows clockwise rotation and a weak easterly flow (0.1 m/sec) in sectors A and B and in the northern part of sector C. This trend gets reversed in the south, with strong westerly flow (0.4 m/sec). At 10 S, this closed circulation pattern becomes mixed with South Equatorial Current (SEC) and sets up an alternating strong northward and southward flow (1 m/sec). Above 200 m, the northward flow is dominant, while at deeper levels the southward flow becomes prominent (Ramesh Babu et al., 2001). Seasonal variability of currents in the IONF was determined from four depth zones: subsurface (450–670 m), intermediate (1150–1370 m), deeper (3450–3670 m) and near-bottom (4270–5100 m) at eight current metre mooring stations (Murty et al., 2001). Of these, three stations were from the northern part of the IONF (10 S, sector A) and the remaining five stations from the southern IONF (15 S, sector D). In the northern IONF, seasonal variations in currents were found at depths of 500 and 1200 m, caused probably by seasonal north-south shift in the westward-flowing SEC. In the southern IONF, a dominant eastward flow at 500 m suggests the occurrence of current shear below the SEC. Low-frequency oscillation of 30–60 days superimposed on high-frequency inertial, diurnal and semi-diurnal,
27
The Indian Ocean Nodule Field
meso-scale fluctuations was noticed at all depths. In the northern IONF, the total kinetic energy (KE) recorded high values during spring–summer and low values during fall–winter. Predominance of zonal flows over meridional flows, intensification of currents and decrease in KE through the water column were some of the interesting features in the IONF. At deeper zones, synoptic-scale oscillation (12–15 days) was responsible for higher energy currents in the northern IONF, while mesoscale oscillation influenced the southern IONF (Table 1.6).
2.2. Chemical characteristics The chemical properties of the IONF water column (Table 1.7) reveal the existence of three north-moving distinct water masses: (1) the water mass between 125- and 200-m depth characterised by high salinity (34.74–34.77 psu) and oxygen minima, associated with weak load of nutrients, (2) the deep oxygen maxima in the depth range of 250–750 m associated with minima in nutrients and relatively high pH and (3) the salinity minima (34.714–34.718 psu) at depths between 800 and 1200 m. The third water mass, in the density range of 27.2–27.5, corresponds to the Antarctic Intermediate Water (AAIW) (De Sousa et al., 2001). Detailed study suggests that changes in water masses are probably the result of mixing of various layers of waters, and the variable oxidation of organic detritus en route. The oxygen maxima water mass occurring between 250 and 750 m shows the least changes in these properties and subsequently moves faster. The variable concentration of dissolved organic carbon (DOC) with depth is related to the biochemical activity as well as to the chemical features of the water column. DOC is least in the oxygen minimum zone and maximum in subsurface waters. The lack of significant correlation between DOC and apparent oxygen utilisation may suggest
Table 1.6 Velocity, direction and kinetic energy of currents in the IONF
Location
Sector A (Sept.–Jan.)
Sector D (Sept.–Jan.)
Water column depth Velocity Direction Total (m) (cm/s) ( )
Kinetic energy (cm2/sec2)
Mean
Eddy
Syno
Meso
0500 1200 3500 4900 0600 1300 3600 4400
13.68 04.30 00.82 01.47 02.44 00.76 00.00 –
159.04 45.85 05.00 07.36 12.76 04.57 00.43 –
145.27 40.42 03.14 05.92 05.91 00.87 00.06 –
13.77 05.43 01.86 01.43 06.85 03.70 00.37 –
5.23 2.93 1.28 1.72 2.21 1.23 0.07 0.11
Source: Murty et al. (2001). Note: IONF ¼ Indian Ocean Nodule Field.
329 269 271 240 076 117 132 277
172.72 50.16 05.82 08.03 15.20 05.33 0.44 –
28 Table 1.7
Mukhopadhyay, Ghosh and Iyer
Average chemical characteristics of water column in the IONF
Water depth (m)
pH
Dissolved oxygen
Nitrate
Phosphate
Salinity
Surface 1000 2000 3000 4000 5000
8.2 7.6 7.66 7.70 7.70 7.70
220 90 125 185 185 185
00 36 35.9 35.5 35.5 35.5
0.10 2.45 2.45 2.40 2.25 1.90
34.64 34.715 34.72 34.72 34.72 34.72
13 S
–
97.5 8.9
36.1 2
2.59 0.24
34.714 0.001
Sources: Modified from De Sousa et al., (2001) and Sardessai and De Sousa (2001). Note: Units for dissolved oxygen, nitrate and phosphate in micrometer, salinity in psu. Sector C is the most promising resource area in the Indian Ocean Nodule Field (IONF), hence shown specially. Water depth at 13 S (Sector C) is 4960 m.
simultaneous consumption of oxygen by other species in the water column (Sardessai and De Sousa, 2001).
2.3. Biological characteristics The benthic community consists of two major members: macrofauna (size >500 mm) and meiofauna (size <500 to 62 mm). The macrofauna is dominated by Polychaetes (65%), followed by Crustaceans (31.8%), Mollusca (1.5%) and Sipunculida (1.4%). The average density and biomass recorded down to a depth of 20 cm within the sediment ranges from 48 to 704 individuals per square metre. However, with the slightest variation in the sedimentary environment, the abundance as well as the community composition of macrofauna changes (Ingole et al., 2001). The meiofauna, on the other hand, dominated by nematods and harpacticoid copepods, varies directly with the total organic and labile matters (Ansari, 2000). The study on the abundance and distribution of meiofauna and macrofauna and total counts of bacteria in sediments from a depth of 5000–5300 m in the IONF records their change in behaviour. The bacteria population ranged from 1010 to 1011 cells per gram of dry sediment, with the highest concentration occurring at a depth of 4–8 cm within the sediment. A decrease in meiofauna, macrofauna, bacteria, labile organic matter (carbohydrates, proteins, lipids), living biomass carbon and lipase has been recorded, contrary to an increase in total organic carbon, sediment enzymes and phosphatase, suggesting the importance of quality food for the IONF benthos (Raghukumar et al., 2001a,b). Some of the major characteristics of the seafloor biology of the IONF are listed in Table 1.8 (Fig. 1.16). The importance of mega benthic community (megafauna ¼ length > 3 cm) lies in the fact that they live in highly stressful environment under extreme hydrological conditions, such as high pressure, low temperature, low oxygen, low salinity, minimum water current, absence of light and a low rate of sedimentation. Because
Table 1.8
Distribution of organisms in sediment layer in the IONF
A. Major macrobenthos (%) Annelida Crustacea B. Major macrofauna (%) Polychaeta Harpacticoida Nematoda Tanaidacea C. Bacterial and organic nutrients
Grab # 191 195 196
Total bacteria (1010/g) 10.11 17.02 39.29
Living biomass (mg/g) 0.059 0.027 0.020
Carbohydrates (mg/g) 2.4 2.4 2.1
0–2 cm 44.5 14.3 0–2 cm 31.6 15.8 38.1 7.90
Protein (mg/g) 0.98 0.87 0.66
Lipids (mg/g) 0.39 0.23 0.01
2–5 cm 7.1 3.3 2–5 cm 30.83 23.05 38.33 7.78
5–10 cm 9.5 6.2 5–10 cm 35.08 24.95 24.95 15.00
10–20 cm 3.9 8.0 10–20 cm 11.67 5.95 17.62 41.19
Labile organic matter (mg/g) 3.787 3.514 2.754
Total organic carbon (mg/g) 5.2 7.3 7.3
Total organic matter (mg/g) 9.4 13.2 13.2
Sources: Ingole et al. (2001) and Raghukumar et al. (2001a,b). Note: Grab locations: 191 (10 10.260 S, 76 00.240 E), 195 (10 05.410 S, 76 05.380 E) and 196 (10 02.450 S, 76 02.520 E). IONF ¼ Indian Ocean Nodule Field.
30
Mukhopadhyay, Ghosh and Iyer
of difficulties in observing and sampling deep-sea organisms, megafauna are normally studied from underwater photographs. However, in an in situ study (Rodrigues et al., 2001), 11 groups of invertebrates—xenophyophores, sea cucumber, holothurians, sponges, sea anemones, sea pens, black corals, shrimps, starfish, brittle star, sea flowers, sea urchins—and one group of vertebrate, namely, fish (Typhlonus nasus), were found on the floor of the IONF. It was recorded that xenophyophores and sea cucumber constitute about 41% and 30%, respectively, of the total megafauna present in the IONF. In addition, a few protuberant molluscs, polychate worms, sea fans and squids were also found.
(b) 0−2
0 2 4 6 8 10 12 14
(a) 0−2
0 2 4 6 8 10 12 14
A
2−4
2−4
4−6
4−6
0−2
15−20
2−4 197
(c) 0−2
20−25 4−6
(d) 8−10
0.12
0.08
0.06
0.04
6−8 0−2
10−15
2−4
194
25−30 0.00
0.12
0.10
0.08
0.06
0.04
0 2 4 6 8 10 12 14
0.00 0.02
15−20
0.02
10−15
10−15
0 2 4 6 8 10 12 14
8−10
8−10
0.10
(e)
0 2 4 6 8 10 12 14
6−8 6−8
2−4 193
15−20
8−10
LOM Biomass
10−15
0.12
0.10
0.08
0.06
0.04
6−8
0.02
4−6 0.00
4−6
6−8 8−10 10−15
TOC
190
188 0.10
0.08
0.06
0.04
0.00 0.02
25−30
Vertical scale refers to depth in sediment core in cm. Top horizontal scale represents concentration of LOM and TOC (mg/g of dry sediment) Bottom horizontal scale measuring concentration of biomass (mg C/g of dry sediment)
Figure 1.16
(Continued)
0.12
0.10
0.08
0.06
0.04
0.00
20−25
0.02
15−20
15−20
31
The Indian Ocean Nodule Field
B (a)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
(b) 0−2
0−2
2−4 2−4 1.2
4−6
4−6 0.0 0.2 0.4 0.6 0.8 1.0
(e) 0−2
6−8 8−10
2−4
10−15
4−6
6−8 8−10 10−15 15−20 20−25
197
0−2
6−8
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 8−10
(d)
0−2
10−15 193
2−4
2−4 0.0 0.5 1.0 1.5 2.0 2.5 3.0
15−20
4−6 6−8
4−6 6−8
8−10 10−15
Protein
15−20
CHO Lipids
8−10 10−15 190
188 15−20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
25−30
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
20−25
194
25−30 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
(c)
2.5 3.0 3.5
0.0 0.5 1.0 1.5 2.0
15−20
Vertical scale refers to depth in sediment core in cm. Top horizontal scale represents concentration of lipids and protein (mg/g of dry sediment) Bottom horizontal scale refers to concentration of carbohydrates (mg/g of dry sediment)
Figure 1.16 Biology of the Central Indian Ocean Basin (CIOB) (Raghukumar et al., 2001a,b), showing (A) concentration of total organic carbon (TOC), labile organic matter (LOM) and biomass-C, and (B) lipids, proteins and carbohydrates in five sediment cores (a ¼ 197, b ¼ 194, c ¼ 188, d ¼ 190, e ¼193).
3. Evolution of the Indian Ocean Nodule Field India has been a central element to any tectonic reconstruction of the world for its unique history of cruising for several thousands of kilometres to join Laurasia after being detached from Gondwanaland. And expectedly there have been schools of theories over the exact nature of dismemberment of Gondwanaland based on geological, geophysical and isotopic signatures, and those established through the distribution of tetrapods, Glossopteris flora and Permo-Carboniferous glacial strata.
32
Mukhopadhyay, Ghosh and Iyer
Since the break-up of Gondwanaland and the evolution of the IORS have influenced the volcanic and tectonic environments of the IONF, some discussion on the evolution of the IONF seems necessary.
3.1. Break-Up of Gondwanaland The reconstruction of Gondwanaland is primarily based on geophysical data, seafloor-spreading magnetic anomalies, transform faults, fracture zones, satellitebased sea surface altimetry and paleomagnetism. Following several studies (e.g. McKenzie and Sclater, 1971; Norton and Sclater, 1979; Reeves and de Wit, 2000; Royer et al., 1992; Schlich, 1982), the major tectonic episodes are recognised as a prelude to the break-up of Gondwanaland and formation of the Indian Ocean (Table 1.9; Fig. 1.17). The break-up of Gondwanaland is believed to have commenced during the Jurassic era (180–200 Ma), with eastern Gondwanaland, comprising Madagascar, Seychelles, India, Antarctica and Australia, separating from western Gondwanaland, made up of South America and Africa. Eastern Gondwanaland broke further in the Late Mesozoic (about 133 Ma), with Madagascar–Seychelles–India (Greater India) rifting away from Antarctica–Australia along the old Eastern Ghat trend of eastern India through a clockwise rotation of the latter by about 20 (Muller et al., 1993). Uplift associated with this rifting tilted peninsular India to the west, resulting in the Table 1.9
Dismemberment of Gondwanaland and evolution of the Indian Ocean
Phase 1 Early part (160–130 Ma): Gondwanaland divides into two groups—Group A comprising South America [SA], Africa [AF], Somalia [SM] and Arabia [AR] and Group B consisting of Madagascar [MD], India [IN], Seychelles [SE], Sri Lanka [SL], Australia [AS] and East Antarctica [EA]. Late part (130–95 Ma): Group A divides further into two sub-groups [SA] and [AF þ SM þ AR], and the Group B also into two sub-groups [MD þ SE þ IN þ SL] and [AS þ EA] Phase 2 Early part (96–65 Ma): [MD] of Group B joins [AF þ SM þ AR] of Group A, while [AS] and [EA] separate from each other. Late part (65–43 Ma): [SE] of Group B separates from [SL þ IN] and joins the [AF þ SM þ AR þ MD] of Group A. Phase 3 Early part (43–10 Ma): [SA] remains separated from [AF þ SM þ AR þ MD þ SE]. [AS] leaves EA to join [IN þ SL]. Late part (10–00 Ma): [AR] separates from [AF þ SM þ AR þ MD þ SE]. [AS] leaves [IN þ SL]. Source: Royer et al. (1992).
33
The Indian Ocean Nodule Field
D
A
EURASIA
ARB
PA
Pa le
o-
A
Panthalassic Ocean
NG
E A
SAM
d u c ti o n z o n e
CI Ne
MM
Te t
AFR
hy s
RH
ER
IA
o-
AFR
Te t
MAD
NAM
S ub
hy s
AUS
IND
spreading centre
AUS ANT
ANT
B
E CIMMERIA ARB Ne
Te t
hy s
AFR
SEY
ARB
AFR
sp ce read ntr in g e
G O N DWA
SAM
o-
RH AUS
NA
L
on
ti uc bd Su ne zo
IND
A
ND
AUS
ANT
C
F EURASIA
ARB ARB AFR
CR
CIR
AFR
IND
AUS
AUS
SW
IR
MAD
RH
SE
IR
Figure 1.17 Dismemberment of Gondwanaland blocks and transhipment of India during geological periods (cf. Bhattacharya and Chaubey, 2001). (A) at >200 Ma, (B) at 150 Ma, (C ) at 84 Ma, (D) at 50 Ma, (E) at 30 Ma, (F) presentday.
34
Mukhopadhyay, Ghosh and Iyer
flow of the major rivers from east to west (Cox and Hart, 1986). Continuous movement across the ridge between Greater India and Australia–Antarctica caused the evolution of the Indian Ocean. Around 118 Ma, ocean-floor spreading between Greater India-Madagascar and Africa stopped. The Middle Cretaceous (95 6 Ma) is widely recognised as the first era of major plate reorganisation in the Indian Ocean, with Australia breaking away from Antarctica and India separating from Madagascar. The slight clockwise rotation of India away from Madagascar amounts to trans-tensional rifting represented then by about 200-km wide wedge of sediment-filled extended crust, which is presently preserved as the western continental shelf of India (Reeves and de Wit, 2000). The separation between Greater India and Madagascar was caused by two phenomena occurring at two different periods. First, the 1000-km southward movement of India away from Madagascar before 90 Ma was caused by prolonged dextral trans-tension along the line of weakness between the east coast of Madagascar and the west coast of India (megashear of Reeves and Leven, 2000). Later, the rapid movement of India towards the north was facilitated by widespread eruption of basalts and rhyolites from the Marion hotspot at about 88–89 Ma. The volcanism was so voluminous that the entire 1500-km long rifted eastern margin (megashear?) of Madagascar was constructed within 6 million years (Storey et al., 1995). The ridge, about which India got separated from Madagascar, is presently believed to exist in the form of abandoned spreading centres on the conjugate crust to the east of Madagascar on the African Plate and in the Laxmi Basin in the eastern Arabian Sea on the Indian Plate (Bhattacharya et al., 1994; see Fig. 1.1). The development of NNW-SSE-trending magnetic lineation, grabens, ridges and basins on the marginal shelf and slope of south-western India between 84 and 65 Ma has been interpreted to represent a two-limbed spreading sequence in the Laxmi Basin. Despite the difference in free-air gravity and isostatic signatures, some workers suggested Laxmi Ridge (having a negative anomaly) as the continental continuation (slivers) of the CLR (showing a positive anomaly) formed by rifting along the margin (Kolla and Coumes, 1990; Naini and Talwani, 1982). However, Todal and Edholm (1998) characterised this enigmatic ridge as a complex marginal high of both continental and oceanic crust, which might have experienced later magmatic and/or tectonic deformation also. Around 84 Ma began the formation of the CIOB (including the IONF). The opening of the Arabian Sea commenced during the Late Cretaceous–Early Tertiary when the ridge system in the Mascarene Basin ceased spreading and jumped to the north to form the CR at about 68–66 Ma, resulting in the separation of Seychelles from India and its transfer to the African Plate (McKenzie and Sclater, 1971). Around this time, one of the largest continental flood basalts, the Deccan Traps, erupted and covered vast areas of the central and western India and the Seychelles. As India was moving towards Eurasia, the Reunion hotspot perforated the crust to give rise to the Chagos–Laccadive–Mascarene Ridge. Between 60 and 50 Ma, the rate of northward movement of India varied from fast (95 mm/year, half rate) to slow (26 mm/year). The slow spreading rate might have been caused by the collision of India with Eurasia. However, the exact timing of this collision is controversial. It is suggested that the first collision between India
The Indian Ocean Nodule Field
35
and Eurasia occurred around 50 Ma (Middle Eocene) with noticeable decrease in spreading rates across the CR, CIR and SEIR (Windley, 1996). A critical appreciation of the seafloor lineaments and the deformation caused by lithospheric compression in a north-south direction suggest an initial touch between India and Eurasia during 58 Ma, and starting of collision during 51 Ma (Mukhopadhyay et al., 1997). However, radiometric dating of garnet and zircon from Himalayan rocks tentatively shows that the huge mountain-building activity of the Himalayas, which resulted from the collision of India with Eurasia, in fact, dates to more than 450 Ma, that is, 9 times older than the previously suggested age (Gehrels et al., 2003). It is proposed that sometime after the collision, India pulled back. Then, around 50 Ma it ploughed into Asia once again, causing episodic mountain-building activity. The second major plate reorganisation event in the Indian Ocean occurred between 45 and 38 Ma. The event saw the spreading between Australia and Antarctica, which started at 45 Ma, resulting in the merging of the India–Antarctic and India–Australia ridges to form the SEIR. Meanwhile, the Chagos-Maldives and Mascarene plateaus broke up around 35.5 Ma, the former becoming a part of the Indian Plate and the latter stationed on the African Plate. During this period spreading across the Wharton Ridge ceased, and later jumped to the south (Liu et al., 1983). The younger tectonic events in the Indian Ocean included opening of the Gulf of Aden at about 10 Ma (Laughton et al., 1970) and onset of deformation between the Indian and the Australian plates at 7–8 Ma in the CIOB, just north of the IONF (Krishna et al., 1998; Weissel et al., 1980).
3.2. Formation of the Indian Ocean Ridge system The evolutionary history of the IORS has been understood largely from the research findings of the last three decades (Table 1.10; Fig. 1.17). Between anomalies 33 and 30, the Indian Ocean Triple Junction (IOTJ) was lying in the eastern Mascarene Basin, about 300 km west of the Vishnu FZ. At anomaly 29, the IOTJ rapidly migrated eastward and was located between the Vishnu FZ and Indrani FZ. The CIR and SEIR were being progressively offset at a rate of 1.4 mm/year, mainly because of their different spreading rates. When the offset was large, the IOTJ jumped eastward and a new CIR segment was created,resulting in an overall constant length for the SEIR, while the CIR and SWIR lengthened. A jump of every 1 Ma created ten 70-kmspaced SWIR fracture zones, including Gallieni and Atlantis II fracture zones. The close association of numerous fracture zones across the SWIR and IOTJ jumps seems to suggest that the physiography of the SWIR records the history of the IOTJ evolution. The Indian Ocean has examples of many ridge jumps. For instance, the India– Madagascar Ridge, earlier existing between India and Madagascar, ceased its activity at about 68 Ma, and took a major jump towards India before opening of the Arabian Sea (Naini and Talwani, 1982). Because of this ridge-jump process, the northern part of the present CIR came into existence about 30 Ma ago, and transferred the Reunion hotspot from the Indian Plate to the African Plate. Similarly, a major ridge jump northward between chrons 31 and 25 transferred the Crozet hotspot from the Indian Plate to the Antarctica Plate (Muller et al., 1993). The CIR has experienced few minor ridge jumps since about 3 Ma (Mitchell, 1991).
36
Mukhopadhyay, Ghosh and Iyer
Table 1.10 The evolving IOTJ Position of IOTJ Period
Age (Ma)
Anomaly
latitude
longitude
Spreading rate (cm/year)
Present Pleistocene Upper Pliocene Middle Pliocene Lower Pliocene Early Oligocene Early Eocene Palaeocene
00 02 04 06 08 38 53 65
00 01 02 03 04 16 22 28
25.7 S 25.7 S 25.7 S 25.7 S 25.7 S 25 S 26.50 S 31 S
70 E 70 E 69.02 E 68.7 E 68 E 63.52 E 60 E 54.6 E
1.84 1.62 2.70 3.78 – 2.88 6.75 –
Sources: Norton and Sclater (1979), Tapascott et al. (1980) and Murthy and Rao (1992). Note: IOTJ ¼ Indian Ocean Triple Junction.
Young oceanic crust forming at the IORS normally consists of tholeiitic basalt (N-MORB) characterised by low concentrations of incompatible elements such as K, Rb, Cs, Ba and the light rare earth elements (LREE). The basalts appear to have been derived from a mantle depleted in these elements. Three major rock types are found in the IORS, each with characteristic phenocrystic assemblage reflecting a distinctive magmatic lineage of differentiation. The rocks have low, moderate and high abundances of Na2O and TiO2 with sequentially lower CaO/Al2O3. The Indian MORB, however, is distinct from the Atlantic and the Pacific MORB in terms of Sr, Nd and Pb isotopic composition (Table 1.5). Isotopic ratios of Sr, Nd and Hf in N-MORB further demonstrate that the incompatible elementdepleted melt is inherited from a portion of the asthenosphere, while other relatively enriched (less depleted), the so-called enriched or transitional, MORB are associated with mantle plumes. The differences in isotopic composition among the rocks from the three oceans may have been possible by any one or a combination of two or more of the following processes: (1) recycling of the ancient oceanic crust (Dupre and Allegre, 1983), (2) contamination of ridge melt by the Kerguelen hotspot [plume] melt (Storey et al., 1989), (3) mixing of sub-Gondwanaland continental lithosphere (Mahoney et al., 1989) and (4) contamination of ridge melt from several hotspot plumes that formed oceanic islands (Mahoney et al., 1993).
C H A P T E R
T W O
Tectonics and Geomorphology
Contents 40 41 43 44 44 45 47 48 49 52 54 56 59 60 61 63
1. Ridge-Normal Lineaments 1.1. The Vishnu Fracture Zone 1.2. The Indrani Fracture Zone 1.3. The Indira Fracture Zone 1.4. Trace of the triple junction 2. Ridge Parallel Lineations and Anomalies 2.1. Seafloor crenulations 2.2. Seafloor faults 2.3. Tectonic, thermal and geoidal anomalies 2.4. India–Eurasia collision event revisited 3. Seamounts 3.1. Spreading rate and seamount distribution 3.2. Structural lineaments and seamount abundance 3.3. Structural lineaments and seamount morphology 3.4. Seamount petrology 3.5. Growth conditions of seamounts
As mentioned in the previous chapter, the Indian Ocean Nodule Field (IONF) contains the world’s second largest and second richest manganese nodule resources after the Equatorial North Pacific nodule belt. The structural boundaries of the IONF, which appear to have an influence on resource characteristics, are also described in the preceding chapter. In this chapter, we focus on tectonic and geomorphological characteristics of this nodule field. The detailed bathymetric information of the IONF was acquired through single (narrow and wide) beam echosounder and multi-beam (59 beams) swath bathymetry systems (Fig. 2.1). These data acquired since the early 1980s helped improve our understanding of seafloor bathymetry compared to that gained through General Bathymetric Chart of the Oceans (GEBCO) maps and other available information. The IONF (area 690 103 km2) interestingly encompasses three types of crust that were generated from the Indian Ocean Ridge System (IORS) during Palaeocene–Eocene time between 60 and 49 Ma, and formed during pre-, syn- and post-collision periods between India and Eurasia. These periods respectively correspond to fast, intermediate and slow rates of generation of crust from the ridge axis. Consequent to such tectonic conditions involving tensional and compressive stresses Handbook of Exploration and Environmental Geochemistry, Volume 10 ISSN 1874-2734, DOI: 10.1016/S1874-2734(07)10002-4
#
2008 Elsevier B.V. All rights reserved.
37
38
Mukhopadhyay, Ghosh and Iyer
caused by the fast- and slow-spreading regimes respectively, the floor of the IONF is riddled with the following features.
Broadly NNE-SSW-oriented ridge-normal ancient fracture zones (FZ, major structural lineaments). Folding, stretching and sometimes faulting of the seafloor along an approximately ESE-WNW direction, which trends parallel to the then mid-ocean ridge (MOR) axis. Numerous volcanic features in the form of seamounts and abyssal hills. However, only a few integrated studies have been made to unravel the relations among the N-S to NNE-SSW-trending regional lineaments, ESE-WNW-oriented local tectonic activities and seamount volcanism (Mukherjee and Iyer, 1999; Mukhopadhyay et al., 2002). These studies brought out new fundamental information regarding basinal dynamics, which probably occurred at two stages: first, the tectonic and volcanic activities associated with the formation of the oceanic crust at the ridge crest between 60 and 49 Ma; and second, mid-plate activities on the said crust during its journey from the ridge crest to the present location in the abyssal region. These works offered possible answers to quite a few outstanding problems, such as how the spreading of the crust and the eruption kinematics altered in the geological past, how the crustal accretion and the presence of fracture zones influence the origin, distribution and morphology of near-axis seamount production and how the seamounts grew in response to mid-plate tectonic activities and local volcanism. A 72E
74
76
78
80 27
Chain E
24
26 Indrani FZ
Chain G
25
26
Chain B
12
Vishn u FZ
25
Chain C
26
Chain A
10 S
Chain H
26 27
25 25
TJT-In
23 24
24
14
16
Figure 2.1 (Continued)
Chain F
Chain D
23
39
Tectonics and Geomorphology
B
1000 S
1030
1100
76E
7620
1230
1300
1330
1130
1400
1200
1430
7600E
7620E
Figure 2.1 (A) Essential tectonic elements in the Indian Ocean Nodule Field (IONF) with seamounts (stars) arranged in eight prominent chains (A to H), trace of the triple junction on the Indian Plate (TJT-In, dashed line), and nearly east-west-running magnetic anomalies dislocated at places by NNE-SSW-running fracture zones (Das et al., 2005).The crust of the IONF was generated at the ridge crest between 60 and 45 Ma. (B) A simplified bathymetry map of a part of the IONF prepared from multi-beam swath survey input shows seamounts (closed contours with numbers) of various dimensions. Seafloor crumpling (folding and faulting) of variable intensities is marked with nearly east-west-oriented bold and thin short lines with tick marks indicating dip direction. Note more crumpling at the southern latitudes, and occurrence of both north-facing normal and south-facing reverse faults (Mukhopadhyay and Batiza,1994).
40
Mukhopadhyay, Ghosh and Iyer
1. Ridge-Normal Lineaments The ridge-normal structural lineaments in the IONF are largely oriented N-S to NNE-SSW, and lay perpendicular to the present-day Southeast Indian Ridge (SEIR). Among the ridge-normal lineaments, the Vishnu FZ (along 73 E), Indrani FZ (along 79 E) and Indira FZ (along 83 E) are important (Figs. 1.2, 2.1). A magmatically pronounced approximately NNE-SSW-oriented trace of the ancient plate boundary [i.e. trace of the triple junction on the Indian Plate (TJT-In)] along 76 300 E meridian has been another important feature. Although located outside the IONF, the influence of Indira FZ on dynamics and resource potential of the field has been considerable and hence included here for discussion. All these structural lineaments have disturbed the seemingly smooth gradient (1:1000 to 1:7000) of the IONF seafloor. The various characteristics of these structural features are tabulated in Table 2.1. The formation algorithm of a fracture zone at the MOR axis is quite interesting in constraining the movement of the plate in response to variable spreading rate and consequent stress regime. Transform faults offset the ridge axes as oceanic plates slip sideways past each other. As the seafloor spreads, the imprint of the transform fault develops on crust on either side of the ridge. Such traces on older crust are known as fracture zones, which are useful in reconstruction of ancient plate motions. The fracture zones normally lie within a single oceanic plate, do not register any relative lateral motion across it and exhibit no major seismic activity. They can be demarcated by offset in magnetic lineation. The orientation and nature of the fracture zone hold fair and accurate evidence of change in spreading direction in the geological past, as these normally offset the magnetic lineation right-laterally. In the CIOB, the progressive northward shift of the oceanic crust interspersed with the fracture zone may have been caused because of ridge jumps on the SEIR, the magnitude of which decreased from east to west (Royer and Schlich, 1988). For example, the Indrani and Indira FZ appear to have undergone a change in trend from N-S to NE-SW as a consequence of the Eocene Table 2.1
Some characteristics of fracture zones in the Indian Ocean Nodule Field
Fracture zone (FZ) Orientation
Width (km)
Central portion
Offset (km)
Vishnu 73 E
13–17
Horst
70
Indrani 79 E
Small
Trough
—
83 E
Small
Graben
160
Indira
Contact with surrounding seafloor Movement West
East
Right lateral Right lateral Right lateral
Sharp
Smooth
Smooth
Uneven
Sharp
Subsided
Sources: Tapscott et al. (1980), Kodagali (1992), Kamesh Raju et al. (1993), Mukhopadhyay et al. (1994) and Kessarkar (1998).
Tectonics and Geomorphology
41
reorientation of the SEIR, which changed direction from E-W to NW-SE during this era (between anomaly 18 and 20) marked by global plate reorganisation between 40 and 44 Ma (Kamesh Raju, 1990). Co-incidentally, the obliquity between the SEIR and the CIR increased from 6 to 12 between magnetic anomaly A26 and A24; decreased to 3 at A23; and again increased to 7 at A22 (Dyment, 1993). The ridge normal lineaments are described below, very briefly.
1.1. The Vishnu Fracture Zone The Vishnu FZ is located on the Indian Plate running parallel to the meridian 73 E (trending approximately N15E in the IONF), while its conjugate part can be traced on the African Plate (Dyment et al., 1999). A detailed multi-beam swath bathymetric study of a part of this fracture zone reveals that this fracture zone is essentially a much wider one compared with other known fracture zones in the world’s oceans. The Vishnu FZ runs for hundreds of kilometres and varies in width from about 13 km in the north to about 17 km in the south. The fracture zone shows very sharply delineated contacts with the abyssal plain along the eastern and western margins (Fig. 2.2). The average throw along the western contact margin ranges between 400 and 500 m, while that along the eastern contact is about 200 m, making the western contact more distinct and pronounced. Between the eastern and western margins of the fracture zone, the crust was domed by about 300 m along its length. This elevated central portion hosts many small abyssal hills with ESE-WNW orientation. The fine-scale bathymetric variations reveal that the average amplitude and wavelength of the seafloor crenulations in the Vishnu FZ has been 233 m (range 40–530 m) and 22.5 km (range 8.1–38.2 km), respectively, and is 5230-m deep in the north. The disposition of magnetic anomalies and nature of ridge-parallel lineation in the surrounding areas suggest formation of the Vishnu FZ on the crust spreading at variable rates, 80 mm/year between 57.9 and 51.3 Ma and 36 mm/year between 51.3 and 46.3 Ma (henceforth spreading rates correspond to half-rates). This fracture zone has offset the magnetic lineation of the same age right-laterally by about 70 km; the offset is larger in the eastern side than in the western side (Kamesh Raju, 1990). The northern and the central parts of the fracture zone trend N15E (Fig. 2.2), while its southern part, beyond the southern limit of the IONF, shows a striking change in trend from N15E to N45E. This change in trend, as mentioned earlier, may have been caused by the global plate reorganisation event at about 42 Ma. Magmatically, the Vishnu FZ appears to be dormant for quite long periods. The rocks dredged from this fracture zone are either largely weathered and/or have thick coatings of ferromanganese oxides—both suggesting lack of recent volcanism. The fracture zone reveals wide zones of brittle deformation with highly fractured, permeable rocks—the cracks possibly penetrating deep down into the crust (Mukhopadhyay et al., 1994). Such opening facilitates mixing of hot circulating seawater within the crust with percolating cool seawater and results in reducing the build-up of pressure within the crust (or magma chamber), inhibiting magma ascension and eruption.
42
Mukhopadhyay, Ghosh and Iyer
7330E
7345
740 948 S
A
Vishnu
Fractu r
e Zone
1000
1015
Vish
nu F ra
cture
Zone
B
11S
1153 73E
74
Figure 2.2 (A) A part of the NNE-SSW-oriented Vishnu Fracture Zone(FZ) along the 73 E. Note the sharp western and comparatively diffused eastern contacts with the seafloor and the presence of several large seamounts (height >500 m). The structural lineations on the seafloor are manifested by east-west-closed contours. Note its bend contacts with the fracture zone, suggesting a much younger age for the Vishnu FZ (Das et al., 2005). (B) Simplified bathymetry of a
43
Tectonics and Geomorphology
1.2. The Indrani Fracture Zone The Indrani FZ lies on either side of the SEIR, and has its southern segment extending northward from the eastern edge of Crozet Island on the Antarctic Plate. This fracture zone runs parallel approximately to 79 E and reveals ridge-trough topography with an elevation difference of about 300 m (Fig. 2.3). The topography is associated with steep gradients in the south and is gentler in the north. The seafloor along the western flank is shallower than that of the eastern flank. Seamounts near this fracture zone are mostly located in the southern part. 79E
7923 14 S
12 S
1417
14
7810E
Indrani Fracture Zon
e
13
79
7925
Figure 2.3 Tectonics and physiography of NNE-SSW-oriented Indrani Fracture Zone along the 79 E (Kamesh Raju et al.,1993). Disposition of a group of seamounts occurring close to the fracture zone is shown in inset (Das et al., 2005).
part of the13^17-km wideVishnu FZ. Contour interval 100 m, with tick marked contour at 500 m. Seafloor lineation of at least three orientations (NW-SE at the south-western part, NNE-SSW at the north-western part and WNW-ESE trends at the eastern part) make the area tectonically complex (Kessarkar,1996).
44
Mukhopadhyay, Ghosh and Iyer
Several east-west-trending ridge-parallel bathymetric lineations curve towards the south at the contact with this fracture zone. This bending of lineations occurs particularly on the crust formed at a fast-spreading rate between 60 and 50 Ma ( anomaly A26–A23) in the Indian Ocean (80 mm/year, spreading half-rate), suggesting a fast slipping transform fault environment during this time. The causative factor for such bending could have been the strong shear coupling at the contact of the fracture zone with the spreading ridge. The free-air gravity values along this fracture zone vary from 0 to 45 mGal. Lithospheric thickness of this fracture zone, deduced from the gravitational edge effect, is estimated to be around 100 km. The elastic limit of the lithosphere underneath, deduced from thermal structure and horizontal heat conduction, has been estimated to be 23 km (Te; Kamesh Raju et al., 1993).
1.3. The Indira Fracture Zone This fracture zone runs in a north-south direction roughly along 83 E between 3 N and 22 S, and offsets the magnetic lineation from A20 to A34 right-laterally by about 160 km. The single channel seismic profile of the southern part of the Indira FZ indicates that the seafloor topography manifests as a discrete U-shaped graben with differing flank relief. The depth to this graben gradually increases from the surrounding flanks to about 1700 m. The axial part of the graben consists of micro-troughs and rises. The western flank of this fracture zone is shallower by 750 m and contains steplike structures. Because of its older age, the eastern flank is deeper than the western one. Faults are found to exist mostly at the subsurface level, though some, reaching to a greater depth, probably developed later than the main fracture zone. The sediments within this fracture zone are distributed heterogeneously and show complex acoustic character and structure. While a thin layer of pelagic sediment carpets the fracture zone without changing the graben’s identity in the south, the northern part is filled and buried by thick sediments. At about 1 S, the fracture zone appears as an anticline and remains fully buried under the Bengal fan sediments. The likely reasons for such structural change could be the inconsistent shape of the fracture zone, which has changed with time depending on the evolutionary conditions such as spreading rates, volcanism near the ridge-transform fault intersection (RTI) and offset between the ridge segments (Krishna, 1996).
1.4. Trace of the triple junction The Indian Ocean Triple Junction (IOTJ) is essentially the meeting point of three spreading ridges, the CIR, the SEIR and the SWIR, and is a magmatically pronounced plate boundary. Although the IOTJ at present is located at 25 S, 70 E, it moved over the geological past and ran approximately parallel to 76 300 E, the course now represented by a linear trace (Fig. 2.4). The northern half of the TJT-In is located between the Vishnu FZ and the Indrani FZ on the Indo-Australian Plate, with its western and southern conjugate parts lying on both sides of the SWIR on the African and the Antarctic plates (Dyment, 1993; Patriat and Segoufin, 1988). The nature of evolution of this triple junction has been unique. For example, the recent data obtained favours a ridge–ridge–ridge (RRR) type configuration for the IOTJ corresponding to magmatic mode, and ridge–ridge–fault (RRF) configuration
45
Tectonics and Geomorphology
75E
78
FZ
12S
FZ
15
>300 nt
200−300 nt
100−200 nt
0−100 nt
0− −100 nt
< −100 nt
Figure 2.4 Magnetic anomalies of a part of the Indian Ocean Nodule Field (IONF), often disturbed by a fracture zone (FZ).The fracture zone was later verified as trace of the triple junction, that is, the meeting point of three ridges on the Indian Plate (Kamesh Raju and Ramprasad,1989; Dyment,1993).
relating to tectonic mode. Because of the slow-spreading rate of the CIR and the intermediate-spreading rate of the SEIR, these ridges are being progressively offset. The CIR changed its orientation with time, for example, from approximately E-W at 57.7 Ma to ESE-WNW at 52.8 Ma, and reverted again to E-W between 52.8 and 47.1 Ma. The SEIR–CIR obliquity was least at 51.3 Ma (3 ) between two elevated zeniths (6 –12 , 7 ) just before and after this period. Propagating rifts appear to have played an important role in the evolution of the triple junction, particularly between the period 63.6 and 46.3 Ma. The volcanic and magmatic potential of this IOTJ trace can be gauged from the occurrence of several volcanic forms and seamounts in its proximity (discussed in Section 2.3).
2. Ridge Parallel Lineations and Anomalies The average depth of the IONF increases from west to east, and the nodule field can be broadly divided longitudinally into three bathymetric areas. The western part (71 –74 E) is extremely rugged with great variations in relief
46
Mukhopadhyay, Ghosh and Iyer
(depth range 2900–5000 m). The eastern part of the field between 79 E and 82 E is a medium relief area, with a narrow depth variation (5000–5500 m), while the central part between the two uneven regimes (74 E to 79 E) is almost a plain area (4900–5100 m). In general, about 90% of the IONF area has a slope angle <3 , but the value could be as high as 18 in seamount-dominated areas. Such bathymetric variations in the IONF appear to have been influenced by the rate and direction of spreading at the ridge axis as well as plate reorganisation events, stress relating to intra-plate deformation at the plate boundaries between Indian and Australian (IAPB) and Australian and Capricorn (ACPB), sediment thickness, midplate volcanism and seamount formation (Fig. 2.5). For example, the increased thickness of sediment in the northern areas of the CIOB camouflages and smoothens much of the bathymetric variations. Further, the seafloor lineations in the central and the eastern parts trend approximately E-W (100 –280 ), while to the west of 73 E the trend is almost NE-SW (145 –325 ). If these lineations are normal topographic expressions of a spreading ridge, then the crust with lineations oriented at 145 –325 (western part of the nodule field) was likely to have been generated from the intermediate-spreading NW-SE-oriented section of the then IORS, while the crust east of 73 E, with E-W-trending lineation, should have formed from the fast-spreading ENE-WSW-oriented section of the IORS. The latter lineations are well spaced, and typical for the floor formed by any fast-spreading ridge (Kessarkar, 1998). 40N
2 kbar 0
40
80S 50E
100
150E
160W
Figure 2.5 Distribution of principal deviatoric horizontal stress in the Indian Ocean. Tensional stress is denoated as ( !) and compression as (^)(Cloetingh and Wortel,1985).
47
Tectonics and Geomorphology
Using the multi-beam bathymetry data, compressive and tensional stress regimes of the IONF were determined. For this purpose, fine-scale bathymetric variations along several short profiles, covering more than 1800-km length of the nodule field, were analysed (Figs. 2.1, 2.5 and 2.6). The regional depth of the IONF was estimated by the least squares fit of adjusted crustal depth as a linear function of the square root of crustal ages following the thermal subsidence model of Parker and Oldenburgh (1977).
2.1. Seafloor crenulations The underlying crust of the IONF shows variable intensity of crumpling and faulting (Fig. 2.1, Tables 2.2 and 2.3). The axis of the majority of the folding trends east-west and has offset the north-south extension of several topographic highs. The flexures are prominent and regular in occurrence in the southern part, where the youngest crust in the IONF (age younger to 50.8 Ma, sector D) records seafloor crenulations of high amplitude (118 m). In contrast, older crusts of sector C in the south-central area (age between 55.9 and 50.8 Ma) and that in the northern sector A (age older to 55.9 Ma) show low-amplitude crenulations (range 62–78 m). These observations suggest that a comparatively intense compressive stress had been acting during the formation of the southern parts of the IONF (Table 2.2). Topographic distortions are also characterised by longer wavelength of crenulations (more than 8500 m) in northern older crusts and shorter ones (5000 m) in southern younger crusts. Such
Table 2.2
Morphotectonic characters of crenulated seafloor in the IONF
Sector
Latitudinal coverage (South) Crustal age (Ma) Spreading half rate (mm/year) Seafloor crenulation: wavelength (km) Seafloor crenulation: amplitude (m) Root mean square roughness (m) Stress condition Indian Plate movement
A
B
09.00 –10.26
C
D
10.96 –13.76
>58 90
10.26 – 10.96 58–56 55
56–61 95
Beyond 13.76 <51 26
8729
5754
5659
5269
62
67
78
118
65
194
90
154
Stretching Compression Stretching Compression Fast movement Soft touch Slip movement First collision
Sources: Malinverno (1991), Mukhopadhyay et al. (1997) and Kessarkar (1996, 1998). Note: IONF ¼ Indian Ocean Nodule Field. Touch and collision are between India and Eurasia.
48 Table 2.3
Mukhopadhyay, Ghosh and Iyer
Distribution and throw characteristics of faults in the IONF
Sector
A
B
C
D
Abundance (%) Average spacing (km) Average length (km) Faults (%) with>100m throw Abundance of normal faults (%) Abundance of reverse faults (%)
19.60 21 23 None ND None
27.88 18 25 33.75 31.32 36.44
22.41 20 23 22.11 40.66 22.03
30.10 08 26 44.12 28.02 41.52
Sources: Mukhopadhyay et al. (1997) and Kessarkar (1996, 1998). Note: IONF ¼ Indian Ocean Nodule Field; ND ¼ Not determinable due to sediment cover.
variations may reflect the low and high degrees of stress, respectively, to which the crust was subjected. It appears that a spectrum of distortion of the seafloor exists, that is, least distorted, older, thicker and stronger crust in the north (sector A) to highly crumpled, relatively thinner, younger and weaker crust in the south (sector D). The relationship between seafloor roughness and spreading rate in the world’s oceans is best expressed by the empirical orthogonal functions (EOF) analysis and by the root-mean square (RMS) inverse equation. The EOF analysis offers a quantitative distinction between the deterministic and the stochastic components of mid-ocean ridge topography, whereas the RMS is a conventional and well-proven procedure. The rapidly accreted areas in the IONF (north and south-central, sectors A and C, respectively) are theoretically calibrated to represent the RMS roughness of the order of 65 and 90 m, respectively (Table 2.2). Similarly, for the intermediate (northcentral, sector B) and slow (southern, sector D) accreted crusts, the RMS roughness was estimated as 194 and 154 m, respectively. However, the seafloor roughness as determined from real-time multi-beam data shows a lower degree of crenulations. For example, the rapidly (fast)-accreted crust of sectors A and C recorded an amplitude of the order of 62–78 m, while for the slowly accreted crust of sector D the amplitude of crenulations is 118 m (Table 2.2; Mukhopadhyay et al., 1997). Thus, the real-time seafloor roughness values of the crusts in the IONF are lower than the calibrated theoretical RMS values (Malinverno, 1991). These lower values may have been caused by various factors, such as the ancient oceanic crust tends to get smoothened with time because of subsidence, loss in thermal budget, sediment overburden, erosion and mass wasting. In summary, the roughness of bathymetry in the IONF differs in values, but not in trend from that of the world’s oceans.
2.2. Seafloor faults In the IONF, a number of ridge parallel east-west trending faults are found, each of which appears to be composed of several smaller faults. Two types of faults are encountered in the IONF—those having their planes facing northward away from the spreading ridge axis from which these have been generated, and those having
Tectonics and Geomorphology
49
their planes facing southwards towards the ridge axis. The north-facing faults are normal faults. In contrast, the south-facing faults , that is, towards the ridge axis, are considered as ‘reverse faults’. The south-facing reverse faults (generally caused by crustal compression) are probably transformed from original north-facing normal faults (formed by crustal tension). The throw direction of some of these normal faults underwent change owing to a shift in the pattern and magnitude of stress, from tension to compression. In general, the density and the total population (number) of faults increases from north to south. The faults are oriented normally in a direction between N80E and N110E. The length of these faults ranges between a little <4 km in the older northern crust (sector A) and about 56 km in the younger southern crust (sector D). The amount of throw along these faults ranges from <50 m to >100 m. A rough estimate suggests that about 40% of faults show throw exceeding 100 m, while about 18% record throw between 50 and 100 m (Table 2.3). Population-wise, the faults with large throw show interesting statistics. For example, the abundance of these faults are maximum in the southern area (sector D, 1528/million km2), followed by those in the north-central area (sector B, 1169/ million km2) and lastly in the south-central area (sector C, 766/million km2). The causal relation between the nature of the faults and spreading kinematics of the IORS is more evident, as difference exists between the crust formed at a fast rate and that formed at a slow rate. Approximately, 75% of the faults occurring in the areas formed at a fast rate (i.e. sectors A and C) are normal faults facing north. In contrast, a substantial number of the faults in sectors B and D face south (i.e. towards the ridge) and appear to be reverse faults. In addition, the faults in sectors B and D are closely spaced, and longer than those on the fast-generated crust of sectors A and C. A high percentage of such reversal of fault planes from north to south (as displayed by throw direction) is encountered in sectors B (43%) and D (49%), against a moderate 26% in sector C and absence of such reversal in the northern area (sector A). This would suggest that crusts of sectors B and D were formed during the periods of crustal compression, while that of sectors A and C were generated during crustal extension. The turnover of the fault planes from north-facing to south-facing in the sectors B and D may have also been caused by some degree of compressive stress acting on the spreading centre during their formation.
2.3. Tectonic, thermal and geoidal anomalies Besides bathymetric-cum-structural variations as detailed in the previous sections, the IONF is also characterised by several other tectonic (seismic, earthquake and stress), thermal (heat flow) and geoidal (gravity and magnetic) anomalies. The compressive and translational stresses in the IONF are distributed abnormally. For example, earthquakes at the plate boundaries between the IAPB and the ACPB, positioned to the north and south, respectively of the IONF, are of higher magnitude than that at the central portion of the IONF (Fig. 2.5; Table 2.4). This can be related to the Himalayan orogeny and crustal adjustments normally associated with any plate boundary dynamics.
50
Mukhopadhyay, Ghosh and Iyer
Table 2.4 Heat flow, thermal conductivity, permeability, subsidence and temperature in the IONF crust
Temperature difference between top and bottom of convective oceanic crust (DT C) Bulk permeability (10–11 cm2) Elevation versus age for the IONF crust: Depth after adjusting sediment loading effect (m) Sector A 5250 Sector B 5200 Sector C 4970 Sector D 4650 Thermal conductivity All sectors: range 75–80 (10–2 Watts/mK) All sectors: range 40–153 Heat flow (milliwatts/m2)
182
2.4
Standard deviation (m) 156 188 169 161 Average 78 Average 84
Source: cf. Anderson et al. (1977). Note: IONF ¼ Indian Ocean Nodule Field.
The maximum depth data of earthquakes offer additional constraints on the thermal and structural regime in the region. For example, in the IAPB area ( just north of the IONF), the earthquakes are dominantly triggered by thrust faulting, and the average focal depth estimated has been 36 4 km below seabed (about 30 km below acoustic basement, Bergman and Solomon, 1980). Although the stress condition in the IONF is to some extent influenced and camouflaged by that of the plate boundary deformations at the IAPB and the ACPB, one should take note of crumpling and faulting of the seafloor of various intensities even within the IONF. Such crumpling, as discussed in the preceding sections, was a consequence of the difference in rate of generation of new crust at the mid-ocean ridge axis and accommodation of old crust through collision at the Himalayas, through subduction at Java and at other deformation zones within the plate. A detailed analysis of the direction of principal stress axes for intra-plate earthquakes in the IONF and their focal mechanism with thrust faulting suggest that the style of deformations is related to the rate of generation of the underlying crusts at the ridge axis, net resistance at the Himalayan collision zone, the suction force acting on the overriding Indian Plate segment at the Tonga–Kermadec trench, and the drag at the base of the lithosphere (Cloetingh and Wortel, 1985). Disposition of fault planes and flexures suggests a dominant north-south-oriented compressive stress field in the IONF that manifests as an increase in local stress values, indicating
51
Tectonics and Geomorphology
accumulation of excess compressed crust. For instance, in sectors A and C, NNWSSE-trending tensional stress of 327 MPa dominates over NE-SW-oriented compressive stress of 145 MPa. In contrast, both NNW-SSE-trending tensional and NE-SW-oriented compressive stress are equally active in sector D, showing stress values of 209 and 218 MPa (Stein et al., 1989). Additional data from this region show that higher amplitudes of flexure (i.e. seafloor with high relief) are oriented in an NNW-SSE direction, suggesting prominence of compressive stress along this direction (Mukhopadhyay et al., 1997). The thermal evolution of any oceanic plate follows the empirical equation of depth–age relationship, that is, as the crust spreads away from the ridge crest to the basinal areas, the depth increases and the heat flow decreases. However, based on a study of 506 heat flow measurements in the Indian Ocean, a deviation from this rule of thumb is recorded, accentuated by a case of ‘missing’ heat flow (Fig. 2.6; Table 2.4). The likely explanation could be that the lost heat has been consumed by circulating seawater in the oceanic crust, which appeared to be quite active in the IONF crust formed between 60 and 40 Ma. The high heat flow (30–50 mWm2, higher than expected from lithospheric cooling products) and the maximum
6 49
30
54
4
2 N
54
57 73
72 48 93
0
155 147
155
144
122 109 A N
2 S
198 78
4
82
44
6
92 68 58
61 91
8
75
10 76E
78
80
82
84
86
88
90
Figure 2.6 Heat flow (mWm2, numbered circles), earthquake epicentres (▲) and acoustic basement highs (þ) and lows () in the India and Australia Plate Boundary (IAPB) and in the northern part of the Indian Ocean Nodule Field (IONF) (Stein and Weissel, 1990). AN ¼ Afanasy Nikitin seamount group.
52
Mukhopadhyay, Ghosh and Iyer
earthquake depth of 40 km in the IONF constrain the minimum depth of deformation and maximum temperature (750 C). Although lack of bathymetric expression for the high thermal anomaly is surprising, several small-scale magmatic intrusions, secondary eruptions and subsurface metal mobilisation may have been caused by the relatively high heat flow (Iyer et al., 1999a; Mukhopadhyay et al., 2002). Water circulation within the oceanic crust and seawater percolation through the sediment, however, seem to have disturbed the IONF sometime during the Palaeocene–Eocene period. This may be possible if sediment becomes less porous and poorly permeable, with change in sediment composition from calcareous/ carbonaceous to siliceous. In addition, general thickening of sediment cover prevented the otherwise normally convective exchange of heat from the oceanic crust to the ocean floor. Such a situation would influence the cause of mid-plate magmatism in the form of local secondary eruptions. Several east-west-trending gravity undulations on the surface as well as in acoustic basement of the IONF are reported. Geoids and gravity maps derived from the Geos 3 and Seasat satellites revealed unsuspected gravity undulations of the order of about 200 mGal wavelength with 10 mGal amplitude and geoidal variations of the order of 0.8 m. These gravity undulations and geoidal variations are distinctly different in orientation by about 40 from those reported in the IAPB zone and, hence, should have a different origin. Based on the elastic–plastic model of rheology, these undulations in the IONF are caused by folding of the oceanic lithosphere due to compressive stress. As discussed earlier, such stress has been accounted for by the formation of ridge-parallel lineations, reverse faults and crumpling of the crust. With several studies made to identify and characterise magnetic anomalies present in the IONF, the evolutionary history of this field as well as of the CIOB is now fairly well understood. Studies carried out in the IONF during the manganese nodule exploration work between 1982 and 2000 have led to the identification of magnetic lineations A21 and A26, and delineation of the Vishnu FZ, the Indrani FZ and the trace of the triple junction. Similarity in topographic signatures is noticed on either side of the fracture zones corresponding to the segments of the same crustal age. It is observed that the evolution of the triple junction and reorientation of the SEIR from east-west to northwest-southeast marked the change in spreading direction. Such change in spreading direction occurred during the global plate reorganisation during 43–38 Ma that has influenced the evolution of the ocean floor in the IONF. The skewness of resultant marine magnetic anomalies can be interpreted as a combination of short period fluctuations and a gradual increase of apparent effective remnant inclination, which however decreased with faster spreading rate (Dyment, 1993).
2.4. India–Eurasia collision event revisited The nature and intensity of flexures and faults in the IONF (Tables 2.2 and 2.3) bear a strong signature of the stress regime that prevailed during Palaeocene–Eocene time. Such stress could possibly be the resultant product of the northward movement of the Indian Plate, the variable rate of spreading at the IORS and the India–Eurasia
Tectonics and Geomorphology
53
collision event. The timing of the collision has been variously calculated by using different methods based on two distinct sources. The first source used reconstruction of the paleo-positions of the Indian continent as recorded in the magnetic reversal pattern on the seafloor. This shows collision at 45 Ma, with the spreading rate decreasing from 110 to 45 mm/year (Dewey et al., 1989). Using magnetic reversal pattern, various collision times such as 53 Ma (Powell and Conaghan, 1973), 50 Ma (Sclater and Fisher, 1974), 40 Ma (Molnar and Tapponier, 1975) and 52 Ma (Patriat and Achache, 1984) have been suggested. The second source from two rather poorly dated subduction-related granitic rock suites suggests collision age as 50 Ma (Searle et al., 1987). However, the rates of thermal cooling of some Himalayan metamorphic rocks in Pakistan, with a mineral cooling age of 39 2 Ma, show that the collision must have been older, taking place before 56 Ma (Hartland et al., 1990) and certainly not at 50 or 45 Ma. Klootwijk et al. (1992) presented paleomagnetic data to suggest that India and Asia collided at around 53 Ma. Meanwhile, new foraminiferal records from Upper Palaeocene strata suggest that the collision must have predated 57 1 Ma (Beck et al., 1995). Recent paleontological evidences from the Indus-Zanksar Tethyan zone in the Himalayas indicate the collision time to be about 50.6 Ma (Mathur and Juyal, 2000). It is also known that at around 50 Ma, throngs of Asian mammals crossed the Bering Strait land bridge to North America. The situation that forced the mammals to leave Asia could have been the sudden unbearable increase in temperature owing to large-scale belching of methane, and volcanism at the northern boundary of the moving Indian Plate. The reason for such large-scale volcanic eruptions, intense global warming and increase in greenhouse effect could possibly have been caused by the collision of India and Eurasia at around 50 Ma. However, a recent study suggests that the birth of the world’s highest mountain chain, the Himalayas, may have occurred at 450 Ma—nine times older than other estimations (Gehrels et al., 2003). This idea is based on the analysis of radioactive uranium in garnet and zircon grains of granites and schists that are believed to have formed during the collision. Hence, the timing of collision between India and Eurasia has drawn sufficient attention and is far from settled. A critical appreciation of the timing of the India– Eurasia collision has come from a study combining multi-beam bathymetry data and that of compressive stress, particularly from the IONF. For example, the lithosphere of sector A, generated prior to 57.9 Ma, shows a nearly flat and smooth bathymetric expression, with occasional presence of latitude parallel east-west-oriented widely spaced normal faults. The disposition of magnetic anomalies in this sector also suggests a consistent fast rate (about 90 mm/year) of formation of the oceanic lithosphere. All these indicate a near-uniform extensional tectonic stress regime that prevailed preceding to 58 Ma. In contrast, the lithosphere of sector B, generated between 57.5 and 56 Ma, shows moderate to intense ridge-parallel flexures. The magnetic anomaly data here suggest reduction in the spreading rate from 90 to 55 mm/year, indicating marginal dominance of compressive stress. These evidences could point to a probable ‘soft touch’ between India and Eurasia at around 57.9 Ma. It seems that an initial touch between the Indian and Eurasian plates occurred at around 57.9 Ma (chron 26) at an angle (DeMets et al., 1990). The continuous push
54
Mukhopadhyay, Ghosh and Iyer
from the spreading ridge in the south caused India to slip fast in an anticlockwise direction towards northwest. This movement resulted in the uplift of the Hindukush mountain range and development and activation of the Owen FZ. Newer geodetic evidences also support oblique slip movement in a left-lateral direction during this period. Consequently, the spreading rate increased from 55 mm/year to a faster rate of 95 mm/year between 56 and 51.7 Ma, during which the crust of sector C was formed. A cessation in the northward movement of the Indian Plate was brought about by another phase of touch or collision between India and Eurasia during the period between 51.7 and 50 Ma (i.e. during chron 23). The spreading rate then plummeted to a mere 26 mm/year from a high of 95 mm/year and resulted in an intense tectonic compression. The crust of sector D, formed during this period, exhibits extensive crumpling and reverse faulting of the seabed. This finding is further corroborated by a study of the CIOB crust (Rajendran and Prakasa Rao, 2000), which found anomaly 23 (of age 51 Ma) to be indistinct and ill-preserved, owing probably to intense flexure and faulting of the seafloor. Similar evidence of reduction in spreading rate is also found from the Arabian Sea spreading anomaly (Chaubey et al., 1993). The predominance of tensional stress during the formation of sectors A and C and the prevalence of a compressive stress regime during the formation of sector D from the MOR axis in the IONF are supported by regional stress data (see earlier section also). Tensional stress dominates over compressive stress in sectors A and C, while compressive stress becomes equally active in sector B. All these probably suggest episodic collision events between India and Eurasia during the period 60 to 49 Ma—a soft touch by the frontal portions at 58 Ma and the first hard collision between 51 and 50 Ma.
3. Seamounts The IONF hosts several seamounts of variable dimensions, which occur either in linear chains or as isolated entities. The study of these seamounts, which are essentially topographically elevated surface manifestations of mantle upwelling, provides a better understanding of crustal and mantle processes. A world ocean survey reveals that seamounts are generally formed at the flanks of MOR, though they are not uncommon at RTI, at overlapping spreading centres (OSC), and at the sites of hotspot volcanism in the basinal regions away from the ridge environment. Besides being a window to the mantle, seamounts are believed to profoundly affect the mixing processes of deep oceans, oceanic circulation, thermohaline structure, biological productivity and formation of eddies. Seamounts also indicate rate and direction of plate movement in a hotspot regime, contribute significantly to the volcanic regime of the basin, and influence the formation, distribution and growth of mineral deposits in the basin. Many of the IONF seamounts are flat-topped, and some have craters at the summit (Fig. 2.7). Mean flatness of seamounts decreases with increasing height.
55
Tectonics and Geomorphology
A
B 5100
520
0
5300
5300
00 52
5300
12 30 S
00
52
4500 5300
12 50 S
500
00 52
5100
510 0
00 50
0
0 500
52
13 00
50
00
12 40
5000
0
530
53
5300
500
0
00 54
00
00
5100 5300
5100 5300
5000
13 10 7435E
7445
7545E
7550
Figure 2.7 Swath bathymetry of (A) a pair of north-south-trending seamount chains and (B) another seamount chain with a tall steep-sided and flat-topped edifice in the north and smaller edifices with gentler slopes to the south (Mukhopadhyay and Batiza, 1994). Contour interval is 100 m (solid line).
They cover about 3–10% of the IONF seafloor. The abundance is around 6000 seamounts per million km2 (Mukhopadhyay, 1998), which is however less than in the Atlantic (7000 seamounts/million km2, Fornari et al., 1987) and in the Pacific oceans (9000 seamounts/million km2, Batiza et al., 1989). The distribution, shape, size, overall style of occurrence, petrology and geochemistry of the IONF seamounts suggest their formation mostly on or near the MOR axis, sharing the same or similar melt, which normally erupts to form the new oceanic crust at the axes. The absolute motion of the IONF during Eocene–Palaeocene time was very similar to its relative motion (Duncan and Richards, 1991). Hence, in the nodule field (age 49–60 Ma), the flow-line parallel seamounts are indistinguishable from those aligned in the direction of absolute motion. Satellite altimetry was used to remotely sense some of these seamounts (Basu et al., 1994). Detection was achieved by passing the sea-surface height data obtained by a satellite altimeter through a matched filter designed from the typical seamount signature model and power spectral density of the background noise. Detection and verification of the ground truth data through trapezoidal model signatures are found
56
Mukhopadhyay, Ghosh and Iyer
easier and more reliable than through the conical model. This method proved reasonably successful with the detection of four seamounts and faithful prediction of another six (Fig. 2.8).
3.1. Spreading rate and seamount distribution Seamounts in the IONF occur parallel to flow lines, that is, along the direction of absolute plate motion (also probably the relative motion) of the Indian Plate. The distribution pattern of seamounts largely depends on spreading conditions at the ridge crest. For instance, it is seen that ridges with high-spreading rates [e.g. East Pacific Rise (EPR)] generate ridge normal, flow-line parallel chains of seamounts, while those formed at the slow-spreading ridges [e.g. Mid-Atlantic Ridge (MAR)] are mostly scattered on the flanks. Against this background, the occurrence of a majority of the flow-line parallel north-south-trending ancient (age >50 Ma) IONF seamounts appears similar in terms of distribution, petrology and origin to the younger seamounts formed at the fast-spreading EPR of recent times (age <10 Ma, Mukhopadhyay et al., 1995). The flow-line parallel ridge normal disposition of the present nodule field seamounts suggests that these are largely formed from the fast-spreading IORS during Palaeocene–Eocene, and their style of distribution is influenced by the spreading rate. The linear disposition of the majority of the seamounts in the IONF is the fundamental characteristic of fast-spreading ridges. If so, this concept could well be extended in terms of identifying any fast-spreading oceanic crust lacking identifiable seafloor spreading magnetic anomalies but having flow-line parallel seamount chains. The abundance and density of seamounts in the IONF with respect to the spreading rate (and subsequent rate of magma upwelling) give interesting results. As mentioned earlier, the period prior to magnetic anomaly 26 is characterised by a fast rate of crustal accretion (90 mm/year), which should normally result in intense magma upwelling and possibly higher production of seamounts. However sector A, representing such a magmatically active regime, shows a low abundance of seamounts (about 6% of total seamounts in the IONF, Table 2.5). It is possible that greater sediment thickness in the northern part of the IONF, derived from nearby landmasses, may have covered some seamounts, partly or wholly, as also evidenced by the subdued topography. In contrast, sector B, limited by chrons A26 and A25, was generated at an intermediate-spreading rate (55 mm/year), and seems to have had moderate intensity of magmatic activity represented by a low-to-moderate population of seamounts (about 10%). Sector C, bounded by chrons A25 and A23, however, was formed in a superfast-spreading regime (rate of accretion 95 mm/year) and, as a consequence, several seamounts formed here because of increased upwelling and magmatic eruptions. This is well documented by the occurrence of about 61% of total IONF seamounts in this sector (Table 2.5). Sector D was generated at a slow rate (26 to 14 mm/year) between chrons 23 and 22, and consequently should have shown a significant reduction in the rate and amount of magma upwelling in comparison to that in sector C. However, an increased occurrence of seamounts in sector D
3.200
Sea surface height (m)
TH = 2.39
3.200 2.560
Filter output
Filter output
2.560
B
1.920 1.280
1.280 0.640
0.000
0.000
−59.600 −71.200 −82.800 −94.400
0.00
410.0
(2.99N, 76.71E)
820.0
1230.0
Distance (km)
1640.0
2050.0
(13.69S, 70.02E)
TH = 2.52
1.920
0.640
Sea surface height (m)
A
−43.400 −58.800 −74.200 −89.600
0.00
560.0
(2.96N, 81.13E)
1120.0
1680.0
Distance (km)
2240.0
2800.0
(19.95S, 71.75E)
Figure 2.8 Geosat altimeter tracks confirm a few Indian Ocean Nodule Field (IONF) seamounts detected earlier by ship-borne bathymetry data (Basu et al.,1994): (A) descending and (B) ascending. Dashed lines show threshold of detection.
58
Mukhopadhyay, Ghosh and Iyer
Table 2.5
Distribution and morphology of seamounts in the Indian Ocean Nodule Field
Sector
A
B
C
D
Seamount population (%) Major Large Small Minor Seamount abundance (% in million km2) Percent volume of lava consumed (km3) Seamount flatness Slope angle ( ) Mid-plate secondary volcanic abundance (%) Taller mid-plate seamount abundance (%)
00 00 3.91 7.24 18.58 2.41 0.14 4.81 00 00
00 19.02 7.49 3.87 23.19 7.71 0.16 5.01 19.00 36.38
87.50 57.69 56.02 63.64 20.06 65.49 0.11 11.82 71.00 22.36
12.50 23.07 32.57 18.18 38.16 24.40 0.10 9.20 10.00 41.25
Sources: Mukhopadhyay and Batiza (1994) and Mukhopadhyay (1998).
compared to that normally expected (about 23%, Table 2.5) is probably due to escape of lava in small volumes through increased seafloor openings, caused by the extreme compression to which this crust was subjected. These findings suggest a positive relation between spreading rate, stress regime, magma upwelling and the formation of near-axis seamounts. Although the variations in the spreading rate have shown some relation with the abundance of seamounts in the IONF, as seen above, the precise physical connection between them is not clear. It is seen that the abundance of seamounts with height 200 m increases steadily over the range of fast- and superfast-spreading rates. If the seamount abundance is normalised to a constant time interval (i.e. the number of seamounts formed along a particular length of the ridge in a fixed period), the spreading rate dependence of seamount abundance would be even greater. This means that faster spreading centres would contain more seamounts per unit area of seafloor than an equivalent seafloor area produced at slower spreading rates. Again, to produce more seamounts around a fast-spreading ridge, the presence of a larger melt volume closer to the spreading axis appears to be critical. Additionally, it is suggested that the ability of the magma to penetrate the overlying plate may also be given due consideration. If this ability varies with spreading rate, the abundance of seamounts might also vary with spreading rate even if the distribution of the melt remains constant at all spreading centres. For example, for a given volume of the melt to penetrate through the entire lithosphere, its ascent time must be shorter than its cooling time. The ascent time is primarily a function of the thickness of the lithosphere to be penetrated and the mode of ascent (whether through diapirs or dikes or some other mechanism). The cooling time, on the other hand, is primarily a function of the size and shape of the melt volume, its temperature and the temperature of the material through which it ascends. The variability in seamount abundance in the world’s oceans with respect to spreading rate thus appears to be broadly controlled by properties of the source of seamount
Tectonics and Geomorphology
59
melt and properties of the lithospheric plate. It can therefore be concluded that seamounts are the normal derivatives of magmatic and tectonic activities at spreading ridges, and the overall style of axial and near-axis volcanism in the IONF seems to vary systematically as a function of spreading rate.
3.2. Structural lineaments and seamount abundance Fracture zones and transform faults are suggested to separate coherent geochemical units along the MOR that are characterised by various degrees of crystal fractionation and partial melting. The coherent magma bodies, at times, give rise to structurally and topographically elevated regions due to the accumulation of hot magma. Previous studies on near-axis seamounts in the Pacific and the Atlantic oceans have brought out contrasting ideas on the role played by fracture zone in influencing the origin and distribution of seamounts. Fracture zones may promote seamount formation through increased magma upwelling (e.g. north of Blanco FZ on Juan De Fuca Ridge, Batiza and Vanko, 1983; Fornari et al., 1987), or hinder seamount formation by making the rising magma cooler and depressing the melt from upwelling (e.g. south of Blanco FZ on Gorda Ridge; Gass et al., 1978), or have no effect on their generation (Epp and Smoot, 1989). These hypotheses may be suitably tested in the light of new data from various geomorphic and tectonic domains of the IONF. The seafloor located close to Vishnu FZ was found to have relatively few seamounts—about 130 km3 of lava erupted to form 30 minor (height 50–99 m), 33 small (height 100–499 m) and one major seamount (height >1000 m). Among these, the major seamount (also the largest one in the entire IONF) is located at 11 090 S and 73 040 E, with its summit height reaching to 1550 m, and has used up about 115 km3 of lava. This left the rest (15 km3) of the lava to form 30 minor and 33 small edifices. This enormous draw of magma by the lone major seamount probably explains the absence of any medium- and large-sized seamounts in the neighbourhood of this fracture zone. A low abundance of seamounts along the Vishnu FZ is, however, inconsistent with the concept that an FZ positively influences seamount formation. It is possible that cool seawater might easily enter through this wide FZ (13- to 17-km) causing a reduction in the pressure and temperature over the magma chamber, thereby diminishing the chances of magma eruption. In addition, the occurrence of thick ferromanganese oxide coatings (6–9-mm) around ancient, depleted, normal-MOR basalt (N-MORB) fragments recovered from this fracture zone would indicate lack of volcanism in the recent past (Mukhopadhyay et al., 1994). Along the Indrani FZ, seamounts are present in the southern part, the summit height rising to 4000-m water depths, while the northern part is conspicuously devoid of seamounts (Kodagali et al., 1992). Again, the seamounts in the western part show a linear trend parallel to the Indrani FZ and tend to be circular, conical and single peaked. Those occurring to the east of this FZ, in contrast, are irregular in shape and multi-peaked. Such variations in the distribution of seamounts across the Indrani FZ may be related to an anomalous thermal input from a nearby source.
60
Mukhopadhyay, Ghosh and Iyer
Abundant seamounts of variable sizes are found close to the NNE-SSW-trending TJT-In (100 20 km on either side of the lineament). In fact, about 70–75% of the IONF seamounts are present in the vicinity of this magmatically active zone (MAZ, Fig. 2.1). Of the total seamounts present in the IONF, about 87% of small and minor seamounts (height 50–499 m), 73% of large (height 500–999 m), and 57% of major (height > 1000 m) seamounts occur within this MAZ. The high abundance and density of seamounts in this zone could be attributed to the excess eruption of magma through faulted, fissured and thin lithosphere that normally occurs in the vicinity of a triple junction. The abundance and distribution of seamounts may thus suggest the possibility of a thermally elevated source at the TJT-In. The seafloor located between the Vishnu FZ and the TJT-In lineament, on the other hand, contains about 133 minor, 90 small and 4 large seamounts that used 244 km3 of lava. This suggests that seamounts can form even in the absence of any structural lineaments (Table 2.5). All these findings prove that the mere presence or absence of a FZ need not necessarily influence the formation and abundance of seamounts and that the magma budget along the FZ/lineament is very important.
3.3. Structural lineaments and seamount morphology Several parameters influence the morphology of seamounts. They include the local and regional tectonic settings, thermal properties of the underlying lithosphere, conduit geometry, magma composition, sediment cover, flow rate of the melt, gravity pull on the ascending magma and magma viscosity. Variations in overall morphology of the IONF seamounts, when seen in this context, reveal that seamounts occurring within 20 km of fracture zone show moderate-to-high basal area, slope angle and volume. These values are, however, less than those occurring between 20 and 50 km across the fracture zone, which generally are the tallest, largest and most conical. Seamounts located between 50 and 100 km from the fracture zone do not show any definite trend but record wide variations in height (100–1201 m), volume (0.4–101 km3), basal area (6–236 km2) and slope angle (2.5 –17.8 , Table 2.5). Although in plan most of these volcanoes are circular to sub-circular, a few show subtle to prominent elongation of the base and the summit. For smaller seamounts, the elongation trend is NW-SE and NE-SW, while for larger edifices the trends are generally N-S and E-W. Flatness (ratio of summit area to basal area) appears to be a pivotal morphological parameter, and it is found that seamounts located away from the fracture zones are more pointed or conical than those occurring close to it. Also, mean flatness of seamounts decreases with increasing height. The taller the seamount, the lower the flatness. Small seamounts located in the MAZ (near TJT-In) particularly show very high flatness values and have slope angles of 8 . Conicality of seamount seems to depend on the amount of erupted lava, conduit geometry, physical state of magma and rate of eruption. For small seamounts, the amount of erupted lava is naturally less, conduits are shallow and narrow and, therefore, a flattened summit is formed. A comparison of height and flatness data between the IONF seamounts and those of the Pacific Ocean (Fig. 2.9) reveals that the bias in seamount distribution is probably related to conduit geometry and eruption style.
61
Tectonics and Geomorphology
Eruption of magma through narrow openings is likely to produce tall seamounts with a pointed summit, while small, flattened volcanic swells are formed by eruption through elongated fissures. Principal component analysis of various morphological parameters (both independent and dependent variables) of the IONF seamounts suggests that summit height is the prime responsible factor (first eigenvector) that accounts for 54% of the total morphological variance. This is followed by flatness as the second eigenvector. Among the factors that control overall seamount morphology in the IONF have been the availability of magma and style of eruption (Mukhopadhyay and Khadge, 1992). Similar studies from the North Atlantic seafloor suggest that volume of plume material and conduit geometry may have controlled the formation of seamounts.
3.4. Seamount petrology The rocks dredged from the IONF seamounts are pillow basalts with phenocrysts largely of plagioclase and a few of olivine. These basalts can be broadly divided into three types. The first type shows fresh glass with microlites of plagioclase and a small amount of olivine set in it. Towards the interior, the rock is holocrystalline with abundant plagioclase, forming typical flow and glomeroporphyritic texture.
B
0.1
Volume (km3)
Flatness
A
0.01
100 10 1
0.001 0 200
600 1000 Height (m)
1400
Conical
600 1000 Height (m)
1400
D 0.80
Old edifice, MORB type, Pacific Recent edifice, K2O rich, Pacific Near-axis edifice, MORB type, IONF
Recent intraplate seamount
0.00 0
1000 2000 Height (m)
3000
103 Volume (km3)
Off-axis seamount
Flatness
Truncate
C
0 200
Off-axis seamount, Pacific
102 10 1 10−1 102
Intraplate seamount, Pacific Near-axis seamount, IONF
103 Height (m)
104
Figure 2.9 Growth characteristics of seamounts: (A) and (B) relations among height, flatness and volume in semi-log term for the Indian Ocean Nodule Field (IONF) seamounts, (C) and (D) similar relations in the IONF seamounts are compared to seamounts in the Pacific Ocean to decipher their origin (Mukhopadhyay and Khadge,1990).
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Mukhopadhyay, Ghosh and Iyer
At places olivine is replaced by iddingsite, while phenocrysts of plagioclase exhibit twinning and zoning. The second type of basalt is partly altered with plagioclase as the dominant mineral, together with olivine and clinopyroxene. The rocks show intergranular, subophitic and intersertal textures. The third type is holohyaline to holocrystalline showing a trachytic texture formed by plagioclase laths. Compositionally, the samples from the IONF seamounts are silica-saturated, show depletion in incompatible elements and appear akin to N-MORB. FeO and TiO2 exhibit an inverse relationship with MgO and Al2O3, while K2O and P2O5 are generally low, and the rocks show high Mg # (62.3; high Mg # is a measure of melt pristinity and consequently represent low degree of melt fractionation). The Mg # is calculated as 100 MgO/Mg þ Feþ2, assuming that Fe2O3/FeO ¼ 0.15. The average composition (in wt%) of these rocks is SiO2 50.6, Al2O3 15, TiO2 1.52, FeO 10.7, MgO 7.34 and CaO 12 (Table 2.6). This primitive nature of seamount lava in the IONF is comparable to the depleted tholeiitic lava from the Pacific and the Atlantic ocean seamounts. It is suggested that the melt that formed the large majority of the IONF seamounts was ancient in nature and probably derived near the ridge crest from a heterogeneous upper mantle silica-saturated source, depleted in large ion lithophile elements. The other circumstantial evidences for the near-axis origin of the IONF seamounts come from the fact that many of the rocks dredged from these seamounts overlie normally polarised oceanic crust, and have a thick coating of ferromanganese oxides accumulated over a long period. In fact, volcanic evolution of any seamount could be ascertained by the type of lava it erupts, with pillow and sheet lava forming the two end members of the evolutionary lineage. In the IONF seamounts, pillow lavas were predominant, indicating that the magma was less evolved, and surfaced Table 2.6 Chemistry of primary and secondary seamount materials from the IONF and other Oceans
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2 O 5
a
b
c
d
e
f
50.53 1.27 15.44 9.81 0.19 7.73 12.30 2.80 0.08 0.11
50.60 1.52 15.03 10.70 na 7.34 12.00 na na na
50.10 1.25 16.73 9.20 na 8.13 11.45 2.79 0.06 0.12
49.89 1.36 15.52 9.42 na 7.76 12.40 3.58 0.10 0.13
50.74 2.27 13.42 13.28 0.21 6.20 10.86 2.74 0.09 0.28
48.37 1.63 16.18 8.44 0.16 6.20 10.08 3.04 0.85 0.29
Sources: a, b, e, f ¼ Mukhopadhyay et al. (2002), c ¼ Batiza et al. (1989) and d ¼ Batiza et al. (1990). Note: a ¼ ancient near-axis seamount, IONF, b ¼ ancient near-axis seamount, IONF; c ¼ young near-axis seamount, Mid-Atlantic Ridge; d ¼ young near-axis seamount, East Pacific Rise; e ¼ secondary eruption at the enlarged portion of the seamount, in ‘a’, IONF, f ¼ alkali basalt from lower flank of a seamount, IONF.
Tectonics and Geomorphology
63
with a low rate of effusion. The petrography and chemistry of these seamounts also suggest eruption of less fractionated near-pristine magma at or near the then moderate- to fast-spreading IORS. In contrast, seamounts occurring in isolation in the IONF were the products of separate batches of magma rising from small and shallow magma chambers.
3.5. Growth conditions of seamounts The mode and type of emplacement of seamounts through the geological ages are generally deduced from the underlying magnetic anomalies, style of disposition, morphology and chemistry. Magnetisation of a seamount, normal or reversed, may yield constraints on the timing of seamount volcanism, particularly if a seamount is magnetised in the opposite direction to the underlying crust. For instance, if a seamount and the underlying crust display the same magnetisation, the seamount may be considered to have formed in the same magnetic epoch as the underlying crust, and if they are oppositely magnetised, the seamount must have formed during a later magnetic epoch. Normally magnetised seamounts show negative anomalies to the north of the seamounts and positive anomalies to their south. Reversibly magnetised seamounts, in contrast, display positive anomalies to the north of the seamounts and negative anomalies to their south. The rule of thumb has been that if a seamount is oppositely magnetised from the underlying crust and is situated more than 25 km from the nearest younger isochron, the seamount may be considered an off-axis edifice (Barr, 1974). Using this argument, it is found that a majority of the IONF seamounts were ancient in origin, and formed at or near the ridge crest between 60 and 49 Ma (on-axis or near-axis origin during Palaeocene–Eocene). As suggested earlier, the growth patterns and evolution of the IONF seamounts are influenced by the regional tectonic activities and volcanic regime that prevailed during their formation. A majority of these seamounts show different trends of basal elongation, consequent probably to the speed at which the underlying crust was generated. For instance, basal elongation of seamounts generated on the fastaccreting crust in sectors A and C (rate of accretion 90–95 mm/year) strikes 050–230, that is, nearly normal to the then IORS trend. This trend of basal elongation appears to be reconciled to the unhindered and fast northward movement of the Indian Plate, and the resultant overall tensional stress conditions prevailing between 60 and 51 Ma. However, the stress regime changed to compressive type once the collision occurred between India and Eurasia at about 51 Ma. As a consequence of this change in nature of stress, basal elongation of seamounts present in sector D and to its south altered to a nearly ridge-parallel direction 090–270 and 135–315. Some striking similarities between the IONF seamounts and those in the tectonic regime of the Pacific and the Atlantic oceans have been recorded. For example, a close similarity exists in the style of distribution between the small, isolated ancient seamounts occurring near the Vishnu FZ in the IONF and those formed at the slowspreading MAR. Such similarity in distribution style was also maintained between the chains of ancient seamounts occurring close to TJT-In in the IONF and those generated presently at the fast-spreading EPR. Further, the density of the seamounts in the IONF also closely resembles the present near-axis volcanoes in the Pacific
64
Mukhopadhyay, Ghosh and Iyer
Ocean. For example, the average seamount density on the ancient IONF crust (age 60–50 Ma) is 5 to 9 seamounts/103 km2 and that of the young Pacific crust (age <10 Ma) is 9 seamounts/103 km2. These data do not indicate any apparent increase in the population of seamounts in the IONF crust after it moved away from the ridge axis. However, this inference is questionable if seen from the perspective of off-axis production rate. The rate of production of the IONF seamounts (i.e. total seamounts/crustal age) has been found to vary between 31 seamounts/Ma and 61 seamounts/Ma, a value more than what a 60–49-Ma-old crust should normally have had (Mukhopadhyay, 1998). The question arises as to why this increase in offaxis edifices occurred in the IONF, and why this is not reflected in the density pattern of the IONF seamounts? Probably an answer to these questions lies in the supply of melt, production rate of volcanoes and spreading rate at the ridge axis. Few off-axis volcanoes in the IONF have tended to develop into individual, isolated and large seamounts. The height of these off-axis volcanic features does not exceed 50 m (the cut-off height used by these workers) and these off-axis volcanic features are largely found to enlarge the volume of the pre-existing near-axis ancient seamounts. In fact, multi-beam bathymetry surveys revealed many seamounts with basal enlargement quite abnormal to their overall structure (Fig. 2.1). Some of the large seamounts in the IONF indeed appear to have formed by several coalesced edifices. This addition of mass at a later stage to the pre-existing volcanic body suggests that seamounts grew when magma ascended through existing weak zones near an ancient seamount and enlarged its basal dimensions (Ghosh and Mukhopadhyay, 1999; Iyer et al., 1997a). This phenomenon of secondary eruption-cum-seamount enlargement has been an interesting feature of the IONF seamounts. The geochemical studies support such secondary emplacement. Rocks dredged from the enlarged part are relatively fresh and evolved N-MORBs. They have low Mg # (50) and contain 0.09% K2O, 2.27% TiO2 and 13.28% FeO in contrast to the main body of the seamount with Mg # 62.3 and 1.52% TiO2, 10.7% FeO, and 2.87% of Na2O þ K2O (Table 2.6). These differences in composition suggest that the new mass could likely be the product of secondary eruptions. The data also hint that the off-axis second-generation volcanisms were weak in nature and, in general, could not penetrate the thick lithosphere on their own. In such cases magma probably ascended through existing weak zones near earlier-formed seamounts and in the process enlarged their dimensions. A morphological finding also suggests secondary emplacement. The ratio between height and the basal diameter provides a rough estimate of growth consistency of a seamount. The seamounts in the Pacific Ocean show an overall fairly constant ratio of 1:8 (Smith and Jordan, 1988). But the IONF seamounts show variable and higher values, ranging from 1:10 to 1:15, suggesting that seamount generation in IONF may involve more than one phase of formation. The mechanism that could have brought about the processes of secondary offaxis eruptions is (1) some melt was trapped beneath the main seamount structures during their formation at the ridge axis and then transported to the basin stored in a horizon of stabilised, narrow but horizontally extended neutral-buoyancy (Ryan, 1994), (2) the trapped magma then underwent fractionation and evolved during the
Table 2.7
Microprobe analyses of basaltic glass from seamounts in the IONF and other oceans Ancient fast generated IONF seamount
Young slow generated MAR seamount
Young fast generated EPR seamount
Sample #
36
4
26
5
7
10
3^7
4^4
F-1^1
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total Mg #
51.02 1.48 15.18 9.97 0.19 7.50 11.62 2.62 0.12 0.11 99.82 61.7
50.38 1.18 15.56 9.77 0.19 7.80 12.66 2.89 0.06 0.12 100.61 63.1
50.18 1.16 15.59 9.68 0.20 7.88 12.62 2.89 0.07 0.09 100.36 63.5
49.74 1.13 17.58 8.98 na 8.42 11.20 2.85 0.04 0.11 100.05 66.7
49.61 1.20 17.33 9.09 na 8.33 11.28 2.84 0.06 0.11 99.85 66.1
50.96 1.43 15.29 9.54 na 7.63 11.88 2.69 0.07 0.13 99.62 63.1
50.13 1.36 14.84 8.30 na 7.73 12.40 2.63 0.06 0.11 99.22 62.4
49.13 1.55 16.22 7.79 na 7.11 11.79 3.08 0.19 0.18 98.59 61.9
50.41 1.18 15.49 9.30 na 8.14 13.00 2.41 0.06 0.11 100.10 63.3
Sources: Mukhopadhyay et al. (1995), Batiza et al. (1989), Batiza and Vanko (1983) and Allan et al. (1987). Note: IONF ¼ Indian Ocean Nodule Field; MAR ¼ Mid-Atlantic Ridge; EPR ¼ East Pacific Rise; Sample 36: western flank of seamount 201; Sample 4: northern flank of seamount 202; Sample 26: western slope of seamount 204; Samples 5 and 7: MAR flank at 26o3300 S; Sample 10: MAR flank at 26o220 S; Sample 3–7 (seamount 1; Fe2O3 ¼ 1.66); Sample 4–4: (seamount 2; Fe2O3 ¼ 1.55); Sample F-1–1: (seamount SASHA, easternmost edifice of Lamont seamount chain). na ¼ data not available.
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plate’s journey, and erupted at a later time through some weak zones at the base of seamounts, (3) the formation of these weak zones could be linked to the mid-plate deformations as a fallout of tectonic reactivation at the IAPB, the ACPB and at the Himalayas. The chemistry of the basaltic glass dredged from the ancient IONF seamounts when compared with that from MAR and EPR seamounts (Table 2.7) indicates that the majority of the IONF seamounts relate closely to the melt normally produced along the present day mid-oceanic ridges and hence may have been formed near the spreading ridge axes (Mukhopadhyay et al., 2002). In summary, the studies on IONF seamounts suggest that a majority of these seamounts were formed near the spreading ridge axis, that seamount abundance is dependent on the availability of a magmatically active zone in the vicinity, that seamount disposition and morphology are spreading-rate dependent, that a few of these seamounts show more than one episode (or stage) of growth, that local midplate secondary eruptions are facilitated by tectonic reactivation, and that the local eruptions mostly enlarge the dimensions of the pre-existing seamounts.
C H A P T E R
T H R E E
Volcanics
Contents 69 69 72 80 82 86 86 103 103 108 112
1. Major Volcanics 1.1. Tholeiitic basalts 1.2. Ferrobasalts 1.3. Spilites 1.4. Pumice 2. Minor Volcanics 2.1. Hydrothermal materials 3. Alteration of Volcanics 3.1. Alteration of basaltic glass 3.2. Zeolitites 4. Tectonics and Volcanics: Interrelations
A study of oceanic rocks helps to understand aspects like magma genesis, magmatic processes, relation between volcanic rocks and seafloor morphology and intensity of volcanic activity. Petrological studies of the oceanic rocks are about 130 years old, since the time when Hall (1876) recovered and detailed the Mid-Atlantic Ridge (MAR) basalts. Similar studies in the Indian Ocean, though scanty, largely centred on characterising its mid-oceanic ridge (MOR) system. Major studies were made under the Deep Sea Drilling Project (DSDP) wherein 21 sites were drilled during legs 22, 24, 26 and 27 (sites 211–216, 231–238, 253, 254, 256, 257 and 259–261), and basalts and related rocks were recovered from the Carlsberg and Central Indian Ridges (CR and CIR), Central Indian Ocean Basin, Somali Basin, Ninetyeast Ridge and other locations (Fig. 3.1). To reconstruct the geological history of the Indian Ocean, the ‘JOIDES ( Joint Oceanographic Institutions for Deep Earth Sampling) Resolution’ examined, in 1987, the volcanic, oceanographic and plate tectonic history of the western and central regions of this ocean. Volcanic activity generally forms a complementary component contributing towards the structural and morphotectonic features on the seafloor. The central eruptions are mainly confined to the bottom of ocean basins and largely create abyssal hills and seamounts, while K-poor tholeiitic normal-mid-ocean ridge basalt (NMORB) erupts along the fissures. Approximately 50 million tons of basalts generated in the world’s oceans every year are utilised to form seamounts, yet this huge volume of basaltic lava corresponds to only 0.2–0.3% of the annual input of terrigenous sediments (25.33 billion tons/year) by the rivers to the oceans and is also considerably less than the basalts formed at the MOR (60 billion tons/year; Lisitzin, 1996). Handbook of Exploration and Environmental Geochemistry, Volume 10 ISSN 1874-2734, DOI: 10.1016/S1874-2734(07)10003-6
#
2008 Elsevier B.V. All rights reserved.
67
68
Mukhopadhyay, Ghosh and Iyer
10⬚ C
AFRICA
0⬚
716
R
711 710 707 706 705
712 713
717−719 215
CIOB
214 757
CIR
709 708
217 758 216
715 218 714
759−761 766
765 764
762,763
253
30⬚ S
756 752−755 254 255
703 704
SW IR
732−735
SEI
736 737
R
742 751 748 750 749 745,746 744 738
60⬚
743 739 741 742 740
30⬚E
60⬚
90⬚
120⬚
Figure 3.1 The Deep Sea Drilling Project (DSDP) and Ocean Drilling Programme (ODP) sites in the Indian Ocean are shown. Note that no sites have been drilled in the Indian Ocean Nodule Field. The ridges bordering the basin are: SWIR ¼ SouthWest Indian Ridge; SEIR ¼ Southeast Indian Ridge; CIR ¼ Central Indian Ridge and CR ¼ Carlsberg Ridge.
Emplacement of layered flows in quick succession has been recorded at the DSDP site 215 (8 7.360 S and 86 47.50 E, Fig. 3.1), east of the Indian Ocean Nodule Field (IONF), and is considered to represent fissure eruptions (Hekinian, 1974). A similar conclusion was made for the basalts in the vicinity of the Indrani Fracture Zone (FZ) (79 E) in the IONF (Karisiddaiah and Iyer, 1992). Geochemically, the oceanic basalts can be divided into three types (Sun et al., 1979): (1) Plume or Enriched (P- or E-type, erupted through deep-seated hotspots), (2) Normal or Depleted (N- or D-type, erupted at the ridge axes) and (3) Transition (T-type). Based on the observation that 17 ridge-centred hotspots with an average length of 1000 km occur along the 65,000-km long MOR, it was estimated that about three quarters of the MOR seems to host N-MORB (Vogt, 1979). The volcanic rocks dredged at depths <1500 m and around seamounts are mostly alkali basalts, similar to those forming oceanic islands, whereas those dredged at greater depths and along spreading ridges are usually N-MORB with a very low K2O content (Engel et al., 1965). The MORB covers 60% of the earth’s surface area and is over 1000 times more abundant than the alkali basalts. The enrichment in incompatible elements [e.g., U, Th, K, Rb, Cs, Ba, Nb, light rare earth elements (LREE), P and Ta] and
69
Volcanics
Sr and Pb isotopes and high La/Sm ratio [(La/Sm)N > 1] in P/E-MORB compared to N-MORB reflects different sources and variable environment of its formation.
1. Major Volcanics Lisitzin (1996) estimated that about 3 billion tons of volcanic components are contributed annually to the oceanic sediments. In the Indian Ocean, this component has been significantly contributed over the last 150 Ma by eruptions at the MOR, hotspots (Reunion and Kerguelen), plate subduction along the Indonesian Volcanic Arc (IVA) and from terrestrial sources (Sykes and Kidd, 1994). Throughout their evolution (110 Ma), the Reunion and the Kerguelen hotspots have been major contributors of tephra to the Indian Ocean, while during the last 5 Ma the IVA has been opined as a source of volcanics (Fagel et al., 1994). In the Indian Ocean, the composition of the volcanics changed with time from basaltic during Late Mesozoic to silicic during Early Cenozoic (Vallier and Kidd, 1977). A large majority of basalts in the IONF occur close to major seamounts (Table 3.1) and have bearing one way or the other on these volcanic bodies. The type, distribution and origin of the major volcanics are presented later.
1.1. Tholeiitic basalts In the IONF, pillow and massive outcrops of basalts are common near topographic highs, while basaltic fragments occur in the abyssal plain. Because of differential cooling of the lava, basalt exhibits three broad textural zones from the glassy top to the interior. The glassy texture in the outer zone, A, is almost holohyaline with sheaf-like radial clusters of plagioclase and scattered phenocrysts or microphenocrysts of olivine and plagioclase in the interstices. The minute incipient feathery crystal-rich transitory intermediate zone, B, grades to the innermost holocrystalline zone, C, that displays intergranular, intersertal and flow textures. The ‘fresh’ basalts have isotropic, yellow-brown glass and well-developed plagioclase, olivine, rarely pyroxene and opaque minerals. Size-wise, the minerals
Table 3.1 Geological characteristics of some major seamounts in the IONF
Seamount
Summit height (m)
Water depth (m)
Basal area (km2)
Summit area (km2)
Volume (km3)
Flatness
Slope angle (degrees)
201 202 204
1201 1076 1125
5375 5400 5400
236 163 58
1.0 0.2 0.3
101 61 23
0.07 0.04 0.07
8.4 8.8 15.8
Source: Mukhopadhyay et al. (1995). Note: IONF ¼ Indian Ocean Nodule Field.
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Mukhopadhyay, Ghosh and Iyer
may be classified into laths or groundmass (<0.25 mm), microphenocrysts or tabular (0.25–0.50 mm), phenocrysts (0.50–1 mm) and megacrysts (>1 mm) (Mislankar and Iyer, 2001). Plagioclase predominantly occurs as laths and in the groundmass, especially in zone A, followed by microphenocrysts, phenocrysts and megacrysts. Olivine, on the other hand, tends to occur as microphenocrysts and rarely as phenocrysts. Augite, when present, occurs as minute crystals (<0.25 mm). In the IONF tholeiites, plagioclase predominates as long, acicular needles, phenocrysts, microphenocrysts and microlites in the groundmass. Sometimes two types of plagioclase occurring in a single sample show different morphology, record variation in the anorthite content (An34–52) and display cross-cutting disposition. In zone A, plagioclase occurs as euhedral or subhedral phenocrysts and as elongated quenched forms in the intermediate and inner zones (B and C). Zoned plagioclase in the IONF basalt is uncommon. Plagioclase at times shows reaction rims and may host groundmass as inclusions; in a few cases, the groundmass extends as embayment into the host plagioclase, producing fine reaction rims. These indicate that the plagioclase phenocrysts have crystallised contemporaneously either with or later than the basaltic groundmass. Normally, olivine in the IONF basalts occurs in zone B as euhedral or subhedral microphenocrysts, which are lattice-like skeletal overgrowths, and as patterns, which are lantern-like and swallowtail forms. Olivine phenocrysts (up to 0.50 mm) are very few, fractured, broken and unzoned. Rarely, fresh olivine forms the sole phase in zone A (Fig. 3.2A), a feature described as ‘topo-concentrations’ (Augustithis, 1978), that is, suggestive of a cumulate origin. Among the other minerals, augite is scarce, less than 0.25 mm in size and of anhedral shape with high extinction angles (38 –45 ). Opaque minerals are mainly stubby magnetite, reddish-brown hematite and rarely spinel/chrome spinel. Sometimes, a sprinkling of opaque minerals occurs as thin, fine needles forming dendrites and trichites. Vesicles are few and are usually empty or partly filled with cryptocrystalline materials. Mineralogically, the IONF basalts are grouped as (1) aphyric, (2) plagioclase dominant, (3) olivine bearing and rarely (4) augite-containing rocks. The rocks represent highly to moderately phyric plagioclase basalts. Besides N-MORB, a few uncommon rocks were obtained from the IONF. These include a sample, collected at the base of a seamount (Sector C, 12 390 S and 76 300 E), with typical granoblastic texture formed by quartz, feldspar and micaceous minerals (Fig. 3.2B). This rock probably represents a thermal metamorphic product and resulted from the interaction of the lava of an erupting seamount with the siliceous sediments (Iyer and Sharma, 1990). One coarse-grained basalt has plagioclase–pyroxene–hornblende showing partial to complete replacement of pyroxene by hornblende and expulsion of the excess iron that formed large opaque minerals (Fig. 3.2C). This sample may have been dredged from a well-jointed dyke outcrop and is analogous to those reported from the pit crater of a seamount in the East Pacific Rise (EPR) (Allan et al., 1987). The mechanisms that control the formation of basalts, among others, are the cooling rate, fluid flow, composition of the liquid, nucleation site, growth rate and density of crystals [Basaltic Volcanism Study Project (BVSP), 1981]. In the IONF, while the flow texture is attributed to the more basic and highly fluid nature of the
71
Volcanics
A
C
B
D
Figure 3.2 (A) Fresh, euhedral olivine present in the glassy veneer ( plane polarised), (B) granoblastic texture in a quartzo-feldspathic rock recovered from the base of a seamount (crossed nicols), (C) coarse grained texture formed by hornblende and pyroxene. Note the expulsion of iron ores near the pyroxene.The rock could probably be a dyke (crossed nicols), (D) unaltered, twinned and zoned plagioclases in a glassy matrix (crossed nicols). Scale bars ¼ 0.05.
basaltic magma, the ophitic texture favours a more static, near-equilibrium condition of the co-existing phases of basaltic lava resulting in cotectic crystallisation. This is more frequent in zone C, where due to lower degrees of undercooling, crystallinity is high, indicating a near-equilibrium of the co-existing phases. The variety of plagioclase in the IONF basalts may represent different stages in the cooling history, characterised by the degree of supercooling (DT ) and the number of nuclei formed (Lofgren, 1980). For instance, low-nucleation sites and high-growth rate would produce large crystals (phenocrysts/megacrysts), while high- nucleation sites and a constant growth rate produce microphenocrysts (BVSP, 1981). The range of the size of plagioclase in the IONF basalts could be related to the nature and the number of nuclei per unit volume present in the parent melt. Further, the abundance of smaller crystals (up to 0.50 mm) indicates their formation at a near-uniform growth rate around a number of nuclei. The differential changes in the cooling temperature could affect the morphotypes of the plagioclase. For instance, it has been shown that for equivalent growth conditions, a plagioclase spherulite will grow at a larger DT or greater departure from equilibrium than a dendritic or skeletal plagioclase, while a smaller DT favours the formation of tabular plagioclase (Lofgren, 1974). The observed crystal sizes in the
72
Mukhopadhyay, Ghosh and Iyer
IONF samples therefore could be related to the changes in DT that resulted in the crystallisation sequence: spherulites > dendrites/skeletal > tabular. The extrusion rate of lava into the seawater may also determine the morphology of the plagioclase and olivine. For instance, sheaf-like radial clusters could have been caused because of rapid spherulitic growth during quenching of the lava (Natland, 1991) and/or due to disequilibria because of resorption with the melt (Best and Bothner, 1971, Fig. 3.2D). The predominance of plagioclase (olivine) and the plagioclase phyric nature of the IONF basalts imply a process of fractional crystallisation of a plagioclaserich magma, an observation that concurs with the other reported MORB (BVSP, 1981). The paucity of zoned crystals and variation in anorthite content (An34–52) in plagioclase in the IONF basalts may suggest a continued slow growth of plagioclase within the melt during its sluggish ascent to the seafloor (BVSP, 1981; Iyer and Karisiddaiah, 1990). Fresh basaltic glass from a few IONF seamounts shows depletion in incompatible elements and a moderate-to-high Mg # (61.7–63.5). FeO and TiO2 are inversely related to MgO, while K2O and P2O5 are generally low. These compositions are typical of N-MORB and comparable to those from the MAR and the EPR seamounts and from other sites in the Indian Ocean. As in the EPR seamounts, the melts that formed the IONF basalts were derived from a heterogeneous source. Since basalts from the ancient IONF seamounts are nearly indistinguishable from the younger, near-axis originated seamounts of the MAR and the EPR (Mukhopadhyay et al., 1995), it can be reasonably presumed that other seamounts in the IONF also have a similar origin since most of them occur in ridge-normal chains (i.e. normal to the present SEIR). Hence, it is conceivable that most of the seamounts may have tapped a common magma chamber.
1.2. Ferrobasalts Ferrobasalts or FeTi basalts (i.e. enriched in Fe and Ti) are significant because of their paragenetic relation with the intensity of magnetic values of the seafloor, which formed the basis for the concept of magnetic telechemistry (Vogt and Johnson, 1973). Ferrobasalts occur either at propagating rifts, high-amplitude magnetic zones (HAM), fast-spreading ridges or in a combination of all these. Ferrobasalts from the IONF, recovered near topographic highs and HAM zones, possibly formed on the then moderate-to-fast-spreading crust (Iyer et al., 1999a). In the IONF, ferrobasalts are found in two clusters in sector C: cluster I (in the north) and cluster II (in the south) (Fig. 3.3, Table 3.2). The two clusters also show differences in terms of geological settings and petrographic features of the ferrobasalts (Table 3.3). The former ferrobasalts overlie the prominent magnetic anomaly A25 and the crust that was generated at a fast rate (190 mm/yr) during the Late Paleocene– Early Eocene time (56 Ma). Also, multi-beam mapping in this area reveals a prominent east-west-trending zone of elongated topographic elevations, narrow faults and fissures. Rocks were dredged from the lower slope and foothill regions of a large seamount (11 -11 120 S and 77 490 -78 E, summit height 1100 m), which has a wide semicircular base and a gentle slope. Another seamount of moderate dimension (height 500 m) is located about 80 km south-southeast from this cluster.
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Volcanics
74⬚
76⬚
78⬚
80⬚
82⬚
84⬚
Z Indrani F
11⬚
TJT-In
Vishnu
FZ
72⬚E 9⬚S
13⬚
15⬚
Figure 3.3 Distribution of different types of volcanics in the Indian Ocean Nodule Field (IONF). Solid circles ¼ seamounts; crosses ¼ normal-mid-ocean ridge basalt (N-MORB), altered basalts and ferrobasalts; solid squares ¼ zeolitites; thick solid rectangle ¼ spilites. Dashed outer boundary approximately limits the spread of pumice. Note overlaps of various volcanics at many sites.
Table 3.2
Ferrobasalts in the IONF: Tectonic regime Cluster I
Ridge-dependent characters 1. Magnetic anomaly (A) 25 2. Crustal age (Ma) 56.5 3. Spreading rate (mm/yr) 110 4. Magnetic intensity (nT) 400 to þ500 5. Stress regime Compression 6. Collision phase Feeble stage of phase I Flexuring of surrounding seafloor (m) 7. RMS flexuring 194 8. Flexuring amplitude 67 9. Wavelength of folds 5754
Cluster II
24 53–52 190 200 to þ400 Tension Inter-collision phase 90 78 6454
Source: Iyer et al. (1999a). Note: IONF ¼ Indian Ocean Nodule Field. RMS (root mean square) flexuring or roughness of the seafloor was calculated as roughness ¼ 1296 spreading rate0.539 (Malinverno, 1991); collision phase represents collision between India and Eurasia.
The ferrobasalts occurring in cluster II encompass anomalies A24a, A24b and A25. Three samples in this area were recovered from the underlying crust bounded by magnetic anomalies A24a and A24b (53– 52 Ma), with two occurring on the trace of the triple junction on the Indian Plate (TJT-In; Dyment, 1993), that is, at
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Table 3.3
Ferrobasalts in the IONF: Geological setting and petrography
Sample no. Location ( S/ E)
Water depth (m)
Cluster I 16
11 000 , 77 550
5250
28
11 000 , 78 000
5300
30
11 120 , 77 490
5400
40
11 050 , 77 500
5300
Cluster II 02
12 330 , 77 120
5210
Seamount cluster (100–800 m height)
06
13 200 , 77 300
5040–5500
Abyssal plain
17
13 000 , 76 300
5400
Abyssal plain þ seamounts (200–1200 m)
Geological setting
Petrographic features
Plagioclase laths dominant, less olivine and opaque minerals; sheaf-like texture, altered Slightly altered Near anomaly plagioclase, A25 at foothill palagonite; of a seamount sheaf-like texture Abyssal plain, near Glass unaltered, plagioclase, A25 at a pyroxene; seamount base intersertal texture Near A25, abyssal Plagioclase, palagonite, plain, away FeMn coating; from sheaf-like seamounts texture, slightly altered Foothill and slope of a large seamount
Plagioclase dominant, olivine rare, opaque minerals, texture hyalopilitic and trachytic Plagioclase, palagonite, opaque minerals, FeMn; medium grained Plagioclase, altered olivine, hematite; sheaf-like texture
x
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Volcanics
Table 3.3 (continued) Sample no. Location ( S/ E)
Water depth (m)
Geological setting
26
12 330 , 77 120
5200
Seamount cluster (100–800 m)
37
12 590 , 76 300
4900–5050
Seamounts, EW lineaments, near TJT-In
50
12 000 , 76 300
5400
Abyssal plain, 17–35 km from nearest seamounts
Petrographic features
Plagioclase, few olivine, FeMn, trachytic texture Plagioclase dominant, opaque minerals, palagonite; fine grained altered Plagioclase dominant, olivine, opaque minerals; intergranular texture, altered
Source: Iyer et al. (1999a). Note: A25 ¼ Magnetic anomaly 25; TJT-In ¼ Trace of the Triple Junction on the Indian Plate. Fe-Mn oxides ¼ Ferromanganese oxides; IONF ¼ Indian Ocean Nodule Field.
the boundary between the crust generated by the CIR and the SEIR, and one sample from a prominent east-west trending lineament. The other ferrobasalts were from the crust generated between anomalies A24 and A25 (i.e., between 53 and 56 Ma), one of which is from the northern slope of an irregular seamount (height 525 m, basal area 22 km2, volume 4.7 km3, slope angle 13.6 ). Magnetic anomaly data suggest that ferrobasalts in clusters I and II were probably emplaced at 56 Ma and between 52 and 56 Ma, respectively. The two clusters also differ in other ways. For instance, the magnetic anomaly data, when compared with synthetic regional magnetic values, indicate large-scale variations in the intensity of magnetisation of the oceanic crust. The synthetic value of magnetic intensity in A25 (cluster I) is between –400 and þ500 nT while in anomalies A24a and A24b (cluster II) it is –200 to þ100 nT and –150 to þ400 nT (Mukhopadhyay et al., 1997). Mineralogically, plagioclase occurs predominantly as microlites and laths in the ferrobasalts while a few phenocrysts and microphenocrysts of plagioclase are set in a holo- to hypo-hyaline groundmass. Olivine, being rare, is anhedral and sometimes altered to iddingsite. Small euhedral magnetite and reddish patchy hematite mainly represent opaque minerals. The rocks commonly show trachytic texture and, in some cases, the plagioclase laths form sheaf-like aggregates with intervening glass. In few cases, the rocks have a thin top veneer of unaltered glass and/or Fe-Mn oxides.
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In terms of their petrography (Table 3.3) and chemistry (Table 3.4), the ferrobasalts are noticeably dissimilar to N-MORB but comparable with other oceanic ferrobasalts. The generally low content of MgO (3.24 and 6.14%) reflects the scarcity of olivine while the high MnO (1.99) in sample # 40 is due to the presence of Fe-Mn oxides. Ferrobasalts of cluster II are slightly enriched in SiO2 (45.87–51.67%) and depleted in TiO2 (1.80–2.12%) and FeO* (i.e. total iron, 11.92–17.09%). The lower MgO (1.76–5.71%), as compared to that in cluster I samples, suggests a greater paucity of olivine. In the FeO*/MgO versus TiO2 plot (Fig. 3.4A), two groups are distinct, with most of the IONF and GSC (Galapagos Spreading Centre) samples in one group and a few IONF samples and the ferrobasalts from global oceanic domains in the other group. Note that the K2O contents in the ferrobasalts at the DSDP site 216, Spiess and Chain ridges and Iceland have values similar to those of IONF samples (Table 3.5). It has also been suggested that the high K2O content may not be due only to alteration but may as well be a characteristic of the ferrobasalts (le Roex et al., 1982; Thompson et al., 1978). Considering the trace elements (Table 3.4), the IONF samples have identical Ba but enhanced Y and Zr contents compared to those of the DSDP sites but values lower than those of the Spiess and Chain samples. Vanadium (V) content in these samples is comparable to the DSDP ones while in the Spiess and Chain ridges’ samples it is less by an order of magnitude. A positive correlation between TiO2 and V suggests incorporation of V in TiO2 phases. The incompatible elements (Ba and Y) show systematic increase with differentiation when plotted against Zr, with Ba (Fig. 3.4B) being slightly scattered than Y (Fig. 3.4C) because of the apparent immobility of Y. The low Cr content indicates the role of crystallisation of clinopyroxene and/or titano-magnetite during the evolution of the lava while decrease in Ni could be either due to these phases and/or olivine. The low Ba and high TiO2 (53– 68 ppm and 2–3%, respectively) and enhanced Fe, K, Ti and P probably indicate the alkaline nature of the IONF ferrobasalts akin to the Iceland samples (Thy, 1989). The IONF ferrobasalts are therefore broadly similar to their global counterparts, as further attested by the paucity of olivine, profuse plagioclase and opaque minerals and enrichment in FeO*, TiO2 and trace elements. Several magmatic processes can be conceived to decipher the formation of these rocks (1) crystal fractionation at the shallow level of a MORB melt, depleted in the LREE (Christie and Sinton, 1981; Vogt and Byerly, 1976); (2) a parent magma inherently enriched in Fe and Ti (Klein et al., 1991); (3) a mixed magma (Wilson et al., 1988); (4) a large molten magma chamber, for example, the moderately fractionated ferrobasalts along the fastest-spreading segment of the EPR (75 mm/ year between 13 S and 23 S; Sinton and Detrick, 1992); (5) variable degrees of shallow crystallisation; and (6) differences in the depths of magma generation (Scheidegger, 1973). In most of the processes mentioned above, it is presumed that the commonly evolved magmatic compositions remain essentially constant over millions of years in a near-steady state, maintained by repeated increments of mixing and fractionation. However, the basic questions that arise are, how and under what conditions can magma be retained at shallow depths for long duration while undergoing continuous differentiation? This could be explained by a mechanism of ‘neutral buoyancy
Table 3.4
Major and trace element contents of ferrobasalts in the IONF Cluster I
Cluster II
Sample no.
16
28
30
40
2
6
17
26
37
50
SiO2 TiO2 Al2O3 FeO* MgO CaO Na2O K2O MnO P2 O 5 Total FeO*/TiO2 FeO*/MgO Ba Co Cr Cu Ni Y V Zn Zr
45.49 2.99 15.01 16.22 4.17 10.20 2.67 0.47 0.39 0.58 98.19 5.42 3.89 54.9 77.4 96.8 143.5 122.1 64.4 471.3 111.6 201.0
48.47 2.54 15.77 15.58 3.44 9.13 2.98 0.84 0.25 0.30 99.30 6.13 4.52 – – – – – – – – –
47.68 2.54 14.98 15.11 6.14 9.25 1.94 0.97 0.24 0.18 99.03 5.95 2.46 – – – – – – – – –
43.37 2.76 14.37 19.13 3.50 7.84 3.97 0.96 1.97 nd 97.87 6.93 5.46 – – – – – – – – –
47.08 2.12 17.33 17.09 1.76 8.27 3.60 0.51 0.15 0.19 98.10 8.06 9.71 – – – – – – – – –
45.87 1.80 16.89 13.95 5.71 10.14 3.03 0.78 0.27 nd 98.44 7.75 2.44 53.1 65.7 138.3 69.4 201.9 59.6 342.1 270.7 192.2
49.43 2.00 17.84 13.40 3.02 8.84 3.19 1.07 0.24 0.23 99.26 6.70 4.44 – – – – – – – – –
49.64 1.93 16.34 12.92 3.41 8.50 3.42 1.64 0.31 0.45 95.56 6.69 3.79 68.4 67.5 86.7 140.4 63.5 56.9 352.1 204.5 190.9
51.67 2.10 16.49 11.92 2.84 9.05 3.51 0.79 0.27 0.30 98.67 5.68 4.20 61.8 81.1 151.8 104.0 78.1 59.6 317.4 168.6 181.3
48.83 2.01 17.09 13.45 3.55 9.50 3.50 0.88 0.20 0.24 99.25 6.69 3.79 – – – – – – – – –
Sources: Iyer et al. (1999a) and Mukhopadhyay et al. (2002). Note: FeO* represents total iron. nd ¼ not determined; IONF ¼ Indian Ocean Nodule Field. Major elements in wt%, trace elements in ppm.
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Mukhopadhyay, Ghosh and Iyer
A
4
TiO2
3 x 2 1
1
3 4 5 FeO*/MgO
170 160
C 70
140
60
120
50
100
Y
Ba
B
2
30
60
20
140 160 180 Zr
200 220
7
40
80
40 100 120
6
10 100
120 140 160 180 Zr
200 220
Figure 3.4 (A) TiO2(%)vs. FeO*/MgO of the Indian Ocean Nodule Field (IONF) ferrobasalts together with those from other global occurrences for comparison. Solid circles and solid triangles ¼ IONF ferrobasalts from clusters I and II, respectively, squares ¼ Galapagos Spreading Centre (GSC); circles with dot ¼ Deep Sea Drilling Project (DSDP); circles with cross ¼ East Pacific Rise (EPR); open triangle ¼ Spiess and Chain ridges; inverted triangle ¼ Iceland. FeO* ¼ total iron; (B) Zr vs. Ba and Zr vs. Y(all in ppm) to show the extent of differentiation in the IONF ferrobasalts. Circles ¼ Spiess and Chain ridges; othersymbols are same as in 3.4A.
zonation’ structure of magma reservoirs, together with the role of fractional crystallisation. The horizon of neutral buoyancy (HNB), defined as that ‘depth interval within which the magma density and the aggregate country rock density are equal’ occurs sublithospherically and has a narrow vertical and a wide lateral extent. Beneath the HNB region, magma ascends because of positive buoyancy and is stabilised at a shallow depth (2–4 km), while above this region the magma descends by negative buoyancy. The HNB of tholeiitic melts thus provides congenial conditions for the long-term stability (over millions of years) of magma reservoirs beneath the crust (Ryan, 1994). The IONF ferrobasalts may have originated when some amount of ascending N-MORB magma was trapped at a shallow depth in the HNB, continuously fractionated and was enriched in Fe and Ti. The recurrent intrusion of magma at the same shallow HNB could possibly have maintained the necessary temperature
Table 3.5
Major and trace element concentration in basalts from the world oceans GSC A
Major elements (wt%) SiO2 50.12 TiO2 3.39 Al2O3 12.09 FeO* 17.01 MgO 4.35 CaO 9.33 Na2O 2.62 K2O 0.17 MnO nd P2O5 0.30 Total 99.38 FeO*/MgO 3.91 FeO*/TiO2 5.02 Trace elements (ppm) Ba Co Cr Cu Ni V Y Zn Zr
– – – – – – – – –
GSC B
49.94 2.09 12.95 13.36 5.84 10.59 2.59 0.15 0.20 0.20 94.82 2.29 6.39 – – – – – – – – –
DSDP 214
48.10 2.35 14.90 14.60 6.45 9.04 2.75 0.37 nd 0.19 98.75 2.26 6.21 45 65 38 na 50 525 26 – 120
DSDP 216
49.50 2.75 13.50 13.80 6.57 8.79 2.57 0.90 nd 0.22 98.60 2.10 5.02 140 53 45 na 44 445 31 – 159
DSDP 254
47.66 2.12 15.37 12.90 8.92 8.53 2.69 0.32 nd 0.24 98.75 1.45 6.08 56 47 469 na 194 342 50 – 156
DSDP 256
50.36 2.36 13.34 13.23 6.58 10.20 2.76 0.25 0.21 0.25 99.54 2.01 5.60 39 36 108 155 93 451 45 – 159
EPR 8oN
Iceland
Spiess Ridge
49.69 2.45 13.19 12.57 6.72 10.22 2.65 0.07 0.20 0.26 98.02 1.87 5.13
46.61 3.99 13.93 14.65 5.55 9.88 3.00 0.74 0.23 0.57 99.15 2.17 3.67
50.74 2.46 14.44 11.53 5.32 9.80 3.71 0.74 0.20 0.39 99.33 1.94 4.67
– – – – – – – – –
– – – – – – – – –
115 38.3 46.17 66.5 32.08 233.5 40.3 114.3 218.7
Chain Ridge
51.89 2.52 14.66 10.58 5.46 9.81 3.12 0.80 0.18 na 99.02 2.64 4.20 162 45 70 60 40 250.5 37 141 221.5
Sources: Anderson et al. (1980), Byerly et al. (1976) and Thompson et al. (1978). Note: Location and other details of the GSC (Galapagos Spreading Centre) and DSDP (Deep Sea Drilling Project) sites: GSC A/B- 00 710 N/85 500 W, depth 2523m, at ridgefault intersection, aphanitic plagioclase, pyroxene; DSDP 214—11 200 N/88 430 E (Ninetyeast Ridge), plagioclase, pyroxene, magnetite, trachytic, olivine absent, vesicular amygdaloidal; DSDP 216—01 280 N/90 120 E (Ninetyeast Ridge), same as DSDP 214; DSDP 254—30 580 S/87 540 E (South tip of the Ninetyeast Ridge), pyroxene, plagioclase, rare olivine, amygdaloidal; DSDP 256—23 270 S/100 460 E (South Wharton Basin), plagioclase, pyroxene, magnetite, strain shadow. EPR ¼ East Pacific Rise. nd ¼ not determined, na ¼ not available.
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Mukhopadhyay, Ghosh and Iyer
and in turn promoted the long residence time. Again, most of the ferrobasalts are associated with topographic highs (and possibly a thick oceanic crust) and consequently individual magma batches would have had an opportunity to experience a greater degree of differentiation during their ascent. The eruption of the ferrobasalts during chrons A25 and A24 could have been facilitated by intense intra-plate tectonic activities caused by the frequent shifting and propagation of the spreading ridges at the intermediate rate of spreading, particularly during the Indo-Eurasia collision event (56–51 Ma).
1.3. Spilites Spilites are not uncommon on the ocean floor but are less abundant worldwide when compared with tholeiitic basalts. In the IONF, spilites occur near the Indrani FZ (Fig. 3.3), which has a throw of 300 m towards the east (Kamesh Raju et al., 1993) and was probably reactivated in the past (Mukhopadhyay and Khadge, 1992). Fresh as well as highly altered spilitic rocks, sometimes exceeding 25 cm in length and with sparse glass coverage, were recovered from this area. The rocks are fine to medium grained with albitic plagioclase (few as phenocrysts), clinopyroxene and olivine, while chlorite, epidote, hematite, other opaque minerals and smectite are minor components. Typical microlitic intersertal texture of spilites, defined by chlorite, and also porphyritic, intersertal and glomeroporphyritic textures are observed. Some spilites are highly altered, as revealed by very low CaO and high K2O, while the others have lower CaO than N-MORB but are analogous to the Sao Paulo basalts, Atlantic Ocean. On binary plots, the sub-alkaline spilitic affinity is noted (Fig. 3.5). The relatively higher than normal K2O content in the IONF spilites (Table 3.6) could be due to seawater alteration subsequent to spilitisation. Given the fact that spilites were mostly found in the neighbourhood of the reactivated Indrani FZ and some major seamounts, a fault-controlled spilitisation of pillows under the influence of hydrothermal solutions cannot be ruled out.
A
B
ALKALINE
8
Spilitic Field
Mildly alkaline
4
Sp ec t
ru m
Sub-alkaline
us
5
2
eo
THOLEIITIC
Ig n
Na2O + K2O
10 6
0 40
0 50 SiO2
60
0
10 K2O K2O + Na2O
20 ⫻100
Figure 3.5 (A) SiO2 vs. total alkalies (after Schwarzer and Rogers, 1974) to depict the subalkaline affinity of the Indian Ocean Nodule Field (IONF) spilites; (B) spilites of the IONF plotted on the Hughes (1972) diagram.
Table 3.6
Composition (wt%) of spilites in the IONF
Sample no.
1
2
3
4
5
6
7
8
9
10
11
SiO2 Al2O3 TiO2 MnO FeO Fe2O3 MgO CaO Na2O K2O P2 O 5 LOI Total
51.05 14.54 2.55 1.40 nd 14.98 2.10 0.29 4.19 2.39 0.31 5.68 99.48
50.40 14.48 1.95 0.36 1.94 15.46 2.59 5.00 4.31 0.82 0.33 1.84 99.48
46.37 14.75 1.90 0.31 nd 18.10 3.79 7.07 5.08 1.00 0.33 1.22 99.92
46.99 18.67 2.05 0.13 3.08 13.68 2.97 5.89 3.58 0.74 0.32 1.48 99.58
46.38 16.30 1.85 0.20 3.22 12.31 3.46 7.07 6.26 0.84 0.33 1.08 99.30
45.41 16.41 2.05 0.27 nd 17.37 4.48 6.48 4.03 0.25 0.35 2.65 99.75
44.41 16.96 2.05 0.27 nd 17.52 4.59 6.77 4.03 0.49 0.35 2.65 100.09
53.70 14.35 1.65 3.50 nd 12.81 2.54 0.59 3.71 1.94 0.44 4.60 99.83
41.98 13.37 2.00 0.64 nd 21.50 1.56 1.77 6.48 2.34 0.29 8.08 100.01
47.96 17.49 1.85 0.28 3.72 12.18 3.92 5.63 3.21 1.76 0.30 1.50 99.80
50.67 13.69 2.18 0.19 13.05 nd 4.81 5.38 4.61 0.80 0.30 nd 95.68
Sources: Karisiddaiah and Iyer (1992); Fodor et al. (1980). Note: nd ¼ not detected; LOI ¼ Loss on ignition; IONF ¼ Indian Ocean Nodule Field. Samples 1–10 from IONF; Sample 11 ¼ Sao Paolo, MAR.
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Mukhopadhyay, Ghosh and Iyer
1.4. Pumice Pumice is widely distributed in the oceanic sediments. In the Indian Ocean, drift pumice has been recorded to occur near Sri Lanka, Madagascar and Reunion. In the IONF, however, a large field of pumice covering an area of about 600,000 km2 between 9 S to 20 S and 72 E to 84 E has been identified (Fig. 3.3). Of the 4083 pumice pieces examined from 1925 locations in the IONF, 94% are between 0.1 and 4 cm with only about 1% being larger than 8 cm in size (largest 36-cm long). The pumice is buff, grey, black and brown in colour and oval, lineated, subrounded, rounded and irregular in shape (Fig. 3.6). Many of the clasts form substrate and are coated either fully or partially by ferromanganese oxides. Pumice shows silicic glassy webs with a few phenocrysts of plagioclase and/or pyroxene, and vesicles that are mainly lineated, with a few rounded. In some instances, quartz, radiolarians, phytoliths, diatoms and ferromanganese micronodules occur within the glassy interstices. The presence of such materials could make the pumice heavy and cause it to sink to the ocean floor. Compositionally, the IONF pumice is of two kinds: (1) trachyandesite (type 1, SiO2 60%) and (2) rhyodacite (type 2, SiO2 70%). The former shows low K2O and nearly constant Na2O as compared to the latter (Table 3.7, Fig. 3.7). The presence of pumice in the IONF may be due to: (1) transportation from the 1883 eruption of Krakatoa volcano (Martin-Barajas and Lallier-Verges, 1993; Mudholkar and Fujii, 1995); (2) in situ eruption on the seafloor (Iyer and Sudhakar, 1993a; Svalnov, 1981); or (3) a combination of (1) and (2) (Mukhopadhyay et al., 2002). Multi-beam maps display a few of the IONF seamounts with summit craters and their occurrence proximal to the spatially distributed pumice may suggest a possible relationship between explosive silicic eruption and the formation of pumice. Experimental studies have shown that the hot pumice sinks faster and nearer to the source as compared to colder pumice (Whitham and Sparks, 1986). The hot pumice absorbs water, becomes heavier and sinks. In the light of these observations, it can be argued that much of the larger pumice clasts in the IONF might have been derived as hot clasts that rapidly sank near their eruptive sources. Although pumice has a tendency to drift, the frequent occurrence of pumice in the IONF and the absence of any favourable surface current circulation pattern that can transport pumice from the IVA or Toba volcanoes seem to favour an in situ origin of the IONF pumice. Evidences that pumice-forming eruptions can occur at great water depth (1500 m or more) come from all oceans, for example, pumice at the plate boundaries (near Tonga Trench, Okinawa Trough, South of Japan) and at intra-plate areas (over Atlantic seamounts in the Atlantic Ocean and the Ninetyeast Ridge), and trachytic pumice flow from intra-plate volcanoes in the Society and Austral hot spot regions. Collectively, all these occurrences suggest that widely distributed pumice, in fact, is a product of recent submarine eruptions. In the light of these findings, an in situ origin of the IONF pumice at abyssal depth could be a possibility. It is also pertinent to note that pumice forms substrate and nucleus for many manganese crusts. Since the oxides accrete at a rate of 1 mm/Ma, it would take about 10 Ma to form a 1-cm thick layer around a nucleus of pumice. This deduction indirectly testifies the coated pumice to be older than the volcanic eruptions of Krakatoa in 1883 and that of Toba 74 ka years ago (Mukherjee and Iyer, 1999).
83
Volcanics
A
0 1 2 3 4 5 cm
B
0
1
2
3
4
5
cm C
0
1
2
3
4
5
cm
Figure 3.6 Hand specimen of pumice from the Indian Ocean Nodule Field (IONF). (A) pumice of different shapes and sizes; (B) pumice coated either partially or fully by ferromanganese oxide; (C) an example of pumice specimens from one sampled site showing recovery of coated and non-coated types.
It has been found that fractional crystallisation of abyssal tholeiites could lead to layered or stratified magma chambers with alkali- and volatile-enriched silicic melts (Hawkins, 1985). A similar situation can be visualised for the IONF pumice since the basinal area is dominated by tholeiitic basalts, which in the course of time may have fractionated to a more silicic melt. The presence of basaltic and silicic volcanics
Table 3.7
Composition (wt%) of the CIOB pumice
Sample
1
2
3
4
5
6
7
Avg.
8
9
10
11
Avg.
SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2 O 5 LOI
59.48 0.51 13.18 7.73 0.20 2.37 5.79 7.05 3.56 0.10 6.23
56.71 0.73 13.25 9.53 0.17 3.37 6.49 6.23 3.33 0.13 12.50
59.58 0.66 13.29 8.19 0.28 2.38 4.46 5.89 5.14 0.14 15.10
61.42 0.46 12.46 5.91 1.34 2.17 5.57 5.87 4.71 0.10 11.20
60.30 0.54 12.44 8.33 0.64 2.55 5.16 5.81 4.10 0.14 15.00
60.59 0.63 13.22 8.71 0.58 2.51 6.12 3.95 3.57 0.12 3.00
63.91 0.97 13.78 6.75 0.21 2.02 2.34 6.63 3.09 0.30 3.03
60.28 0.64 13.09 7.88 0.49 2.48 5.13 5.92 3.93 0.15 9.43
68.57 0.17 10.97 3.95 0.28 1.11 3.31 5.10 6.50 0.05 12.30
73.75 0.28 13.05 1.74 0.13 0.46 2.33 3.96 4.23 0.08 5.61
69.20 0.97 14.03 3.76 0.41 1.33 2.52 4.84 2.75 0.17 12.73
69.52 0.77 14.24 5.51 0.35 1.97 0.56 3.81 3.04 0.21 6.50
70.26 0.55 13.09 3.74 0.29 1.22 2.18 4.43 4.13 0.13 9.28
Source: Iyer (1995). Note: LOI ¼ Loss on Ignition; Avg. ¼ Average; FeO* ¼ Total iron as ferrous iron; CIOB ¼ Central Indian Ocean Basin. For groups see text. Samples 1–7 ¼ Trachyandesite; Samples 8–11 ¼ Rhyodacite.
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Volcanics
8
Na2O
4
4
0
0
0.8
8
K2O
TiO2
FeO
8
0.4
4
0
0 22
8
MgO
Al2O3
18
14
4
0
10 80
72
CaO
SiO2
8
64
56
4
0 4
8
12 SI
16
4
8
12
16
SI
Figure 3.7 Variation diagrams for the CIOB pumice. SI = solidification index (100MgO/ MgOþNa2OþK2OþFeO*); filled circles ¼ trachyandesite pumice; open circles ¼ rhyodacitic pumice; open triangles ¼ data from Frick and Kent (1984); solid triangles ¼ data from Martin-Barajas and Lallier-Verges (1993).
86
Mukhopadhyay, Ghosh and Iyer
may point to a process of simple fractionation of the basaltic magmas to form the pumice. Hence, it is plausible that the IONF pumice and glass shards may have jointly been contributed from the cataclysmic Krakatoa eruption of 1883, Toba eruption at 74 ka and from in situ volcanism (i.e., local mid-plate basinal eruptions).
2. Minor Volcanics 2.1. Hydrothermal materials Volcanic-hydrothermal materials (vhm), discovered in the IONF (Iyer et al., 1997a,b, 1999b), have important implications on the structural and volcanic histories of the field. Coarse fractions from surface and sediment column from various sedimentary regimes were investigated for vhm. Two occurrences of vhm have been reported, one from the base of a seamount, about 45 km from the TJT-In in the siliceous sediment domain; the other scooped up from a red clay sediment domain (Fig. 3.8). The seamount, located at a water depth of 4440 m and overlying magnetic anomaly A 23b (¼age 50.81– 50.64 Ma), is 800-m high, covers an area of 37.56 km2 (length ¼ 5.77 km, basal width ¼ 6.51 km) and has a summit width of 0.18 km. The coarse fraction contains 2200–2500 particles of vhm per gram. The vhm from the red clay sediment domain, however, contains only 25 spherules per gram. This location is about 53 km from a major seamount. Individual entities of native Al spherules and particles occur associated with the vhm in the vicinity of seamounts (Iyer et al., 2007a). Radiolarians and diatoms were identified from the various subsamples of the vhm-hosted sediments so as to determine a possible age. Two radiolarian zones, Buccinosphaera invaginata and Collosphaera tuberosa, are present with a distinct boundary between them at 30 cm down core. This defines the first appearance datum (FAD) level of B. invaginata (Gupta, 1988) and are paleomagnetically dated to be synchronous at 180 20 ka in nearby cores (RC 14–22 and VM 34–53, Caulet et al., 1993). Factor analysis of percentage data of 47 species in the IONF helped to recognise three conspicuous radiolarian assemblages, which were related to the overlying sea surface temperature (Gupta, 1996). The assemblages were quantified, and mutual ratios were plotted (Fig. 3.9) to identify datum within the latest Quaternary period. In the vhm from a siliceous sedimentary domain, the ratios of transitional to warm and transitional to cold fauna show three prominent peaks, which, on the Pleistocene climatic scale of Martinson et al. (1987), correspond to 130, 70 and 10 ka ages for the 28, 16 and 6 cm (dominant vhm) depth, respectively. Cacsinodiscus nodulifer diatoms are generally absent in most of the upper sections of the core but occur abundantly between 20 and 28 cm and at 36 cm core depths (Fig. 3.9). Burckle and McLaughlin (1977) found that this species (>120 mm) proliferate in warmer climates, suggesting the signature of the last interglacial age (130 ka) between 20 and 28 cm in this area. In contrast, the vhm from red clay domain show absence of the index fossil B. invaginata (FAD level at 180 ka; Gupta, 1988), but presence of a number of Stylatractus universus (extinction ¼ Last Appearance Datum level at 425 ka;
87
Volcanics
30⬚ N
5⬚ S
F/98/3
10⬚ N
F/99/3
10⬚ S
IONF
40⬚E
F/101/0
F/154/0
S/120/1 S/128/1
Indrani FZ
TJT-In
Z S/106/0 S/124/2
80⬚
100⬚
F/156/4
F/155/1 F/152/0 F/153/0
Vishnu F
10⬚
60⬚
S/139/0
S/129/0 F/81/2
S/120/2 S/126/2 F/56/1 15⬚
S/231/1
* S/89/2200 S/657
S/206/1 S/210/5
S/241/2
S/94 20⬚ 70⬚E
75⬚
80⬚
85⬚
Figure 3.8 Distribution of volcanogenic sediments in the Indian Ocean Nodule Field (IONF). Open stars ¼ seamounts; solid dots ¼ sediments examined for volcanogenic hydrothermal materials (vhm); solid triangle ¼ S657 and asterik ¼ S89 near a large seamount; circle with dot ¼ S94, the southernmost sample with vhm.The last numerals after station number (e.g. S/124 and S/ 210/5) represent the number of magnetic particles present in 1 g of the sediment coarse fraction.
Hayes and Shackleton, 1976) and Collosphaera Orthoconus (FAD level at 650 ka; Johnson et al., 1989). From these two dates, the age of the vhm in the IONF seems to range between 425 and 650 ka. Older sediment exposures at this site, as confirmed by the presence of Australasian microtektites of 770 ka age (Prasad, 1994), suggest intensive scouring by local bottom currents. This is also evident by the presence of 1–2 cm-sized spherical manganese nodules that testify to periods of no sedimentation or erosion (Iyer, 2005). Generally, three types of vhm occur in the IONF: (1) ochrous metalliferous sediment (2) magnetite spherule and (3) glass shard (Iyer, 2005).
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Mukhopadhyay, Ghosh and Iyer
A 1
Ratios 2
3
0 Sample depth in core (cm)
4
10 ka
8 12 70 ka
16 20 24
130 ka
28
180 ka
32 36
Transitional v/s warm fauna
FAD of B. invaginta Transitional v/s cold fauna
B 0
Glass shards in 1000/slide 2.5 5.0
0
5
Magnetics ( ⫻ 100) 10 15 20 25 30
35
40
0 4 8 12 16 20 24 28 32 36 0
50 100 No. of diatoms/slide
Figure 3.9 Down-core variation in sediment core S657 to show (A) the occurrence of radiolarian species and ratios of transitional to warm water fauna (solid triangle) and transitional to cold fauna (open circle); FAD = First Appearance Datum and (B) the occurrence of glass shards per slide (hatched portion) and diatoms per slide (dotted portion). The panel on the right shows the number of magnetic particles.
2.1.1. Ochrous metalliferous sediment The magnetic fractions in the vhm consist of highly porous to indurated material, interpreted as ‘ochrous precipitates’ of hydrothermal origin, that displays moss-like pyroclastic texture (Fig. 3.10A). The smallest precipitate is only a few microns in diameter and the largest, 3.35 mm 1.45 mm, is oval shaped, weighs 30 mg, and has a depressed narrow end and overturned rims that formed possibly because of melt flow. The precipitate also hosts a bunch of spherules and has glass shards
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Volcanics
A
C
B
D
Figure 3.10 Scanning electron microscope (SEM) images of the ochrous sediment particles and spherulesç(A) similar to moss-like morphology of Heiken and Wohletz (1985); scale bar = 10 mm: (B) condensed particle and spherules similar to the experimentally produced ones of Wohletz and McQueen (1984); scale bar = 100 mm; (C) nontronite showing striations recovered in core S94; scale bar = 100 mm; and (D) magnetite [spherules] and titanium [irregular] particles in core S94; scale bar = 10 mm.
protruding from it. The ochrous precipitates exhibit various properties and hues, from highly porous, light yellow and orange to more condensed variety (Fig. 3.10B). The porous ochrous precipitates do not have any distinctive mineralogy vis-a`-vis the condensed particles with magnetite and maghemite as the main phases. Compositionally, the metalliferous sediments are a mixture of SiO2 (16–30%) and FeO (54–73%) with variable contents of other oxides (Table 3.8). The inclusions of Ni and S in the metalliferous sediment are generally between 0.02 and 1.40% and 0.21 and 4.65%, respectively, with the highest Ni (up to 2.24%) and S contents (up to 9.82%) detected as inclusions in one place (Table 3.9). The four possible modes of formation of metalliferous sediments are (1) derivation from hydrothermal emanations (Zelenov, 1965), (2) reaction of seawater with hot lava (Corliss, 1971; Honnorez, 1981), (3) hydrothermal precipitation (Alt et al., 1987; Hekinian et al., 1993) and (4) contributions due to bacterial processes (Alt, 1988). Interestingly, the IONF samples resemble Fe-Si oxyhydroxides of the Pacific Ocean (Si/Fe ratio 0.14–0.33), and are also comparable to Fe-oxide mud of inactive deposits of the Red Seamount in the Pacific Ocean (Table 3.8). It therefore appears that
90 Table 3.8
Microprobe analyses (wt%) of metalliferous sediments in the IONF
Serial no.
Samples
SiO2
TiO2
Al2O3
FeO
1 2 3 4
89 89 89 89
16.31 16.18 29.99 25.87
0.33 0.46 0.14 0.17
3.02 3.23 6.62 4.06
71.01 69.32 53.87 58.79
1.35 1.19 0.37 0.13
0.65 0.63 0.05 0.19
0.11 0.30 0.30 0.03
0.28 0.13 – –
97.30 94.82 91.80 90.93
5
89
25.75
0.04
5.07
60.49
0.40 0.46 1.21 0.25 0.20
0.09
–
93.96
6
89
26.11
0.10
4.70
58.56
0.18 1.53 2.52 0.51 0.73
0.01
0.34 95.29
7
89
25.02
0.02
6.43
58.59
0.27 1.49 1.57 0.40 0.79
0.29
0.35 95.22
8 9 10 11
657 657 657 EPR
20.77 22.86 21.93 31.67
0.10 0.05 – –
0.34 2.25 0.23 –
71.06 59.26 73.50 41.20
0.08 – 0.29 0.18
0.19 1.38 – –
0.11 0.20 0.07 –
MnO MgO CaO Na2O K2O
1.66 1.63 0.28 0.46
1.13 6.15 1.12 2.45
0.92 1.01 0.14 0.98
1.59 6.29 1.18 2.37
1.66 0.74 0.04 0.25
0.05 0.28 0.16 1.23
0.26 0.23 0.20 0.52
Cr2O3 BaO Total
95.68 98.95 99.31 79.62
Remarks
T-1, Si/Fe ¼ 0.14 T-1, Si/Fe ¼ 0.14 T-3, Si/Fe ¼ 0.33 T-1/3, Si/Fe ¼ 0.26 T-1/3, Si/Fe ¼ 0.26 T-1/3, Si/Fe ¼ 0.27 T-1/3, Si/Fe ¼ 0.26 T-1, Si/Fe ¼ 0.18 T-1, Si/Fe ¼ 0.23 T-1, Si/Fe ¼ 0.18 Fe hydroxide from seamounts, T-3, Si/Fe ¼ 0.46
12
Society
17.24
–
6.84
56.22
0.03
–
–
–
–
0.02
–
80.35
13
EPR
41.82
–
1.06
26.63
0.65
–
–
–
–
–
–
70.16
14
Red 8.25 Seamount
0.04
0.50
56.06
0.01 0.68 0.86 0.67 0.26
0.002 0.005 67.34
15
Red 17.27 Seamount
0.05
0.85
56.45
0.18 0.90 1.04 0.86 0.39
0.002
16
Galapagos
47.06
–
0.18
32.72
0.31 2.44 0.74 1.54 1.78
–
17
Fukujin
53.91
0.41
0.71
26.65
1.04 1.88 0.84 2.02 1.50
–
18
Red 47.50 Seamount
0.04
0.56
32.16
0.02 1.92 0.09 3.16 1.90
0.03 78.02
–
86.77
0.04 89.00
0.001 0.005 87.36
Fe hydroxide. T-1, S ¼ 0.3%; Si/Fe ¼ 0.18 Fe-Si hydroxide. T-3, S ¼ 0.4%; Si/Fe ¼ 0.95 Fe-oxide mud of active vent, Pacific Ocean (3),Si/Fe ¼ 0.09 Fe-oxide mud of inactive deposit, Pacific Ocean (3), Si/Fe ¼ 0.19 Nontronite (3), Si/Fe ¼ 0.86 Nontronite, Si/Fe ¼ 1.22 Nontronite (2), Si/Fe ¼ 0.89
Note: EPR ¼ East Pacific Rise; IONF ¼ Indian Ocean Nodule Field; 1–10, S89 and S657; Iyer et al. (1997a); 11, Alt et al. (1987); 12,13 and type T-1, T-3, T-1/3, Hekinian et al. (1993); 14,15 and 18, Alt (1988); 16, Corliss et al. (1978); 17, McMurty et al. (1983). Serial number 11 to 15 represents Fe-Si oxyhydroxides and 16–18 Pacific Ocean nontronite. The number of analyes is in parenthesis.
91
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Mukhopadhyay, Ghosh and Iyer
Table 3.9 Microprobe analyses (wt%) for Nickel and Sulphur Inclusions in Volcanic Magnetite Spherules and Metalliferous Sediments in the IONF Serial no.
Sample no.
Nickel
Inclusions in volcanic magnetite spherules 1 89 0.13 2 89 0.02 3 89 0.11 4 89 0.14 5 89 – 6 89 – 7 657 0.11 Inclusions in metalliferous sediments 8 89 0.02 9 89 0.06 10 89 0.03 11 89 0.08 12 657 1.40 13 657 1.09
Sulphur
Remarks
0.08 0.04 0.06 – 0.01 1.55 0.54
Metallic matrix Oval-shaped spherule Tear drop spherule (3) Matrix Matrix Edge of matrix Matrix (2)
4.65 0.53 0.23 0.21 0.37 0.43
Metallic (3) Metallic (2) Metallic Metallic (2) Metallic (2) Metallic (4)
Source: Iyer et al. (1997a). Note: Serial number 8 and 13 show the highest S (9.82%) and Ni (2.24%) contents at a single probe spot, respectively. The number of analyses is in parentheses. IONF ¼ Indian Ocean Nodule Field.
favourable conditions exist in the IONF for hydrothermal solutions to percolate through the basaltic crust, leach the rocks and mix with the seawater for subsequent cooling and oxidation, resulting in FeOOH precipitates. It is possible that during the discharge of hydrothermal solutions from the igneous crust into the sediments, metal deposits formed either within the sediment column or at the sediment–water interface, resulting in the formation of intra-sedimentary hydrothermal deposits (Bonatti, 1983). The Fe-rich metalliferous sediments in the IONF are sometimes associated with nontronite-like particles (Fig. 3.10C). Nontronite, a typical hydrothermal mineral, shows similar Si and Fe enrichment but has higher than normal Al2O3 and MgO content as compared to other oceanic occurrences. As hydrothermal solution percolates through the sediment, it cools slowly under reducing conditions to form Fe and Si precipitates that later react with biogenic silica to form nontronite (Murnane and Clague, 1983). The composition of the IONF metalliferous sediments and their geologic setting affirm their derivation as in situ precipitates from hydrothermal exhalations (cf. Harrison and Peterson, 1965). Restricted occurrence of these sediments near seamounts and absence of any strong bottom currents in the IONF, as corroborated by several hundreds of underwater photographs, also support their in situ origin. 2.1.2. Magnetite Spherules The volcanic spherules found in the metalliferous sediments are fresh and display metallic luster (Fig. 3.10D). They range in size from a few microns to 475 mm and are mostly spherical, but dumb-bell, tear-drop and oval-shaped spherules also occur, albeit in lower proportions. Many of the spherules have a reddish-brown ochrous
93
Volcanics
coating (Fig. 3.11A). Electron microscopy of the spherules shows the predominant presence of magnetite that forms an array of surface textures, such as brickwork, interwoven, corkscrew type and dendritic (Fig. 3.11B–D). Some spherules have a smooth surface with no apparent crystallinity, while others are polytextured. The textures are caused by quenching, that is, melting followed by immediate supercooling, a process commonly experienced by cosmic dust. In contrast to cosmic spherules, the crystalline structure of the IONF spherules is restricted to the outer zone. In a few spherules, Si-rich blobs either protrude from an A
B
C
D
E
Figure 3.11 Photomicrograph of volcanic magnetite spherules from core S657: (A) adhering ochrous sediments on a few of the spherules; scale bar = 50 mm; (B) well-developed large euhedral magnetite crystals on a spherule; scale bar = 10 mm; (C) quench texture on a magnetite spherule; scale bar = 100 mm; (D) a broken magnetite spherule with linearly arranged crystal edge; scale bar = 10 mm; and (E) polished sections of spherules showing a central Si-rich capsule with a thick wall and enclosing magnetite crystals; scale bar = 100 mm.
94
Mukhopadhyay, Ghosh and Iyer
otherwise Fe-rich matrix or appear as inclusions (Fig. 3.11E). Vacuoles present within the spherules are located concentrically or eccentrically (Fig. 3.11C) and may have formed because of blowout made by escaping volatiles. Magnetite is ubiquitous in these spherules, together with minor proportions of ilmenite, hematite and maghemite (Table 3.10). In core S89, Ti-rich spherules and grains were also recovered (Fig. 3.10D). The spherules are non-welded, probably due to their low population per unit area or due to the rapid formation of a thin, rigid skin on the surface (cf. Heiken and Lofgren, 1971). Except for their sizes, the IONF spherules are similar to the archetypal terrestrial volcanic spherules, and to the magnetite spherules described from the 45 N MAR. The reported size of terrestrial volcanic spherules from different volcanic areas is between 1 and 300 mm (Iyer et al., 1997b) vis-a`-vis the maximum diameter of IONF spherules (475 mm). Microprobe analysis of spherules (Table 3.11) shows high FeO (87–99%; average 95%, Fe 74%) and minor amounts of Ti and Mn, while sectioned spherules show inclusions enhanced in Mg and Al. One of these is hyalosideritic olivine with no crystallinity, while others show orthopyroxene-like composition (bronzite-enstatite). Besides these, the inclusions enriched in Si, Al, Fe, Ca and Na indicate the presence of feldspar but the Fe content is too high for a pure plagioclase. A few inclusions in the spherules are substantially enriched in Ti (52–60%), Ni (0.02–0.14%) and S (0.01– 1.55%, Tables 3.11 and 3.12). The spherules from a red clay domain display Fe content of 65 to 78% (average 76%), while Ni is between 0.03 and 4.46%, and S between 0.02 and 0.17%. Besides these, Ti-rich spherules and inclusions also occur that could represent either ilmenite or titaniferous-magnetite. Different sources can be ascribed for the formation of magnetite spherules (Iyer et al., 1997b and references therein) such as (1) industrial, (2) diagenetic, (3) biological, (4) extraterrestrial and (5) volcanic. An assessment of the composition of the IONF spherules rules out the first three options as sources, as does their likeness to droplets of basaltic glass of Kilauea lki, Hawaii, basaltic hydromagmatic ash, basaltic microspherules from the Eastern Pacific Ocean, and microlapilli occurring near the Vityaz FZ, Indian Ocean. Moreover, an extraterrestrial source is unlikely since the IONF spherules lack a Ni-rich core, a common feature of extraterrestrial spherules (Blanchard et al., 1980). On the contrary, the dominance of magnetite associated with silicate fragments and the presence of magmaphile and diagnostic elements like Ti and Mn in the IONF spherules suggest that they are volcanogenic (Iyer et al., 1997a,b). To understand the mechanism of the formation of volcanic spherules the ‘fuelcoolant interaction (FCI)’ process, in which a rapid vaporisation of water occurs when in contact with hot molten materials, was advocated (e.g. Peckover et al., 1973; Wohletz et al., 1995). When magma contacts water, it is essential that the two mix thoroughly to create vapour explosions and the resulting products fragment before explosion. Fragmentation increases the contact area of melt and water causing heat exchange at explosive rates. To study the formation of spherules, Wohletz and McQueen (1984) made laboratory-scale experiments using thermite melt as an equivalent to basalt. Ignition of a mixture of thermite (Al þ Fe3O4), quartzofeldspathic materials and water in steel containers resulted in the formation of
95
Volcanics
Table 3.10
Gandolifi X-ray diffractometry of the magnetite particles in the IONF
Sample
Size (mm)
S89
275
S89
S657
410
800
Description
Black, metallic spherule
Black, metallic spherule with orange coloured adhered material
Black, metallic botryoidal particle with quench droplets
Exposure time (hours) dA
Intensity
Minerals present
32
2.9827
MS
Ilmenite
S MS W MS MS MS MS MS W W
Magnetite
80
2.7300 2.5363 2.2306 2.1204 1.8596 1.7241 1.6313 1.5063 1.4777 3.6792
VS VS MS VW MS S W W W W
Magnetite
80
2.6861 2.5130 2.1905 2.0891 1.8409 1.6926 1.6099 1.4873 1.4554 2.9307
2.6968 2.5050 2.1188 2.0814 1.6988 1.6099 1.5126 1.4821 1.1159 1.0866
VW VS W W W S VW S W MS
Hematite
Hematite
Maghemite
(continued)
96
Mukhopadhyay, Ghosh and Iyer
Table 3.10 (continued)
Sample
Size (mm)
Description
Exposure time (hours) dA
1.0642 0.9685 0.9548 0.9345
Intensity
Minerals present
W W VW W
Source: Iyer et al. (1997b). Note: VS ¼ very strong; S ¼ strong; MS ¼ medium strong; VW ¼ very weak; W ¼ weak; IONF ¼ Indian Ocean Nodule Field.
hydrovolcanic particles of 1–2 mm after weak interaction, and 1–100 mm following strong explosive interaction. The resultant quenched particles were irregular aggregates, spheriods or blocky conforming to hydrovolcanic ash. Iron and aluminum oxides were formed that resemble a basaltic melt (Table 3.13). Although the compositions are still very different from natural silicate magmas, their similar density, liquidus temperature and viscosity were considered to be viable working analogues (Iyer et al., 1997b; Wohletz et al., 1995). Considering congenial conditions represented by abundant siliceous sediment, predominance of tholeiitic basalt and evidence of hydrothermal activities, a natural field set-up in the IONF seems to have prevailed, similar to the above-mentioned laboratory-scale experiments. It is possible that lavas or hydrothermal emanations enriched in iron might have got admixed with the siliceous sediments resulting in localised hydrovolcanic events(s) and the production of the spherules. The locations where volcanic spherules are found in the IONF overlie major magnetic anomalies close to seamounts. As discussed in the section on ferrobasalts, enrichment of Fe and Ti in the IONF spherules can be related to high remnant magnetisations influenced by strong magnetic anomalies. Iyer et al. (1997a,b) explained the chemogenesis (i.e. the relative enrichment and depletion of Fe and Si) of the IONF spherules based on a process of liquid immiscibility coupled with oxygen fugacity (fO2). It was suggested that oxygen controls the fractionation of magma and results in Fe-rich and Fe-depleted liquids, which on cooling precipitate magnetite (Osborn, 1979). As the temperature of the cooling liquid falls, iron oxidises faster and combines with oxygen to form magnetite while silica forms a glassy microlayer for the Fe-rich liquid cooling in the interior of the spherules, where euhedral magnetite crystallises. With a more complete oxidation, unmixing of the phases occurs so that Si is either pushed aside or else is separated from the Fe matrix as blobs (Del Monte et al., 1974). The observed internal structure and the presence of vesicles in the IONF spherules may be better explained by a mechanism of cooling and solidification resulting from an abrupt decrease in the solubility of oxygen in the oxidised particles, leading to oxygen concentration in the inner, still-molten region of the particle.
Table 3.11
#
Microprobe analyses (wt%) of volcanic magnetite spherules, and inclusions within the spherules in the IONF
Sample no.
SiO2
Magnetite spherules 1 89 and 1.91 657
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
Cr2O3 BaO
Total
Remarks
Magnetite spherules (24) Ti-rich areas in spherules (5) Ti-rich areas in spherules (2)
0.60
0.68
94.98
0.46
1.04
0.45
0.39
0.08
0.16
0.13
100.88
2
89
15.73
52.12
5.30
12.32
6.41
1.43
2.45
0.50
2.66
0.63
0.76
100.31
3
89
2.81
60.76
4.41
23.92
6.82
0.40
0.65
–
0.09
0.03
1.04
100.93
Inclusions in spherules 4 89 38.68
0.26
1.76
35.48
0.18
24.02
0.40
0.14
0.08
0.18
–
101.18
5
89
57.71
0.20
3.26
14.23
0.07
23.42
0.23
0.10
0.45
0.24
0.11
100.02
6
657
59.32
–
2.00
9.15
–
28.76
0.08
0.03
0.02
0.60
0.03
99.99
7 8 9
89 89 89
43.26 59.85 48.14
– 0.27 0.29
36.76 25.76 25.18
8.88 2.60 21.25
0.38 – 0.28
0.51 0.05 0.50
4.58 3.55 0.74
5.23 6.40 0.48
0.23 0.93 1.53
0.19 – 0.08
– – 0.29
100.02 94.41 98.76
Source: Iyer et al. (1997a). Note: Figures within brackets are the number of grains analysed.
Hyalosideritic olivine Bronziteenstatite Bronziteenstatite Feldspar Feldspar Feldspar
Table 3.12
Microprobe analyses (wt%) of volcanic magnetite spherules and hydrothermal precipitates from the IONF
98
Specimen #
Si
Ti
Al
Fe
Mn
Mg
Ca
Na
K
Cr
Ba
Ni
S
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
0.34 1.60 0.17 0.20 0.01 0.07 nd 0.09 nd 0.27 0.02 0.07 0.15 0.16 0.13 0.15 0.08 0.25 0.89 13.38 26.37 11.84 15.03 4.90 1.72 1.93
0.25 0.56 0.02 nd nd 0.03 nd 0.04 0.05 0.13 nd 0.03 0.07 nd 0.05 0.09 0.41 nd 0.36 0.26 nd nd 0.07 0.11 0.07 0.06
nd 4.49 nd 0.03 nd nd nd nd nd nd 0.01 nd 0.66 nd 2.10 nd nd nd 0.36 3.81 13.07 0.98 8.37 35.30 8.26 23.23
74.8 64.9 75.9 75.2 77.6 77.1 76.8 77.1 77.8 76.2 77.3 78.6 76.4 77.4 74.4 77.3 76.9 76.7 73.8 30.9 nd 51.6 37.0 11.2 61.4 26.5
0.43 nd 1.32 1.47 0.08 0.33 0.15 0.37 0.03 0.64 0.03 0.18 0.05 nd 0.04 nd nd 0.01 0.36 0.39 0.19 0.09 0.55 0.10 0.07 0.03
nd 1.21 nd nd nd nd nd nd nd nd nd 0.03 nd nd nd nd nd nd 0.63 11.59 0.75 1.24 0.07 0.27 0.15 0.38
nd 0.43 0.13 0.02 0.01 nd 0.04 0.07 nd nd 0.01 nd nd 0.03 nd 0.04 0.05 0.39 0.32 0.31 6.03 1.77 0.18 0.65 0.26 7.40
0.07 0.19 0.02 0.09 nd 0.01 nd 0.04 0.02 nd 0.07 0.01 nd 0.04 nd nd 0.04 0.05 0.29 0.11 6.22 0.48 0.22 0.18 0.07 0.05
0.02 0.18 0.07 0.02 0.04 0.03 0.05 0.07 0.05 0.07 nd 0.07 nd nd nd nd nd nd 0.07 0.14 0.47 0.82 0.07 0.15 0.13 0.12
0.01 0.11 nd nd nd nd 0.16 0.11 0.34 0.06 nd ndd ndd ndd ndd ndd ndd ndd 0.11 1.38 ndd 0.06 1.80 4.21 0.64 4.32
0.01 0.18 nd 0.34 nd 0.16 0.02 0.01 0.10 0.18 nd ndd ndd ndd ndd ndd ndd ndd 0.12 ndd ndd 0.06 0.15 0.17 ndd 0.22
0.03 nd 3.27 nd 0.04 nd 0.04 nd 4.46 0.07 0.09 ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd
0.07 nd 0.17 nd nd nd 0.07 0.05 nd 0.03 0.02 ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd ndd
Source: Iyer et al. (1999b). Note: na ¼ not available, nd ¼ not detected, ndd ¼ not determined; IONF ¼ Indian Ocean Nodule Field. In sediment cores S89 and S657, Ni and S contents range from 0.02 to 0.14% and 0.01 to 1.55%, respectively, in spherules, and 0.02 to 1.4% and 0.21 to 4.65%, respectively, in hydrothermal precipitates. #1–17 ¼ Volcanic magnetite spherules of S94; 18 ¼ Metallic grain of S94; 19 ¼ Average of 24 spherules from S89 and S657 (Iyer et al. 1997a); 20 ¼ Olivine inclusion of S94; 21 ¼ Plagioclase inclusion of S94; 22–26 ¼ Si-Fe-Al-enriched phase of S94.
Table 3.13 #
Composition (wt%) of magnetite spherules, hyalosidertic olivine and Ti-rich particles from the IONF
Si
Ti
Al
Fe
Mn
Mg
Ca
Na
K
Remarks
1 2 3
0.86 18.08 7.35
0.30 0.16 31.25
0.21 0.82 2.80
73.83 27.58 9.58
0.36 0.14 4.96
0.39 14.49 0.86
0.26 0.29 1.75
0.22 0.10 0.37
0.07 0.07 2.21
4 5
1.31 0.23
36.43 tr
2.33 0.10
18.59 72.05
5.28 na
0.24 tr
0.46 tr
na na
0.07 na
6
0.15
tr
0.09
71.91
na
tr
tr
na
na
7
0.07
tr
0.05
72.10
na
tr
tr
na
na
8
0.25
tr
0.16
71.01
0.05
tr
tr
na
na
9 10
na 8.20
na na
na na
71.50 42.90
0.37 0.25
na na
na na
na na
na na
11
23.67
0.65
7.33
10.04
na
3.90
6.85
1.97
0.27
12
23.50
1.27
7.44
10.10
na
4.20
7.54
1.98
0.30
13
21.86
0.70
7.41
7.15
0.15
5.40
9.39
na
0.36
Avg. of 24 IONF spherules Hylosidertic olivine from the IONF Avg. of 5 Ti-rich areas of a IONF spherule Avg. of 2 Ti-rich areas in the inclusion Magnetite spherule, 30–100 mm, Mt. Etna Magnetite spherule, 30–100 mm, Mt. Lipari Magnetite spherules, 30–100 mm, Mt. Vesuvius Magnetite spherules, 30–100 mm, Mt. Bracciano Volcanic spherules from the MAR Irregular Si-rich channels in MAR spherules Avg. of 57 basaltic spherules from DSDP site 32, NE Pacific Ocean Avg. of 14 microlapilli (300–2000 mm) from DSDP site 32, NE Pacific Ocean Avg. of basaltic microlapilli from two cores near Vityaz Fracture Zone, Indian Ocean
14
15.00
7.00
7.00
31.00
tr
na
3.00
na
1.00 (continued)
Table 3.13 (continued) #
Si
Ti
Al
Fe
Mn
Mg
Ca
Na
K
15
23.00
3.00
10.00
20.00
tr
na
6.00
na
1.00
16 17 18 19 20
4.71 5.02 6.70 17.08 8.53
na na 1.35 0.86 1.14
16.63 13.28 6.01 18.21 22.47
45.04 49.89 44.31 13.46 18.10
na na 1.22 0.74 1.03
na na 3.88 2.23 3.52
0.32 na 1.46 1.18 1.36
na na 2.28 1.47 2.37
0.26 na 1.59 1.65 1.54
Remarks
Avg. of 50 volcanic spheroids of avg. 101 mm from 5 Pacific coast volcanoes Avg. of 95 volcanic spheroids of avg. 54 mm from Irazu, Kilauea Iki, Ubinas and Huainaputina Large Fe-Al sphere Small Fe-Al sphere Blocky Fe particle Coating on Fe particle Fe-Al spindle
Source: Iyer et al. (1997b). Note: # ¼ 1–4 ¼ S89 and S657; #5–8 ¼ Del Monte et al. (1975); #9, 10 ¼ Aumento and Mitchell (1975); #11 ¼ Melson et al. (1988); #12 ¼ Vallier et al. (1977); #13 ¼ Nath and Iyer (1989); #14 ¼ Hodge and Wright (1964); #15 ¼ Wright and Hodge (1965); #16–20 ¼ experimental products (Heiken and Wohletz, 1985). avg ¼ average, na ¼ not available, tr ¼ trace; IONF ¼ Indian Ocean Nodule Field. In #1, 2 and 4 the Cr values are 0.08, 0.43 and 0.02% and Ba 0.08, 0.68 and 0.93%, respectively. Cr in #14 is up to 1% and in traces in #6 and 8.
Volcanics
101
2.1.3. Glass shards Glass shards are common in the IONF sediments (Fig. 3.12). Abundant platy glass shards of 150–250-mm length occur within the ochrous sediments or as individual fragments. Their number gradually decreases down-core from a maximum of 7000 per slide at 1–2 cm to about 2500 per slide between 5- and 6-cm depth and decreasing further down-core. The shards are rhyolitic with SiO2 varying between 74 and 81.5% (average 77.5%; Table 3.14A). The source of the IONF shards is not yet convincingly deciphered. For instance, the supereruption of Toba volcano in northern Sumatra about 74 ka ago (Rose and Chesner, 1987) resulted in widespread fallout of tephra to more than 3000 km in a northwesterly direction (Dehn et al., 1991; Ninkovich et al., 1978). Glass shards (28–470-mm long) from a few sediment cores in the IONF compositionally correspond, in fact, to the Younger Toba Tuff (YTT) (Pattan et al., 1999), which is suggested to have been dispersed up to 14 S. However, the Ti/Al ratio seems to indicate that the IONF shards originated as distal fallout from the IVA (Martin-Barajas and Lallier-Verges, 1993). Sukumaran et al. (1999) suggest that the shards could not have been carried 3000 km from the eruptive source, either by surface or by northward-flowing deepsea currents, and thus rule out Toba as a source. Based on 230Thxs values and radiolarian biostratigraphy, the authors favour the deposition of the shards consequent to suboceanic volcanic activities during periods of elevated glaciation. The major Indonesian volcanic activities are dated to have erupted at 74,000, 450,000 and 840,000 years and at 1.2 Ma (Rose and Chesner, 1987). These dates do not correspond to the 10 ka age of the vhm with which IONF shards coexist. Again, significant dissimilarities appear to exist when certain elemental concentrations and their ratios of the IONF shards and for the Toba shards are compared (Tables 3.14B). Hence, it is yet unclear whether the shards are (1) formed contemporaneously with the metalliferous sediments, or (2) pre-existing entities in the IONF sediments, or
Figure 3.12 Scanning electron microscope (SEM) image of platy glass shards recovered with the magnetite spherules from core S657. Scale bar ¼ 10 mm.
Table 3.14
Elemental abundance (wt%) in volcanic glass shards from the IONF and Toba, Indonesia
A. Composition of glass shards Author
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
Cr2O3
BaO
Remarks
I
77.48
1.27
9.28
6.87
0.28
0.32
1.08
0.95
2.33
0.27
0.14
S P
77.10 76.81
0.05 0.07
12.60 12.77
0.90 0.92
0.07 0.06
0.05 0.05
0.76 0.79
3.25 3.41
5.04 5.08
na na
na na
Rhyolitic shards (6) YTT (81) IONF cores (91)
B. Representative elemental concentrations (wt%) and ratios
Si Ti Al Ti/Al Si/Al K
1
2
3
4
5
36.22 0.19 4.91 0.04 7.38 1.94
33.75 0.07 6.83 0.01 4.94 4.23
33.56 0.08 6.93 0.01 4.84 4.23
36.59 0.04 6.72 0.01 5.44 3.54
25.39 1.01 9.24 0.11 2.75 2.87
Source: Iyer et al. (1997a). Note: Number of shards analysed are in parentheses. YTT ¼ Youngest Toba Tuff, I and S ¼ Shane et al. (1995), P¼ Pattan et al. (1999). na ¼ not available; IONF ¼ Indian Ocean Nodule Field. Note: 1 ¼ Iyer et al. (1997a); 2, 3 ¼ Toba glass shards (Ninkovich et al., 1978); 4 ¼ Average of 88 Toba glass shards (Rose and Chesner, 1987); 5 ¼ IONF glass shards (MartinBarajas and Lallier-Verges, 1993).
Volcanics
103
(3) from earlier IVA eruptions, or (4) attrition products of the pumice. Geochemical studies by Mascarenhas-Pereira et al. (2006) have provided proof of intra-plate volcanism as a source for the glass shards.
3. Alteration of Volcanics Low-temperature alteration of deep-sea volcanics is a ubiquitous process that produces various authigenic minerals. Such alterations involve a precursor, one or more processes and the end products. The IONF basalts are most commonly altered to palagonite, clay minerals (montmorillonite and smectite), zeolites and iron oxides and hydroxides.
3.1. Alteration of basaltic glass The main processes in the alteration of the oceanic crust are related to the availability of weak zones, composition of seafloor rock, seawater weathering, metamorphism, hydrothermal circulation and deuteric compositional changes brought about by magmatic solidification. The effects of seawater–rock interaction are the result of two processes: (1) low-temperature weathering (or halmyrolysis) and (2) hightemperature alteration. The former occurs at <70 C, is pervasive and continues for long periods (>106 years). The latter process is hydrothermal, occurs in the temperature range of 70–400 C or higher, is generally restricted to sites near accreting plate margins or mid-plate volcanic areas and spans relatively for short intervals (102–104 years; Thompson, 1991). Altered basalts are a source of Si, Mg and Ca and a sink for Ti, K, P, Mn, total Fe and Na. Water and potassium are the most effective and noticeable parameters of alteration. It is generally agreed that the magnitude of changes in the trace elements is always greatest in the altered glass. For instance, Ba is enriched and Zn is quite constant during palagonitisation of basaltic glass. Basaltic lavas erupted on the seafloor are quenched to form a glassy skin that preserves the nearly pristine magma chemistry. One notable alteration product of this glass is palagonite, supposedly formed at high temperatures almost instantaneously during eruption (Bonatti, 1965). However, it is now accepted that the palagonitisation occurs gradually after the lava is emplaced (Moore, 1966). Three stages of palagonitisation of basaltic glass have been recognised (Honnorez, 1981): Initial—coexistence of fresh glass relics with residual altered glass (palagonite), increase in K and Mg and loss of Ca from altered rocks. Mature—alteration of fresh glass and in situ replacement of residual glass by zeolites and smectites. Final—complete replacement of residual glass by authigenic minerals. Dredged basalts recovered from various sites in the IONF (Fig. 3.3) were examined for the presence of glass and its state of alteration (Fig. 3.13). The samples are of various sizes and non-uniformly altered to yellowish-red palagonitic materials.
104
Mukhopadhyay, Ghosh and Iyer
Ferromanganese oxides of variable thickness may overlie the basalts while the inner zone below is partly altered. Petrographic studies revealed the alteration of glass to palagonite (Fig. 3.14), plagioclase to sericite, olivine to iddingsite and appearance of limonite (Table 3.15). X-ray diffraction (XRD) analysis of a few samples shows phillipsite, the formation of which during the halmyrolysis of glass is well documented (e.g. Bonatti, 1965; Honnorez, 1981; Thompson, 1991). The major, minor and trace element concentrations in the altered basalts of the IONF samples (Table 3.16) are comparable to those from other oceans (Table 3.17). Removal or addition of certain elements during the alteration can be deciphered through binary plots. For example, with increasing K2O, CaO þ MgO decreases
Figure 3.13 Holohyaline and glass-flow textures of seamount basalt in the Indian Ocean Nodule Field (IONF). Note the alteration and devitrification of glass. Scale bar = 0.05 mm.
Figure 3.14 Photomicrograph of altered basalt with sub-microscopic sheaves of plagioclase. Small elliptical blebs occur around nucleus of plagioclase microlites. Scale bar = 0.05 mm.
105
Volcanics
Table 3.15
Petrography of altered basalt in the IONF
Location ( S/ E)
Depth (m)
Description
13 000 / 76 290 0 13 21 / 77 300
5346
Altered glass
5270
12 280 / 76 520
na
12 570 / 75 000
5348
12 330 / 77 120
5200
12 300 / 78 120
5280
12 130 / 76 110
5100
0.5 cm glass with rust coloured stains. 3 distinct zones: altered glass (A), glass þ interior (B) and interior (C). Zone B has sub-microscopically intergrown plagioclase and a few pyroxenes. Zone C is partly altered and has plagioclase microphenocrysts. Zeolites, limonite and smectite occur. 2 layers of glass. The upper unaltered glass has olivine and is separated from the lower layer by a band of greenish-yellow palagonite, between the bands of palagonite zeolites occur. Highly altered, 0.5 cm glass with rust coloured stains. Interior is buff to light gray, 2 glassy layers separated by palagonite. Uppermost glass hosts well-developed and fresh plagioclase. Lower glassy layer is oxidised and has some fresh plagioclase. The inner layer contains plagioclase and glass showing intersertal texture. RFA is also present. Glass of 0.5 cm with a veneer of sediment and Fe-Mn oxides. Plagioclase form trachytic texture. A few olivine grains present. Altered glass with patches of Fe-Mn oxides. A few altered grains of olivine, abundant vesicles and sheaves of plagioclase, RFA. Altered glass with patches of Fe-Mn oxides in a matrix of fibrous palagonite. Abundant plagioclase microlites as sheaves and in trachytic forms in glassy groundmass. Occurrence of a few fresh euhedral plagioclase phenocrysts, altered and cracked euhedral plagioclase, and olivine. Sub-ophitic and RFA.
Source: Iyer (1999a). Note: na ¼ not available; RFA ¼ Red feathery alteration texture; IONF, Indian Ocean Nodule Field (cf. Baragar et al., 1977).
reflecting the alteration of plagioclase and olivine and the formation of smectite. Decrease in CaO and MgO and increase in K2O, with increasing loss on ignition (LOI) (Fig. 3.15), could represent a nearly isomolar exchange during the alteration of basaltic glass to palagonite, an observation in accordance with Staudigel and Hart’s (1983) study. Although MgO shows higher concentration in less altered materials for a given LOI content, the variation in MgO is small. The relation also shows K2O to increase linearly with increasing alteration, except in those samples having higher MnO. CaO has a strong negative trend, indicating depletion of lime with increasing alteration (Iyer, 1999a).
106 Table 3.16
Mukhopadhyay, Ghosh and Iyer
Composition of altered basalt in the IONF
Sample no.
81
6
14
24
26
85
87
SiO2 TiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O P2 O 5 MnO LOI Ba Co Cu Cr Ni V Y Zn Zr
43.64 0.68 14.80 7.58 0.97 1.56 5.42 3.74 0.39 3.71 15.79 784 160 1203 32 1519 191 96 342 334
39.59 2.02 14.29 15.39 3.41 3.66 2.58 1.97 0.10 1.71 12.39 190 146 442 151 588 190 33 295 206
41.72 1.09 15.34 13.93 4.81 3.08 2.77 1.91 nd 0.30 12.14 83 90 302 519 217 220 23 270 128
41.18 1.63 17.84 13.35 1.84 1.34 2.59 2.82 nd 0.25 15.41 81 67 321 373 101 100 22 310 186
38.28 1.79 13.16 14.68 1.49 1.53 2.92 3.18 0.10 3.89 17.34 237 486 408 84 900 182 54 375 275
37.28 1.31 13.86 11.71 1.52 1.19 3.18 3.01 nd 6.75 17.98 604 459 1406 92 2470 154 60 420 240
37.10 1.14 14.08 12.44 1.58 1.22 3.60 2.94 nd 7.29 16.97 547 429 1290 83 2224 123 72 390 272
Source: Iyer (1999a). Note: nd ¼ not determined; LOI ¼ Loss on ignition; IONF ¼ Indian Ocean Nodule Field. Major elements in wt%, Trace elements in ppm.
Table 3.17 Composition (wt%) of palagonite, most altered basalt material and red feathery altered (RFA) materials from the world oceans Oxides
1
2
3
4
5
6
7
8
SiO2 TiO2 Al2O3 FeO* MgO CaO
42.05 2.13 11.43 19.31 3.95 1.48
53.42 2.38 15.60 20.58 5.15 1.11
44.19 2.81 15.42 15.72 2.82 0.60
41.80 1.90 12.15 19.96 4.06 0.72
37.88 1.21 15.38 11.97 7.56 5.44
41.96 1.63 16.06 9.39 3.69 1.49
47.11 1.55 18.37 8.91 1.84 2.34
47.65 0.42 19.45 9.80 3.44 11.03
Source: Iyer (1999a). Note: 1¼ DSDP leg 37 (Scarfe and Smith, 1977, average of 4 samples); 2 ¼ DSDP leg 37 (Andrews, 1977, average of 6 samples); 3 ¼ average of 3 samples (Melson, 1973); 4 ¼ average of 6 samples (Baragar et al., 1977); 5 ¼ average of 4 samples (Furnes, 1978); 6, 7 ¼ Swallow Bank, Atlantic Ocean (Matthews, 1971, 1 sample each); 8 ¼ RFA material from DSDP sites 332 and 335, MAR basalts (Baragar et al., 1977, average of 6 samples).
107
Volcanics
B
A
4
81 26
5
3
85
K2O
4 K2O
x 3
x
87
24 2
6 14
2
1
1 0 0
2
4
6
8
0
10 12 14 16
10
8
CaO + MgO C 4.0
14
12
16
18
LOI
81
H
3.5 26
85
K2O
3.0 24
87
2.5
L 6
2.0
14
1.5 2
4
6 8 10 CaO + MgO + MnO
12
D
100
81 H
2
87
6 26 85 87
TiO2
L 14
1
85
60
26
Y
24
H 6 24 L 14
81 20
50
150
250 Zr
350
50
150
250
350
Zr
Figure 3.15 Inter-element relations in the Indian Ocean Nodule Field (IONF) basalts. (A) K2O vs. CaO þ MgO for palagonite and altered basalts. Palagonite: solid dots ¼ Iyer (1999a); cross ¼ Scarfe and Smith (1977); circle with cross ¼ Andrews (1977); open circle ¼ Melson (1973); square and plus sign ¼ Barager et al. (1977); inverted triangle ¼ Matthews (1971). Altered basalts: circle with dot ¼ Melson (1973); triangle ¼ Barager et al. (1977); (B) loss on ignition (%)vs. K2O (%)for the altered basaltic glass of the IONF and other oceanic occurrences. Symbols as in (A); (C) K2O vs. CaO þ MgO þ MnO of the altered basalts. L and H represent samples with <2% and >2% MnO content, respectively; (D) Zr vs.TiO2, and Zr vs.Y for the altered basalts. See text for explanation. Zr and Y in ppm, rest in wt%.
108
Mukhopadhyay, Ghosh and Iyer
Barium seems to relate more closely with MnO than with K2O and H2O, indicating its association with the biogenic debris or aluminosilicates. Similarly, Co, Cu, V, Ni and Zn contents are higher in MnO-enriched samples, a feature common in manganese nodules. Chromium correlates with MgO signifying Cr to generally occur in olivine or be supplied from the seawater. Depending on the MnO contents, inter-elemental plots made for Y, Zr and Ti (elements less sensitive to weathering and non-mobile) point to two clusters (Fig. 3.15). Samples with >2% MnO have high Ba, Co, Cu, Ni, Y, Zn, Zr, TiO2, Co and low Cr, (cluster H) while those with lower MnO content show a reverse trend (cluster L). Strong positive correlations between Y and Zr in both the clusters and between Zr and TiO2 in cluster L (Fig. 3.15) suggest that though both Zr and Ti are immobile, some amount of Zr, which probably admixed with the phosphatic phase (fish debris?), is removed during the leaching of P. The composition of palagonite, most altered basalt material and red feathery alteration (RFA) from the world oceans are furnished in Table 3.17. Based on the above pattern of chemical exchanges, the following process is envisaged for alteration, leading to palagonitisation of the IONF basalts. Basaltic eruptions that occurred about 60 and 45 Ma ago, chilled to form pillow basalts with a quenched glassy exterior and holocrystalline interior. Subsequent interaction with seawater caused a gradual breakdown of the homogeneous glass that shows a distinctly wider range of elemental change vis-a`-vis the polyphase crystalline interior. The mineralogical changes noted are the presence of palagonite, clays, iron oxides and phillipsite. Compositionally, Si, Mg and Ca were leached while Na and K were incorporated, with Fe and Ti remaining immobile. These characteristics typify an initial-to-intermediate stage of palagontisation of the IONF basalts under low-temperature oxidising conditions. Later, hydrogenous precipitation of ferromanganese oxides formed a protective veneer over the basalts and arrested the progress of alteration (Iyer, 1999a,b).
3.2. Zeolitites Marine authigenic phases such as phosphorites, manganese nodules, feldspar, metalrich sediments and zeolites are of considerable importance, as they play key roles in the geochemical cycling of different elements, both in seawater and in sediments. Zeolites may constitute up to 80% of oceanic and volcaniclastic sediments and their composition depends, among other factors, on (1) reaction time for devitrification, (2) crystallochemical transformations of glass of different Si/Al ratios at low temperature and pressure, (3) sedimentation rates, (4) sediment type and (5) Eh–pH conditions. A few studies that have been carried out concerning the Indian Ocean zeolites are by Arrhenius (1963) and Bonatti (1963) who reported natrolite occurrence, while Kastner and Stonecipher (1978) and Iijima (1978) detailed the presence of clinoptilolite (59%), phillipsite (22%) and analcime (17%). Kolla and Biscaye (1973) concluded that the alteration of basic and silicic volcanics in the Indian Ocean could have formed phillipsite and clinoptilolite, respectively. Besides these, phillipsite occurs in the IONF as well developed diagenetically formed crystals within manganese micronodules (Banerjee and Iyer, 1991) and in one case, as large crystal (21 10 8 mm) forming the nucleus of a manganese nodule (Ghosh and
109
Volcanics
Mukhopadhyay, 1995). Interestingly, there are indurated zeolitic slabs in the IONF, as in the Pacific Ocean (Morgenstein, 1967), which are described later. From the IONF, a few unusual porous and semi- to highly-indurated specimens were recovered, mostly from the siliceous sediment though, a few were from red clay domain (Table 3.18). The samples are between 3- and 20-cm long and may occur proximal to seamounts and fracture zones (Fig. 3.3). The samples are ‘fresh’, light pink to yellowish in colour and unaltered, though weathered ones are common. A few among these occur as the nucleus of nodules and at times are bioturbated (Fig. 3.16) (Banerjee, 2000; Iyer and Sudhakar, 1993b). Binocular microscopy (Fig. 3.17A) of the samples displays irregularly shaped pores, cavities, burrows and tubes that may be lined with ferromanganese oxides. Some of the samples show initiation of nodule growth-cusp around a zeolitic nucleus. Some others show a distinct entrapment of pumice in association with fine needles of zeolite. Electron microscopy (Fig. 3.17B) reveals platelets of clay minerals and phillipsite crystals. Although plagioclase, sometimes as an impurity, obscures the XRD peaks (Sheppard and Gude, 1983) the prominent XRD peaks (d ¼ 3.15 A˚) are similar to those for harmotome (Fig. 3.18A) and to the typical ‘Indian Ocean phillipsite’ (dA ¼ 3.19; location—23 160 S/75 590 E, Gottardi and Galli, 1985). This was confirmed by differential scanning calorimetry (DSC) profiles that Table 3.18
Geological setting of a few zeolitites from the IONF
Sample
Description (size in cm)
1
25 21.5, palagonite and Fe-Mn oxides 7.5 4.5, hard, compact
2 3 4 5 6 7 8 9 10 11
4.5 2.5, flat, ‘laterite-like’ with Fe-Mn oxides length 10 cm, highly altered, yellow 9.5 7.5, yellowish, flat a) 9 6, tabular, yellowish with Fe-Mn oxides b) 5.5 5, altered with Fe-Mn oxides(0.3) 12.5 7.5, ‘fresh’, red 7 4, flat, gritty with Fe-Mn. 5 2.5, flat, black 12 7.5, flat, black to buff, burrows
Position ( S/ E)
Depth (m)
Topography
13/75.7
4275
Seamount
12.6/ 76.1 11.2/73
5300
Seamount
5340
12.4/ 76.8 12.7/ 79.5 11.3/ 73.2 12.4/ 76.3 13/75.7 17/76.9 16/83 14/79.4
5050 5070
Fracture zone/ seamount Flank of seamount Fracture zone
5100
Seamount
4800
Seamount
4275 5100 4872 5172
Seamount Abyssal plain Abyssal plain Seamount
Source: Iyer and Sudhakar (1993a,b). Note: Samples 1–8 from siliceous sediment; 9 and 10 from red clay and 11 from siliceous-red clay, Fe-Mn oxides ¼ Ferromanganese oxides; IONF ¼ Indian Ocean Nodule Field.
110
Mukhopadhyay, Ghosh and Iyer
A
B
0
1
2
cm
3
4
5
Figure 3.16 Specimen of bioturbated zeolitites recovered from the Indian Ocean Nodule Field (IONF). Note the presence of ferromanganese oxides on the lowerspecimens. Scale bar in ‘A’ = 2 cm.
showed exothermic water loss at 157 C and 238 C (Fig. 3.18B) similar to that of the phillipsite–harmotome family (cf. Gottardi and Galli, 1985). As the samples are formed mostly of zeolites and following Deffeyes’s (1959) definition, the term ‘zeolitites’ has been proposed (Iyer and Sudhakar, 1993b). The two possible precursors for the formation of IONF zeolitites are: (1) a continuous layer of volcanic ash and/or altered fragments of pumice, or (2) devitrification of basaltic glass. Ash layers in the IONF (Martin-Barajas and Lallier-Verges, 1993; Vallier and Kidd, 1977), which act as substrates for hardgrounds and pavements (Gupta, 1991), may have been disintegrated by mass movements, erosion by underwater currents, solution and stirring by organisms and tectonic reactivation (Iyer and Sudhakar, 1993b; Menard, 1960) and subsequently were wholly or partly indurated and altered to zeolites. Phillipsite and clinoptilolite, respectively, are the dominant or sole zeolite in silicic tuffs and in mafic ash layers. Interestingly, the IONF zeolitites are associated
111
Volcanics
with both mafic and silicic precursors. They occur in a large field of pumice (600,000 km2 in area) with the pumice fragments sometimes entrapped within the zeolitic slabs, an observation that supports siliceous precursors. Mafic precursors are evidenced by the presence of numerous altered basaltic fragments. Under favourable conditions, the mafic precursors are transformed into zeolites through an intermediate stage of palagonite formation (see earlier section, Iyer et al., 2007b). A
a
b
c
Figure 3.17 (Continued)
112
Mukhopadhyay, Ghosh and Iyer
Figure 3.17 (A) Photomicrographs of polished sections of zeolitites:(a) dendrites and spicules of ferromanganese oxides in a matrix of dominantly zeolitic material; scale bar = 300 mm; (b) cusp formation and initiation of nodule growth with zeolitic material as nucleus; scale bar = 300 mm; (c) a fragment of pumice within the zeolitic material, observe the fine needle-like crystals of phillipsite at the centre; scale bar = 500 mm; (B) Scanning electron microscope (SEM) images of polished zeolititesç(a) platelets of clay minerals; scale bar = 2 mm; (b) well-formed stubby crystals of haromotome (cf. Sheppard and Gude, 1983); scale bar = 4 mm; (c) well-developed prismatic crystals of phillipsite; scale bar = 6 mm; and (d) crystals of zeolites of varied nature; scale bar = 4 mm.
Most of the IONF zeolitites occur near the major fracture zones (Vishnu FZ along 73 E, 75 450 E FZ and Indrani FZ along 79 E) and seamounts. Such locales possibly are conducive for zeolite formation by the alteration of the glass, palagonite and volcanic debris by dilute, low-temperature hydrothermal fluids. The IONF samples are not only typical examples for a process of complete zeolitisation but also the presence of the ferromanganese oxides accounts for later localised induration.
4. Tectonics and Volcanics: Interrelations It is now seen that the majority of the IONF seamounts were formed at the ridge axis between 60 and 45 Ma and were transported to the present sites along with the underlying crust. Many of these seamounts have an east-west ‘bulged’ elongation, which can be attributed to later addition of magma along weak zones after the seamounts moved to the present sites from the MOR (also see Chapter 2) and this is suggestive of multi-episodic volcanism in the basin. The weak zones may
113
Volcanics
A
(041)
PI (140)
(151) (103) (320) (323)
36⬚
(001)
(131)
(321)
(021) (120)
PI (121)
30⬚
20⬚
10⬚
2q B
−3 −4
98.31⬚C
−5 Heat flow (mW)
I −6
II 211.64⬚C
−7 −8 −9 157.10⬚C
238.04⬚C
−10 20
70
120
170
220 270 320 Temperature (⬚C)
370
420
470
Figure 3.18 (A) X-ray profile of zeolitites. PI ¼ Plagioclase impurity; (B) differential scanning calorimetry (DSC) profiles of zeolitites. See text for explanation.
have resulted during mid-plate tectonic reactivation (owing to tensional or compressional stress) of diverse intensity. Volcanic-hydrothermal materials (vhm) as well as altered basalts (zeolitites and palagonites) generally occur near to these ‘bulged’ seamounts and weak zones. These morphotectonic features probably
114
Mukhopadhyay, Ghosh and Iyer
facilitated the circulation of low-temperature fluids, and brought about hydrothermal alteration. High concentrations of vhm in the IONF (metalliferous sediments, volcanic spherules and glass shards), associated with sediments of 10 ka and 425–650 ka age, are likely to have formed in situ from volcanic emanations or hydrothermal solutions. It is possible that the laboratory experiments (interacting thermite melt with watersaturated quartzo-feldspathic sands) have been mimicked by local volcanic eruptions in the IONF. Plausibly, the interaction between the Fe-rich lavas or hydrothermal emanations and the quartzo-feldspathic sands (equivalent to abundant siliceous sediments in the IONF) may have resulted in localised hydrovolcanic events and production of magnetite spherules. However, considering the hydrostatic pressure at a depth in excess of 5000 m, which is well above the critical pressure of generation of a vapour phase, a more detailed investigation is required to explain hydrovolcanic events at abyssal depth. Hence, the presence of a variety of major and minor volcanics in the IONF may imply that this field is not tectonically and volcanically inactive (Iyer and Sudhakar, 1995). This fact may have a strong bearing on the formation, distribution and enrichment of ferromanganese deposits in the IONF (see Chapter 5).
C H A P T E R
F O U R
Sediments
Contents 117 117 124 135 137 139 146 149 151
1. Distribution and Source 1.1. Distribution 1.2. Source 2. Sedimentary Processes 2.1. Dissolution of carbonate 2.2. Bottom water mass and sedimentation 2.3. Depositional environment 2.4. Diagenesis 2.5. Sediment consolidation
Sediments are formed from disintegrated rocks as a result of physical and chemical weathering. The action of various agents, like ice, wind, water and variable temperature, helps fragment the rocks into smaller particles and leach the more soluble minerals. Weathering also results from reaction of seawater with basalt, erupting at the crests of the mid-oceanic ridges and at other submarine volcanic features, contributing in the process considerable amounts of materials to seawater. Another significant contributor has been extraterrestrial matter. Transported either in a dissolved or suspended state, all these materials ultimately form four types of sediment—biogenous and hydrogenous through precipitation, and lithogenous and cosmogenous in a clastic detrital state. Among the biogenous deposits are calcite, aragonite, opal, phosphorites and organics. The hydrogenous types are evaporites, zeolites, manganese nodules and polymetallic sulphides, while silica, feldspar and rock fragments constitute deposits of lithogenous type. The contributors to cosmogenous deposits have been the cosmic spherules, microtektites and minitektites (Chester, 1993; Lisitzin, 1996). In general, mechanical weathering dominates in temperate climates at high latitudes, where water in the form of ice is the chief weathering agent. Chemical weathering (i.e. leaching), on the other hand, is favoured by high rainfall, variable temperature and dominates in tropical areas. Hence, any sedimentary basin reflects the palaeo- and present environmental conditions of sedimentation including the processes and provenances. The distribution of various sediment types in the world’s oceans has been recorded with reliable accuracy (Fig. 4.1; Table 4.1). There have been considerable activities in recent years to understand the sediment distribution and sedimentary processes in the Central Indian Ocean Basin (CIOB), a basin which Handbook of Exploration and Environmental Geochemistry, Volume 10 ISSN 1874-2734, DOI: 10.1016/S1874-2734(07)10004-8
#
2008 Elsevier B.V. All rights reserved.
115
116
Mukhopadhyay, Ghosh and Iyer
160W
100
40W
0
40E
100E
60 N
30
0
30
60 S
Ice rafted
Carbonate
Siliceous
Red clay
Terrigenous
Siliceous/red clay
Figure 4.1 Sediment distribution in the world oceans (Kolla and Kidd, 1982; The Open University,1995).
Table 4.1
Distribution (%) of various sediment types in the world oceans
Major type
Minor type
Red clay Siliceous ooze
Diatom ooze Radiolarian ooze Calcareous ooze Foraminiferal ooze Pteropod ooze
Atlantic
Indian
Pacific
All oceans
26 07 – 65 02
25 20 0.5 54 0.1
49 10 05 36 –
38 12 03 47 0.5
Sources: Berzukov (1960) and Stowe (1996).
hosts the sediments of the two largest rivers of the world, the Ganges and the Bramhaputra, and also holds the second richest and second largest manganese nodule field in the world oceans. Hence, the sediments described throughout this chapter, if not specifically mentioned, are essentially those of the CIOB, which also includes the Indian Ocean Nodule Field (IONF).
117
Sediments
1. Distribution and Source 1.1. Distribution The nature and distribution of seafloor sediments in the Indian Ocean are principally controlled by five interrelated factors: (1) climatic and current pattern, (2) nutrient and organic production in surface waters, (3) relative solubility of calcite and silica, (4) submarine topography and (5) detrital input. Based on their interactions, four major types of sediments occur—terrigenous, calcareous, siliceous and pelagic (Fig. 4.2). Covering about 35% of the CIOB, calcareous sediments, with a sedimentation rate of 4–6 mm/103 year, are common along the equatorial highproductivity areas and at lesser depths, such as near the seismic and aseismic ridges (the Chagos Ridge, the Ninetyeast Ridge) and the shallow areas of seamount
60E
70
80
90 10 N
0
10 S
20
30
Calcareous Ooze
Siliceous Clay
Calcareous Clay
Brown Clay
Terrigenous Clay
Terri-Siliceous Clay
Mixture of Terrigenous, Calcareous, Siliceous Clay Southern limit of Indonesian Volcanic Tephra
Figure 4.2
Sediment distribution in the Indian Ocean (Udintsev,1975; Kolla and Kidd,1982).
118
Mukhopadhyay, Ghosh and Iyer
summits. Siliceous clay (and ooze) and red-brown clay, on the other hand, are the dominant sediment types in the deeper parts of the basins in the CIOB, and cover about 35 and 16% of the surface area, respectively. These sediments are found in areas where the rate of sedimentation is low (<2 mm/103 year) and terrigenous contribution is absent. The red-brown clay is eupelagic and contains <25% of the coarser fraction of either lithogenous or volcanic derivation. Terrigenous sediments, on the other hand, are hemipelagic and contain more than 25% terrigenous or coarse volcanic material. This sediment type, with the highest rate of sedimentation, is largely detrital brought about from the continents by the large rivers and covers 14% of the CIOB (Chester, 1993; Lisitzin, 1996). In the CIOB, the broad distribution of sedimentary facies is now available based purely on qualitative information from sediment samples obtained through box cores, Pettersson grabs and Van Veen grabs (Table 4.2). The enormous load of terrigenous materials (about 1.67 million tons/year) in the central and the eastern Indian Ocean, including the northernmost part of the IONF, is contributed mainly by the Ganges and the Bramhaputra rivers. This amounts to about 24% of total load carried by all world rivers (Milliman and Meade, 1983). These two rivers bring materials largely from the mighty Himalayas and from the Indo-Gangetic plains. The southern extent of terrigenous sediment distribution in the CIOB appears intriguing. Kolla and Biscaye (1973) identified terrigenous influence up to 10 S, based on the clay mineralogy and up to 8 S, based on chemical analyses of surface sediments (Nath et al., 1989). The major element ratio Al/(Al þ Fe þ Mn) was used to determine terrigenous extension. Four long-gravity cores of sediment, one each from the four sectors in the IONF were studied in detail to trace the limit of terrigenous influence—vertically and horizontally. Decreasing contents of detrital components such as Al, Ti, K and Fe and a decreasing ratio of Al/(Al þ Fe þ Mn) towards the southern sectors indicate diminishing influence of terrigenous influx. The ratio lowers the cut-off limit of 0.65 at around 10 S. The Al/(Al þ Fe þ Mn) ratio varies from 0.5 to 0.77 in these sediment cores and is lower (<0.65) near the top and base of the cores, with the ratio reaching a maximum value (0.77) at an intermediate depth corresponding to about 140–100 ka at 14 S (Mudholkar et al., 1993) and to the last interglacial maxima (Curray and Moore, 1971). All these studies in the CIOB approximately constrain the extension of terrigenous sediment in the northern part up to about 7 S, siliceous in the central part between 8 S and 15 S, calcareous sediments to the west of 74 E bordering mid-oceanic and Chagos ridges and brown clay in the southern part south of 15 300 S (Banerjee, 1998; Kolla and Kidd, 1982; Nath, 2001). The areas roughly between 7 S and 8 S and between 15 S and 15 300 S are composed of mixed terrigenous–siliceous sediments and siliceous–pelagic clays, respectively. Among other materials reported to occur in the CIOB are volcanic ash (tephra), pumice, aerosols and extraterrestrial materials. Much of the volcanic ash contributed by erupting volcanoes and carried by wind is finely dispersed, and sometimes forms several centimetre thick layers in deep-sea sediments. Some of these ash layers are correlatable over great distances, marking periods of major volcanic eruptions. A well-known example is the ash that the Toba supereruption spewed out in
Table 4.2
Location of sediment sampling stations in the IONF Longitude ( E)
Latitude ( S)
0
10 15 –10 26
10 00
10 00
10 00
12 00
12 30
0
0
75 59.5 –76 15
75 15
75 37
76 00
76 07.50
75 52.50
0
Sampler type
Water depth (m)
BC
5400
BC
5400
BC
BC
BC
BC
5400
5400
–
–
Sample ID by author
T1
R1
A1
R2
T2
Type of studies made
▪ 12 nos. core in the region ▪ Physical properties ▪ Grain size þ clay mineralogy ▪ Geotechnical properties ▪ Grain size þ clay mineralogy ▪ Geotechnical properties ▪ Grain size þ clay mineralogy ▪ Geotechnical properties ▪ Grain size þ clay mineralogy ▪ Geotechnical properties ▪ Grain size þ clay mineralogy ▪ Geotechnical properties
Reference
Sector
a
C
b
B
a,c
B
b
B
a,c
B
b
B
a,c
B
b
C
a,c
C
b
C
a,c
C
(continued)
120
Table 4.2 (continued) Latitude ( S)
Longitude ( E)
Sampler type
Water depth (m)
Sample ID by author
Type of studies made
15 300
73 00
SC þ PG
4650
129
12 00
75 300
SC þ PG
–
128
11 00
75 450
SC þ PG
5039
287
12 010
73 00
SC þ PG
5000
124
11 300
81 280
SC þ PG
–
139
15 00
73 300
SC þ PG
4900
231
12 010
73 020
SC þ PG
5075
121
14 00
72 00
SC þ PG
5000
126
14 00
73 00
SC þ PG
4389
56
12 500
78 00
SC þ PG
4977
81
13 500
74 00
SC þ PG
5150
47
11 15.60
75 00.70
BC
–
AAS 4/1
12 00.10
75 29.90
BC
–
AAS 4/2
▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Biogenic silica concentration ▪ Tektites, minitektites ▪ Tektites, minitektites
Reference
Sector
d
D
d
C
d
C
d
C
d
C
d
D
d
C
d
D
d
D
d
C
d
D
e
C
e
C
12 30.50
76 30.90
BC
–
AAS 4/5A
12 36.90
78 30.70
BC
–
AAS 4/6
12 370
78 300
BC
–
AAS 4/6
11 44.60
73 59.70
PG
4896
22
14 470 –15 59.80
73 440 –76 59.70
PG
4969–5161
7 sites
13 080
75 010
SC
5270
SK226
14 00
9 990
76 00
77 920
SC
SC
5050
5250
SS657
NR-1
▪ Tektites, minitektites ▪ Tektites, minitektites ▪ Tektites, minitektites ▪ Geochemistry, mineralogy ▪ Geochemistry, mineralogy ▪ General geochemistry ▪ Major elements, Toba tuff ▪ Glass shards, excess Al ▪ General geochemistry ▪ Major elements, Toba tuff ▪ Glass shards, excess Al ▪ REE concentration ▪ U, Th isotope, transition ▪ Major elements, Toba tuff
e
C
e
C
e
C
f
C
f
D
g
C
h i g
D
h i j j
A/B
h
121
(continued)
122
Table 4.2 (continued) Latitude ( S)
11 00
11 970
10 00–15 00
10 00–15 00
15 00–16 00
Longitude ( E)
78 490
78 490
72 00–74 00
75 00–80 00
72 00–82 00
Sampler type
SC
SC
PG
PG
PG
Water depth (m)
5325
5450
–
–
–
Sample ID by author
NR-21
NR-35
21, 22, 23, 33
1, 17, 19, 26, 27, 29, 32
2, 30, 31
Type of studies made
▪ Glass shards, excess Al ▪ U, Th isotope, transition ▪ Major elements, Toba tuff ▪ Glass shards, excess Al ▪ U, Th isotope, transition ▪ Major elements, Toba tuff ▪ Glass shards, excess Al ▪ XRD, infrared, DTA, clay ▪ Major elements ▪ REE analyses ▪ XRD, infrared, DTA, clay ▪ Major elements ▪ REE analyses ▪ XRD, infrared, DTA, clay ▪ Major elements ▪ REE analyses
Reference
Sector
i
C
j h i j
C
h i k
B/C/D
l m k
B/C/D
l m k
D
l m
11 00–15 00.50
72 01.60 –81 490
PG, BM
4300–5380
12 00
76 300
SC
5450
12 00
77 00
SC
5430
– F200B
F88B
14 00
74 00
SC
5240
SK176
12 00
76 400
SC
5300
GR1–120E
12 500
76 000
SC
5250
SS667
9 00–14 00
75 590 –76 010
SC
5158–5352
SPC14–19
9 00
77 00
BC
5400
–
13 030
75 440
BC
5099
GC02
▪ Radiolarian zonation ▪ U, Th, biogenic Si, chem. ▪ Radolarian zonation ▪ U, Th, biogenic Si, chem. ▪ Radolarian zonation ▪ U, Th, biogenic Si, chem. ▪ U, Th, biogenic Si, chem. ▪ U, Th, biogenic Si, chem. ▪ Geochemistry assessing biogenic and detrital influence ▪ Geotechnical properties
n
C
o,p
C
REE
q o,p
C
q o,p
D
o,p
C
o,p
C
r
A/B/ C/D
s
A
t
C
Sources: a, Khadge (2002); b, Valsangkar and Ambre (2000); c, Khadge (2000); d, Pattan et al. (1992); e, Prasad and Khedekar (2003); f, Banerjee (1998); g, Mudholkar et al. (1993); h, Pattan et al. (1999); i, Pattan and Shane (1999); j, Pattan and Banakar (1997); k, Rao and Nath (1988); l, Nath et al. (1989); m, Nath et al. (1992); n, Gupta (1996); o, Borole (1993a); p, Borole (1993b); q, Gupta (1988); r, Banakar et al. (1998); s, Khadge (1998); t, Pattan et al. (2005). Note: BC, box core; PG, Pettersson grab; SC, spade core; BM, boomerang core. Refer figure for sector reference. DTA, differential thermal analysis; IONF, Indian Ocean Nodule Field; REE, rare earth elements; XRD, X-ray diffraction.
123
124
Mukhopadhyay, Ghosh and Iyer
Sumatra (Indonesia) at 74 ka, which spread over a distance ranging between 1500 and 3500 km (Pattan et al., 2002). Aeolian contributions from the Australian, African and Asian landmasses are also considerable.
1.2. Source 1.2.1. Mineralogical indicators The distribution of various clay minerals in the sediments of the CIOB can be used to determine the source of the sediments. The clay minerals along with lithogenous and hydrogenous components form the dominating part of the four types of sediments available in the basin. Detailed study of clay minerals of different grain sizes in the surface as well as subsurface layers of sediment from 140 locations in the CIOB was made. The major clay minerals in the nodule field are smectite, illite, chlorite, kaolinite and mixed-layer derivatives. Other minerals are feldspar, pyroxene, quartz (lithogenous), zeolite, ferromanganese oxides and hydroxides (hydrogenous or authigenic). The hydroxides provide much of the amorphous iron-oxide minerals, which impart red to brown colour to the sediment. Grain size distribution and clay mineral analyses were carried out on 21 box core sediments collected broadly from two latitudes—15 cores from around 10 S in sector A and 6 cores from around 12 S in sector C (Tables 4.3 and 4.4). The lengths of the cores were at least 35 cm. Lithological and particle size investigations show that sediments are predominantly clayey silt, with mean grain size (M2F) of 7.0–8.6 for surface and 6.6–8.6 for subsurface sediment. The eastward increase of smectite and kaolinite and the southward decrease of illite have been noted (Valsangkar and Ambre, 2000). Variations in distribution and mineralogy of clay minerals in different size fractions of the IONF sediments are interesting (Table 4.4). The conventional treatment of the separation of clay minerals through settling velocity, followed by differential thermal analysis (DTA) suggests that the concentration of illite, kaolinite and chlorite in <2 mm fractions is higher in the northern part of the nodule field (sector A), while the concentration decreases in the southern part (sector D). The abundance of these three minerals in the terrigenous and siliceous sediments indicates influence of continental influx. The concentration of these minerals decreases towards the pelagic clay-dominated sedimentary regime in the south with a corresponding increase in smectite (derived from local young eruptions) and montmorillonite [resulting from weathering of ancient ridge volcanics; Banerjee (1998)]. The higher concentration of montmorillonite in the pelagic clay-rich domain in the southern areas of the IONF appears to have been caused by submarine weathering of basic volcanic rocks (halmyrolysis). Iron-rich montmorillonite can also be found in <1 and 1–2 mm fractions of the siliceous, and 1 mm fraction of the pelagic clays. It is possible that Fe-rich montmorillonite might have been formed by the interaction between iron hydroxides and biogenic silica (opal-CT) during early diagenesis in siliceous sediments having high contents of biogenic silica. In calcareous sediments, there is, however, an absence of iron-rich montmorillonite and predominance of diagenetically mobilised minerals following higher biogenic sedimentation. While montmorillonite has been abundant in <1 and 1–2 mm
Table 4.3
Grain size distribution in the IONF sediments
Sector and location ( S/ E)
Water depth (m)
Sediment layer (cm)
Graphic mean
Standard deviation
Graphic skewness
Graphic kurtosis
Sector B 10 000 /75 150 Sector B 10 000 /75 450 Sector B 10 000 /76 000 Sector C 12 000 /76 150 Sector C 12 22.50 /75 450
5150–5340
Surface 05–10 depth Surface 05–10 depth Surface 05–10 depth Surface 05–10 depth Surface 05–10 depth
7.61 7.62 7.70 7.84 7.59 7.53 7.29 7.13 7.72 7.25
2.43 2.45 2.58 2.37 2.55 2.80 2.52 2.94 2.41 2.27
–0.24 –0.22 –0.68 –0.17 –0.06 0.11 –0.06 0.24 –0.14 –0.15
0.71 0.80 0.80 0.81 0.39 0.79 0.64 0.77 0.59 0.66
5200–5400 5180–5400 5200–5380 5120–5320
Source: Valsangkar and Ambre (2000). Note: Sector B values are averages of five sediment cores from each location, while sector C values are averages of three sediment cores from each location. IONF: Indian Ocean Nodule Field.
Table 4.4 Distribution of clay minerals (%) in the IONF sediments Sector and location ( S/ E)
Sediment layer depth (cm)
Smectite average (range)
Illite average (range)
Kaolinite average (range)
Chlorite average (range)
Sector B 10 000 /75 150 Sector B 10 000 /75 450 Sector B 10 000 /76 000 Sector C 12 000 /76 150 Sector C 12 22.50 /75 450
Surface 05–10 depth Surface 05–10 depth Surface 05–10 depth Surface 05–10 depth Surface 05–10 depth
34.9 (32.4–37.4) 40.5 (–) 34.5 (21.1–53.0) 33.1 (23.1–43.1) 47.0 (36.5–55.2) 40.5 (23.4–57.8) 13.9 (5.66–23.7) 12.0 (10.2–15.2) 16.9 (13.8–21.4) 9.16 (5.17–13.3)
35.6 (35.2–36.0) 27.97 (–) 30.4 (21.4–40.3) 36.0 (27.3–53.3) 25.9 (14.5–35.2) 27.8 (17.1–36.8) 41.3 (37.7–49.2) 41.7 (35.7–49.2) 32.3 (25.0–37.5) 42.7 (34.5–53.3)
09.4 (6.10–12.6) 17.16 (–) 13.5 (8.55–17.8) 13.5 (11.7–15.4) 12.7 (6.30–20.6) 13.2 (9.26–20.6) 13.7 (7.89–23.6) 12.5 (9.42–14.8) 17.0 (15.5–17.8) 19.9 (12.5–25.3)
20.1 (18.9–21.4) 14.30 (–) 21.5 (17.1–30.0) 18.1 (10.3–28.1) 17.1 (10.6–23.1) 18.5 (9.26–29.6) 31.1 (28.8–33.0) 33.7 (29.5–40.2) 33.7 (29.3–35.7) 28.2 (20.8–38.4)
Source: Valsangkar and Ambre (2000). Note: Sector B values are averages of five sediment cores from each location, while sector C values are averages of three sediment cores from each location. IONF: Indian Ocean Nodule Field.
Sediments
127
fractions, suggesting their crystallisation and segregation preference in these sizes, the percentage of illite, on the other hand, increases with increasing size. Illite shows low crystallinity in finer sediments caused by early diagenesis (Rao and Nath, 1988). 1.2.2. Geochemical indicators During the last few decades, valuable contributions have been made to improve our understanding of how the Indian Ocean works as a chemical system. The R-mode factor analysis of the geochemical data of sediment samples showed five important sources of supply for the various elements in the IONF sediment—detrital (loaded with Fe, Ti, Al, P and K), combined hydrogenous and diagenetic (Mn, Ni, Cu and Co), biogenic (Si), sea salt (Na and Mg) and dissolution residue (Ba). Geochemical and isotopic investigations [high Th/Ta ratio (12.8–21.1) and strontium-neodymium (Sr-Nd) data] suggest Himalayan rocks and materials of the Indo-Gangetic plains as the chief source area for CIOB sediments (Fagel et al., 1997). The major elements in the sediment samples from 140 locations covering the four sediment domains of the CIOB (including the IONF)—terrigenous, pelagic, calcareous and siliceous—were determined (Table 4.5; Fig. 4.3). The concentration of major elements, inter-element correlation, their possible source and mechanism of precipitation are briefly discussed here. Organic carbon concentration in the sediment is also given in Fig. 4.3. Aluminium, believed to be an index element for the land-derived alumino-silicates (terrigenous source) and also contributed by weathering of basalts and hydrothermal derivatives, varies from 8 to 13% in the IONF sediments. It is higher in sediments from the northern sectors (sectors A and B, 11–13%) than in the southern sector sediments (sectors C and D, 8–10%). This indicates a decrease in terrigenous input towards the south. Al shows good correlation with Ti (r ¼ 0.50), supporting its detrital origin (Mudholkar et al., 1993). Bulk and excess element concentration in the IONF sediments are shown in Table 4.6. Silica, which is normally transported into the marine environment from landderived alumino-silicates and from skeletons of diatoms, radiolarians, sponges and silico-flagellates, varies between 48 and 68%. Pattan et al. (1992) reported that 50% of silica in cores 226 (sector C) and 657 (sector D) is of biogenic origin, attributable to the higher surface productivity and better preservation. The higher content of SiO2 at 8–10-, 18–20- and 26–28-cm depths in core 226 is due to the probable presence of pumice at these levels. Silica, in general, does not show any correlation with Al and Ti, indicating dominance of a biogenic source. The average SiO2/ Al2O3 ratio and excess SiO2 values, which help in understanding the origin of the elements (Bischoff et al., 1979), are highest (6.7 and 27.97, respectively) in siliceous sediment, followed by intermixed siliceous and terrigenous areas (between 7 S and 8 S, 4.54 and 14.04) and pelagic and siliceous areas (between 15 S and 16 S, 4.25 and 12.27) areas. The major contributor for excess SiO2 has been biogenic silica, which was forthcoming due to the proximity of the northern IONF to the subequatorial region of high biological productivity (Nath et al., 1989). Titanium is an extremely immobile element in the marine environment, and shows input largely from the continents and sometimes from the weathering of oceanic basalts. The TiO2 content in the CIOB sediment varies from 0.28 to 0.44%
128 Table 4.5
Mukhopadhyay, Ghosh and Iyer
Concentration of major, trace and excess elements in the IONF sediments Calcareous ooze Element
Excess
Siliceous ooze Element
Major (%) and excess element (ppm) Si 12.75 – 27.4 Al 2.55 38 5.44 Fe 1.60 35 3.24 Mn 0.26 92 0.64 Ca 25.50 99 0.69 Na 0.96 – 1.67 K 0.46 29 1.62 P 0.07 77 0.10 Ti 0.12 – 0.23 Trace (ppm) and excess element (ppm) Ba 1494 87 3056 Co 24 85 64 Cu 141 92 305 Ni 110 88 220 Pb 20 82 44 Sr 958 95 175 Zn 83 78 160
Pelagic clay
Terrigenous clay
Excess
Element
Excess
Element
– 33 39 93 50 – 40 78 –
20.09 6.60 4.10 1.41 1.41 1.82 1.88 0.17 0.27
– 31 44 97 64 – 30 76 –
– 7.6 5.6 0.56 1.9 – – 0.08 0.47
90 87 93 89 89 50 79
2483 163 556 600 109 138 226
86 93 97 97 97 20 78
1662 46 235 – – 226 –
Sources: Nath et al. (1989), Borole (1993 a,b), Banakar and Jauhari (1994), Pattan et al. (1994), Banakar et al. (1998) and Jauhari and Pattan (2000). Note: IONF, Indian Ocean Nodule Field.
and decreases from the northern to the southern locations, indicating again a reduction in terrigenous input towards the south. It does not show much variation with depth except for some high values at intermediate depth, suggesting increased supply of terrigenous material. Ti generally shows a linear relationship with Al (r ¼ 0.91), Fe (r ¼ 0.72) and Mg (r ¼ 0.69), indicating its homogeneous distribution in the clay-hydrolysate fraction. Iron is normally found fractionated between pelagic clay, hydrogenous metals and metalliferous components. The average Fe2O3/Al2O3 ratio varies between 0.40 and 0.44, which is much lower than those in the average pelagic clays (0.58). A lack of excess Fe2O3 and a good correlation of Fe with Al (r ¼ 0.88) and Ti (r ¼ 0.72) suggest that iron is derived either from a hydrogenous source or from detrital clays. There are, however, large amounts of Fe-rich montmorillonite in the sediment (Rao and Nath, 1988). Potassium in the CIOB sediment (average content 1.88%, Mudholkar et al., 1993) is preferentially absorbed into clays. The presence of large quantities of illite and phillipsite and other products of diagenetic origin suggests that these components act as favourable sinks for potassium (Shankar et al., 1987). The abundance of illite is greater in siliceous-terrigenous area (north of sector A, 46–51%), compared to its characteristic lower abundance in siliceous clay (sectors B and C, 35–40%) and pelagic–siliceous clay (south of sector D, 33–36%). The average K2O content for
129
Sediments
2.0
20
Laxs (ppm)
Fexs (%)
2.5
1.5 2.0 0.5
15 10 5
1.5
Pxs (%)
Alxs (%)
2.0
2.0 0.5
Baxs (ppm)
Opal (%)
0.6 0.2
35 25 15 5
3500 2500 1500 500 80
Coxs (ppm)
1400
Srxs (ppm)
1
1000 600 200
60 40 20
400
Cuxs (ppm)
60 40 20
300 200 100
20
0.9
15
0.7
Mnxs (%)
C.F. (%)
CaCo3 (%)
80
10 5
0.5 0.3 0.1
4N 2
0
2
4
6
8
10 12 14S
4N 2
0
Latitude
2
4
6
8
10 12 14S
Latitude
Figure 4.3 Latitudinal variations in excess elements, opal, coarse fraction (CF) and carbonate in the Indian Ocean Nodule Field (IONF) sediments (Banakar et al.,1998). Table 4.6
Concentration of bulk and excess elements in the IONF sediments
Bulk Excess Excess: bulk
Al
Fe
Mn
P
Ba
La
Ce
Yb
5.9 1.7 29
3.6 1.6 44
0.73 0.68 93
0.13 0.09 69
2763 2521 91
29 10 34
80 32 40
3.5 1.9 54
Source: Banakar et al. (1998). Note: Al to P in %, Ba to Yb in ppm. IONF, Indian Ocean Nodule Field.
130
Mukhopadhyay, Ghosh and Iyer
these three domains is 2.49, 2.16 and 2.16%, respectively, which roughly follows the trends of illite distribution. The excellent positive relationship between K and Al in most of the cases supports a homogeneous nature of the lithogenous material. The average sodium content of the sediments derived from continental weathering of rocks is moderate (Na2O, 1.97%). The presence of biogenically derived calcium in these sediments depends mainly on the position of the calcite compensation depth (CCD, also known as carbonate compensation depth, or carbonate line). Other sources of calcium include clay minerals and feldspars (Rao and Nath, 1988). Magnesium occupies the lattice of montmorillonite. The average MgO content in the IONF sediments is 2.26% showing a good correlation with Ti (r ¼ 0.69) and Fe (r ¼ 0.57). The MgO content decreases from north (2.36%) to south (2.05%) of the IONF, with a positive correlation with Al2O3 (r ¼ 0.73). Phosphate concentration increases along the sediment depths, as the mobilisation of iron oxyhydroxides (forming smectite) does facilitate an upwards diffusive flux of phosphate into the overlying water. A close relationship is found between the dissolved phosphate concentration and the organic carbon content along the sediment column (Nath and Mudholkar, 1989). In general, the presence of higher Al excess in both siliceous (where carbonate is absent) and calcareous (where opal is absent) sediments in the IONF and its strong positive correlation with biogenic matter (both opal and carbonate) suggest association of excess aluminium (Alex) with surface water productivity (Fig. 4.4). The average opal content in carbonate ooze and detrital clay to the north of sector A is about 5%, whereas in the siliceous ooze area (sectors B, C and part of sector D) it is 26%. Again, Al/Ti values in the subsurface sediment reach 48.5, an unusual increase, three times higher than average shale and potential crustal sources. The importance of a biogenic contribution for enhancing the Al/Ti ratio appears possible, which is also supported by sediment trap data showing that trap materials comprise 90–95% biogenic components. The upper sediment traps have average concentrations of 0.12% Al and 341 ppm Ti (Al/Ti ratio ¼ 3.6), while bottom sediment traps have average concentrations of 0.4% Al and 619 ppm Ti (Al/Ti ¼ 6.5, Pattan et al. (1992)). It is also seen that the highest Al/Ti and Alex values have a bearing on the sediments enriched in volcanic glass. These volcanic glass shards in ash layers were recovered from eight sediment cores at and around 14 S in the CIOB. The shards are fresh, colourless, with no signs of alteration, and constitute 60–70% of the coarse fraction. They are rich in SiO2 (76.8%) and total alkalis (8.4%), suggesting rhyolitic composition, with very low MnO concentration (0.05%). Electron microprobe analyses of 60 of glass shards show average concentrations of 6.74% Al and 0.04% Ti. The Al:Ti ratio of glass varies from 125 to 188 with a mean value of 175. The composition is consistent with that of the glass shards produced by the youngest Toba eruption in northern Sumatra (age 74 ka), suggesting such eruptions as a possible source of CIOB glass shards (see also Chapter 3). The volcanic ash (tephra) built aprons of several tens of metres, supporting bihemispheric dispersal of the ash cloud and dispersal of gas and aerosols into the CIOB (Pattan et al., 1999). Additionally, occurrence of metalliferous sediments and high concentration of volcanic spherules associated with sediments of 10 ka in the CIOB are suggested to have
131
Sediments
A
B
1.0 0.8 0.6
1.0
0.2
0.5 0.0 0.0
0.5
C 4000
1.0 1.5 Alxs
2.0
0
2.5
10
20 Opal
30
40
20 Opal
30
40
20
30
40
D 4000
n = 19 g = 0.77
n = 19 g = 0.77
3000 Baxs
3000 Baxs
1.5
0.4
0.0
2000 1000
2000 1000
0.0 E 4000
0.2
0.4 0.6 Mnxs
0.8
0
1.0 F 2.5
n = 18 g = 0.61
Alxs
2000 1000
10
n = 18 g = 0.88
2.0
3000 Baxs
n = 19 g = 0.77
2.0 Alxs
Mnxs
2.5
n = 19 g = 0.59
1.5 1.0 0.5 0.0
0
10
20 OpalCFB
30
40
0
10
OpalCFB
Figure 4.4 Inter-element relationship among opal and excesses of Al (wt%), Ba (ppm) and Mn (wt%). Note possible burial of Ba through Mn phase following a direct relation (see C), and a strong direct relation between Alexcess and productivity (biogenic silica) (see F). CFB ¼ Carbonate free basis (Banakar et al.,1998).
derived from mid-plate volcanism and hydrothermal activities at a local scale (Iyer et al., 1997a,b). The other material falling (and sinking) in the CIOB is pumice—a product of volcanic eruptions (local as well as distal). Being highly porous, it can float and move with ocean currents for long distances, even transporting attached organisms. In addition, the aerosol particles brought by wind are the other contributors.
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Mukhopadhyay, Ghosh and Iyer
The aerosols flying into the northern portion of the Indian Ocean (size 0.1 mm to <0.001 mm) are delivered from central Asia, the Arabian Peninsula and the Indian Subcontinent and are characterised by high concentrations of biogenic components (Corg up to 20%, in certain cases up to 40%; Lisitzin, 1996). Among the extraterrestrial materials are tektites (size 1–1.5 mm), microtektites (size 200–900 mm) and minitektites (size 1–1.7 mm), which are found in the CIOB in various shapes—club, dumbbell, spherical, elongated, oval and so on. (Fig. 4.5; Table 4.7). The source materials of CIOB sediments are suggested to be mostly derived from the Himalayas, with minor contributions of illite-rich sediment from southern continental India, slumped sediment from the Indonesian arc and kaolinite-rich wind-blown particles from Australia (Wijayananda and Cronan, 1994). Thus, two major fractions (lithogenous detritus and calcium carbonate) and two minor fractions (authigenic Fe-Mn hydroxide colloids and biogenic detritus) constitute the sediment in the CIOB (Table 4.8). The detrital material derived from the Himalayas contributed to the bulk of sedimentation during 400 to 1100 ka (Pattan et al., 2005). Banakar and Jauhari (1994) divided the CIOB into six tentative geochemical zones from north to south (Fig. 4.6): (1) carbonate-dominated low Mn, low Fe sediment close to the equator; (2) carbonate-free high Al, high Ba siliceous sediment; (3) siliceous sediment with low Al, low Fe; (4) carbonate ooze with low Al, Ti and Fe (west of 74 E); (5) carbonate-dominated red clay with high Mn; and (6) terrigenous sediment with high Al, Ti and Fe to the north of the IONF. In addition, the interelement correlation (Figs. 4.3 and 4.4) suggests three strong positive mutual associations in the CIOB sediments, all having r > 0.7. These are (1) Ba and Mn, Ca and Sr; (2) Al, Mg, Fe, Ti and Sc and (3) Mn, Cu, Co and Ba. These associations may suggest well-defined carriers of the metals to the sediments (Banakar et al., 1998). The uranium and thorium contents in the IONF sediments are given in Table 4.8. The high Th/Ta ratio (12.8–21.1) in the CIOB sediments is indicative of orogenic calc-alkaline source rocks of Himalayan origin. The Sr-Nd isotope data are scattered between two-end members—the Himalayas and the seawater (Fagel et al., 1997; Nath, 2001). The Nd-isotope studies suggest supply of sediments from two major sources in the CIOB—the Indian shield (low E0 values between –14 and –17) and the Indonesian arc (E0 > 0; Fig. 4.7). From the barium distribution in this sediment (high barium suggesting higher productivity), Schmitz (1987) assumes that the sediment would record higher Ba concentration when passing through the high-productivity region and therefore will give an idea of plate movement. Accordingly, positions of the Indian Subcontinent at different geological times have been deduced from seafloor magnetic and biogenic barium data (Fig. 4.8). This deduction may also suggest that the Indian– Eurasian collision occurred during Late Eocene to Oligocene and uplift of the Himalayas reflected by a high-spreading rate, occurred during the Late Miocene. The source of volcanic ash layers in the CIOB sediments remains questionable. These ash layers have been reported from various depths in the sediment cores and may have different sources (Mascarenhas–Pereira et al., 2006). For example, the ash layers might be the product of secondary eruptions and have formed locally in situ in the basin, or may have been erupted through Indonesian arc volcanism (IVA) and later transported to the CIOB by seawater and air. There are many evidences in favour of in situ formation of ash layers in this magmatically active basin:
133
Sediments
A
c
b
a
d
e
g
f
B
a
b
c
d
h
Figure 4.5 Tektites in the Indian Ocean Nodule Field (IONF) sediment: (A) various shapes and dimensions of mini- and microtektites. (B) Different types of crater (pit, sandblasted) formed on the surface of tektites (Prasad et al., 2003).
Table 4.7
Composition of different tektites, and distribution of microtektites in the IONF
A. Compositional range (wt%)a
SiO2 Al2O3 FeO MgO CaO K2O Na2O TiO2
Microtektite
Tektite Australasian
IONF
CIOB
Minitektite CIOB
62.2–79.7 8.90–17.7 3.57–8.63 1.31–7.95 1.37–9.77 0.90–2.81 0.62–3.91 0.49–1.00
56.26–72.52 12.22–19.01 4.39–9.16 2.38–10.62 2.48–4.14 0.38–2.02 0.43–1.04 0.71–1.03
49.6–77.0 7.50–22.1 3.00–8.10 1.90–17.1 1.00–5.80 0.10–3.70 0.20–2.80 0.50–1.0
66.2–72.4 13.0–18.1 5.00–6.20 2.80–5.40 2.60–3.50 0.60–2.20 0.20–0.60 0.70–0.90
B. Distribution of microtektitesb Microtektite abundance
a
Sediment core
125^250 mm
>250 mm
Number of microtektites with impacts
AAS 4/1 AAS 4/2 AAS 4/5A AAS 4/6
659 379 662 217
445 277 401 364
73 19 28 30
Cassidy et al. (1969), Prasad and Sudhakar (1999) and Prasad et al. (2003). Prasad and Khedekar (2003). IONF: Indian Ocean Nodule Field. b
Size (mm) of impacted microtektite Range
Average
300–1400 280–1725 370–2000 295–2400
558 666 821 810
135
Sediments
Table 4.8 Uranium and thorium concentrations in representative the IONF sediments Sectors A þ B Sample 232 230 Th Th interval (dpm/g) (dpm/g) (cm)
238
232
230
238
232
230 U Th Th U Th Th (dpm/g) (dpm/g) (dpm/g) (dpm/g) (dpm/g) (dpm/g)
00–10 10–20 20–30 30–40 40–50 50–60 60–70 70–80 80–90
1.28 1.67 1.79 0.75 0.64 0.78 0.78 0.78 0.77
2.81 3.06 1.98 2.99 2.56 2.35 2.42 2.43 2.61
47.39 12.31 1.79 1.30 1.85 0.93 0.93 0.81 0.73
0.88 1.07 1.10 1.51 – – 1.08 – 0.81
3.88 4.35 3.60 3.28 – – 3.20 – 1.80
1.97 1.45 1.33 – – – – – –
28.16 16.38 9.16 – – – – – –
Sector C
Sector D
1.37 1.32 1.08 1.10 – – 0.97 – 0.63
Sources: Borole (1993a) and Banakar et al. (1998). Note: One core from each sector (A þ B, C and D)—NR-1, F88B and SK176, respectively—is furnished here. IONF: Indian Ocean Nodule Field.
(1) the association of ash layers with 10-ka-old radiolarian assemblage (Gupta, 1988), (2) occurrence of a large field of ‘in situ formed’ pumice associated with ash layers (Iyer and Sudhakar, 1993a,b), (3) confirmation of several episodes of local eruptions of fractionated mid-ocean ridge basalt (MORB) melt (see secondary eruptions in Chapter 2; Mukhopadhyay et al., 1995, 1997), (4) discovery of 10-ka-old high concentrations of volcanogenic hydrothermal material including spherules and rhyolitic glass shards (Iyer et al., 1997a,b) and (5) geochemical signatures and analyses of shards suggest an intra-plate volcanic event (Mascarenhas–Pereira et al., 2006). However, the fibrous and cuspate shape morphology and geochemistry [Ti/Al ratio, enrichment of Zr and light rare earth element (LREE), chondrite normalised pattern] of some of these tephra layers suggest their formation from Sumatra/Java IVA (Martin-Barajas and Lallier-Verges, 1993) with some from the Youngest Toba Tuff (YTT at 74 ka; Pattan et al., 2002). Also the ‘fresh’ pumice which was associated with these ash layers in the IONF is suggested to be chemically more akin to the IVA (Mudholkar and Fujii, 1995). Nevertheless, the enigma that an episode having an age of 74 ka is associated with 10-ka-old ash layers or with 200-ka-old sediments keeps the ‘IVA-transport theory’ under question (Nath, 2001). On the other hand, it is more likely that both the ‘in situ eruptions’ as well as the ‘transported eruptions from IVA’ contributed to the addition of secondary volcanics in the CIOB.
2. Sedimentary Processes The inter- and intra-oceanic variables that generally control the sedimentation pattern in deep-sea environments are: (1) sediment type and grain size; (2) eustatic and local sea-level changes; (3) tectonics; (4) rates of sediment supply and accumulation; (5) geometry and size of receiving basin; and (6) ocean current circulation patterns,
136
Mukhopadhyay, Ghosh and Iyer
A
B 80E
20N
100
80E
100
Fe (%)
Al (%)
20N
0
0
20S
20S > 6.5 6.5 - 5.5 5.5 - 4.5 4.5 - 3.5
> 5.5 5.5 - 4.5 4.5 - 3.5 3.5 - 2.5
3.5 - 2.5 2.5 - 1.5 1.5 - 0.5 < 0.5
C
2.5 - 1.5 1.5 - 0.5 < 0.5
D
20N
Smectite (%)
>70 50 - 70 30 - 50 10 - 30 <10
MYANMAR
INDIA
2
1
20N
3 4
4 Sri Lanka
3
0
0
00
40
3
00
40
5
3 5
20S
20S 80E
100
80E
100
Figure 4.6 Geochemical zones in the Indian Ocean Nodule Field (IONF) sediments showing distribution of (A) Al, (B) Fe and (C) smectite. The overall sediment distribution process (Wijayananda and Cronan, 1994) is sketched in (D): 1 ¼ Mg, Al, Ti, V, Cr, Fe and smectite-rich subcontinental sediments, 2 ¼ Himalayan range sediments, 3 ¼ sediments from subduction zone, 4 ¼ terrigenous sediments, 5 ¼ Mn, Co, Cu and Ni-rich hydrogenous sediments (after Nath, 2001).
governed in part by the Coriolis effect (cf. Blatt et al., 1980). These six variables are further governed by even larger-scale processes such as the relative rates of generation and destruction of oceanic crust, the disposition of the continents, global climatic changes and possibly Milankovitch cyclicity. The factors that may control the sedimentation in the CIOB are discussed later.
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30N INDIA 20 −14
AFRICA
−14.7 Old continental area particles
−7.9
−7.2
−7.2 −1.9 0 −2.6 −7.3 −2.8 −3.1 −7.0 10
20 −7.7
−5.8 −5.9
−10.8
40 00
30
40
+2.9 −4.9
Antarctic waters
20
10
−7.2
−12.2
−7.4 +0.06
−2 −3
−7.8 −7.7
−8.4
−12 Atlantic waters
10E
−17.4
−7.2
−9
30
40
50
−6.8
50
60S
−18.9
60
70
80
90
100 110E
Figure 4.7 Inferred sources of sediments in the Indian Ocean Nodule Field (IONF) based on Neodymium (Nd) isotopes (expressed as e0 units; Dia et al.,1992).
2.1. Dissolution of carbonate The primary factors influencing the nature of deep-sea sediments have been productivity and preservation, that is, the production and supply of planktonic organisms, along with the depth of carbonate dissolution. Planktonic organisms, in fact, are the major supplier of materials for the formation of deep-sea biogenous sediments (siliceous and calcareous clays). About half of the world’s deep-ocean floor is covered by biogenous ooze composed of coccoliths (5–30 mm), foraminifer tests (50–500 mm), diatom frustules (5–50 mm) and radiolarian frustules (40–150 mm). Some contribution also comes from seawater by chemical processes, in particular, by the precipitation of dissolved material. These sediments are called chemogeneoushydrogenous (Lisitzin, 1996), are subject to favourable physico-chemical conditions and facilitate the formation of ferromanganese oxide deposits in the world oceans including the IONF. From the surface, calcareous skeletal parts descend rapidly down the water column and are ultimately deposited as carbonate sediment (calcareous ooze). However, in deeper areas in excess of 4000 m, in general, the seawater becomes sufficiently acidic (with the increase in concentration of CO2) and becomes undersaturated with respect to CaCO3. As a result, calcareous material begins to dissolve.
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62 Ma
53 Ma
10N
IN IN
0
DI
17 Ma
6 Ma
217 DI
A
217
217 IN
DI
6 Ma
A
216
217
A
216
216 215
10
216
217
217
20
215
216
215 214
216 30
40 Ma
215 214
214
213
215
213
214
213
214 213 213
215 214
40S
9.9 cm/yr
5.6 cm/yr
2.3 cm/yr
1.4 cm/yr
11.3 cm/yr
213 −217 = DSDP cores
Figure 4.8 Paleo-position of the Indian Subcontinent deduced from biogenic barium and seafloor magnetic data (Schmitz, 1987). Sediment used for this experiment came from 5 DSDP cores 213 to 217.
Dissolution of shells is enhanced by high hydrostatic pressure and low water temperature. The depth at which calcium carbonate shows an accelerated dissolution is called the lysocline and the depth at which the proportion of carbonate falls below 20% is known as CCD. The siliceous ooze are found commonly in deep abyssal plains at water depths of about 5000 m. This indicates that the distribution and composition of deep-sea sediments are controlled by the productivity and preservation of planktonic organisms. The production and supply of such organisms are guaranteed close to the zone of high biological productivity (best in equatorial region), while preservation of skeletons is influenced by the CCD. As the solubility of skeletal remains is deep and temperature dependent, the CCD is depressed close to the equator, along with the elevated mid-oceanic ridge regimes and other shallow areas in the ocean, where biological productivity is generally greater than that in the open ocean. The average level of CCD is an indicator of the rate of removal of atmospheric CO2 to the deep sea, which has been variable over the geological past (Fig. 4.9). There are two principal ways by which CO2 gets into the ocean: (1) directly, through solution of CO2 from the atmosphere; and (2) indirectly, through transportation of weathered land products by rainwater as carbonic acid. If the first process is dominant, the ocean becomes more acidic and both lysocline and the CCD rise; if the indirect process of CO2 intake is more active, the opposite happens. Fluctuation in CCD through geological time and its effect on sedimentation pattern and distribution in the Indian Ocean are very pronounced, and this helps determine paleoclimatic conditions fairly accurately (Pickering et al., 1989).
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Paleo - CCD (Peterson and Backman, 1990) ~ 26S, 58E
Paleo - depth (km)
3
Paleo - depth curve 4
~22S 62E
5
~16S 75E
~12.5S 78E
~20S 69E
~18S 71E
Oxyhydroxide drought
2
1
6 0 PS
10 PL LM
20 MM
EM
30 LO EO LE
40
50 ME
EE
60 Ma P
Epoch
Figure 4.9 Position of the ancient calcite compensation depth (CCD) in the Indian Ocean vis-a-vis paleo-depth of the crust (Banakar and Hein, 2000). P ¼ Palaeocene, EE ¼ Early Eocene, ME ¼ Middle Eocene, LE ¼ Late Eocene, EO ¼ Early Oligocene, LO ¼ Late Oligocene, EM ¼ Early Miocene, MM ¼ Middle Miocene, LM ¼ Late Miocene, PL ¼ Pliocene, PS ¼ Pleistocene, 1 ¼ seamounts formation began, 2 ¼ crustal accretion commenced.
Despite the fluctuations in the global sea level since Cambrian (Fig. 4.10), the global average of CCD (Fig. 4.10) appears to be deeper now than at any time in the past 250 million years. In the Atlantic, the CCD has lowered from 3500 m during Jurassic– Cretaceous time. In the Indian Ocean, the change has also been stark, mean CCD deepened from about 3900 m at 135 Ma to 4000 m at 35 Ma and further to 5100 m today (Kolla and Kidd, 1982). A much lower CCD level (5700 m) is recorded from a 3-m calcareous clay sediment core recovered from a location adjacent to 73 E fracture zone (Vishnu FZ, Mukhopadhyay et al., 1994). The Pacific Ocean, however, does not show much variation, with the CCD lowered by only 200 m since Jurassic period. Paleo-levels of CCD could be ascertained from long sediment cores and from the age– depth relationship of oceanic crust (as oceanic crust is believed to be subsiding with time because of cooling and contraction, The Open University, 1995).
2.2. Bottom water mass and sedimentation The sedimentary process in the CIOB is highlighted by interaction of various alien water masses influencing the precipitation, erosion and non-deposition of sediments over the geological past. For example, the mixing of two bottom water masses, namely, nutrient-rich Antarctic bottom water (AABW) and the nutrient-poor North Atlantic deep water (NADW), appears to have influenced circulation and distribution of nutrients, both in vertical and in horizontal directions (Fig. 4.11). There is evidence that as Antarctica moved to its present polar position during the Eocene, there was a general deterioration of climate, ultimately resulting in glaciation near the end of the Eocene or beginning of the Oligocene. This resulted in the production of copious amounts of cold, aggressive bottom water that must
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0
2
4
6
570 Ma Cambrian 500 Ordovician 430 Silurian 395 Devonian 345 Carboniferous 280 Permian 225 Triassic 190 Jurassic 135 Cretaceous Paleoc. and Eocene Oligocene Miocene
0
2
4
6
65 38 25 5 Ma
Plio - pleistocene
100's of m above present sea - level
Figure 4.10 Estimated global eustatic sea level since 570 Ma (Hallam,1984).
have moved northward into the Indian Ocean through numerous fractures in the Southeast Indian Ridge (Kennet et al., 1975). This bottom water slowly grew into a vigorous circulating force, the AABW and consequently inhibited sedimentation and caused erosion at many places. The strength of the circulating water probably decreased by the Oligocene, drawing an end to the hiatus and permitting sedimentation to resume in most places. The AABW, which entered the CIOB through the saddles along the Ninetyeast Ridge at 5 S, sank into the basin and made a westward movement, possibly playing a double role—acting as an erosive agent to remove the top thin veneer of sediment and enriching the sediment and the nodules with additional metal input (Johnson and Nigrini, 1982; Warren, 1982). The NADW, on the other hand, entered through the major fracture zones across the central Indian Ridge on the west leaving a trail of influence on the water circulation pattern in the CIOB. The turbulence in the deep-water movement
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created due to its low stability in this basin (Levitus, 1982) might be responsible for the intense erosion of younger sediments. To understand the nature of sedimentation in the CIOB over the last 300 ka, the isotope decay rates were measured using @ 13C and 230Th. Variations in 13C data (Geochemical Ocean Sections Study, GEOSECS project) in benthic foraminifera from different oceans suggest that the southern ocean is an ideal region to monitor fluctuations in the global influence of the NADW. The nutrient properties of circumpolar deep water reflect the mixing of deeper water masses of the world. The bottom water mass causes sedimentation as well as erosion. To quantify the erosional capability of the bottom water mass, the distribution of 230Th and the index radiolarian species in the sediment cores in the IONF were studied. For this, three sediment cores, NR-1 (sector A, 9.99 S, 77.92 E), NR-21 (sector B, 11.00 S, 78.49 E) and NR-35 (sector C, 11.97 S, 78.49 E) were retrieved using a spade corer (dimensions 20 cm 30 cm 45 cm). The lengths of the recovered sediment columns were 28 cm (NR-1), 32 cm (NR-21) and 23.5 cm (NR-35) from water depths of 5250, 5325 and 5450 m, respectively. The bottom sediment at these localities is siliceous ooze, with <3% carbonate (Nath et al., 1989). Some chemistry of sediments down these three cores and that from a nearby core is given in Fig. 4.12. The exponential decay of 230Th is clearly evident in cores NR-1 and NR-35, which help in determining their average accumulation rates (NR-21 shows A
Warm surface current
Figure 4.11
(Continued)
Deep cold current
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B
Present day Productivity
Depth (km)
0
728 3
752
OMZ
721
754
757 722 731
6 Late Miocene−Early Pliocene Productivity
Depth (km)
0
721 728 3
752
OMZ
754
757
722 731
6 0
4000 Distance (km)
8000
Figure 4.11 (A) Distribution and exchange of warm surface and colder deep water along the conveyor belt in the world’s oceans (Einsele, 2000). (B) Fluctuation in oxygen minimum zone (OMZ): present day and at Late Miocene (Dickens and Owen,1994).
a non-uniform distribution). Again, the ratio of the 230Th flux in the sediments (Fa) to its production rate (Fp) in the overlying water column provides an evidence for sediment erosion on the seafloor (Mangini et al., 1982). The production rate (Fp) of 230Th and the flux of the 230Th to the sediments (Fa) is 2.4 dpm/cm2 ka for core NR-1 and 3.1 dpm/cm2 ka for core NR-35. The respective Fa/Fp ratios (0.18 and 0.22) are far less than that expected under ideal conditions (Fa/Fp ¼ 1) of nonerosion and non-deposition. These extremely low Fa/Fp ratios indicate removal of the younger sediments by some dynamic agent. The expected surface activities of 230Th under ideal conditions of non-erosion and non-deposition are 210 and 161 dpm/g for NR-1 and NR-35, respectively, assuming a constant flux of 230Th. By extrapolating the best-fit decay curves for these values, the total thickness of the sediment column removed should be around 30 and 37 cm, respectively, for the NR-1 (sector A) and NR-35 (sector C) cores. With an average accumulation rate of 2 mm/ka, the effective chronological record thus eroded can be estimated to be around 175 ka (Banakar et al., 1991). Additional evidences of erosion were obtained by studying the distribution of index Neogene radiolarian zone in the NR-1 core. Biostratigraphically important radiolarian species such as Collosphaera invaginata (first appearance datum, FAD, 0.15–0.2 Ma), Collosphaera tuberosa (FAD, 0.4–0.5 Ma), Collosphaera orthoconus
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(FAD, 0.65 Ma), Stylatractus universus (last appearance datum, LAD, 0.42 Ma) and Lamprocystis nigriniae (FAD, 0.9 Ma) were encountered in subsamples of NR-1 core. C. invaginata, the index species of the very first and topmost zone of the Neogene radiolarian biostratigraphy, is absent not only in the upper subsamples but also in the entire length of core NR-1 (Gupta, 1988, 2000; Johnson et al., 1989). C. orthoconus and C. tuberosa normally co-occur in the 0–25-cm depth interval. However, the disappearance of the top zone (Neogene radiolarian zone 2), characterised by C. tuberosa until the 25-cm depth and the absence of S. universus (LAD, 0.42 Ma) throughout core NR-1, indicate that the top sediment layers were eroded. The sediment in the core spans a period between 200 and 400 ka. The age derived for the top 25-cm depth in this core with an accumulation rate of 1.6 mm/ka is around 155 ka. Considering the 175 ka erosion of the sedimentary record based on the Fa/Fp ratio, the time span thus derived for this core, using the radiochemical method, is between 175 and 350 ka, which agrees well with the biostratigraphic age derivation (Banakar et al., 1991).
0 10
Al/Ti (g/g) 10 30
0
Alex (%) 20 40
Biogenic opal (%) Terrg. matter (wt%) 20 30 40 20 25 30
Volcanic glass Abundant
Pass
A
20
Rare
30
Absent
40 NR -1
50 10
30
50 20
40
60
20
30
40
20
30
40
10 20
Rare Pass
Core depth (cm)
0
Abundant
30 NR -21 40 0
10
20
30
20
40
60
20
30
40
20
30
40
20
Pass
Rare 10
30
Figure 4.12 (Continued )
Abundant NR -35
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B
20
Fe/Ti
16 12 8 4 0
Mn/Ti
3 2 1
Ce-anomaly
0 0.10 0.05 0.00
−0.05 −0.10 0
100
200
300
400
500
Core depth (cm)
Figure 4.12 (A) Distribution and concentration of terrigenous material Al, biogenic opal and volcanic glass from coarse fraction in three sediment cores NR-1, NR-21 and NR-35 [PAAS has Al/Ti ratio as 16.7] (Pattan and Shane., 1999). (B) Distribution of cerium (Ce) anomalies and Mn/ Ti and Fe/Ti ratio in the Indian Ocean Nodule Field (IONF) sediment (Pattan et al., 2005).
Mottled zones were encountered between the depths of 6 and 14 cm in the NR-1 and NR-35 cores. Nearly horizontal 230Th decay patterns in these mottled zones can be attributed to sediment mixing due to bioturbation. Similar mottled bioturbated zones between the depths of 10 and 20 cm have been noticed in core NR-21 also (Nath and Mudholkar, 1989). A sharp decrease in nitrate values and high levels of organic carbon in these not-so-infrequent mottled zones in the IONF suggest anoxic conditions. Thus, it can be said that the AABW-induced sediment erosion as recorded in NR-1 and NR-35 and intense bioturbation as shown by all three cores might have provided progressively favourable conditions for maintaining the ferromanganese nodules in the IONF at the sediment–water interface. However, it is difficult to ascertain whether there was a single erosional event lasting nearly 175 ka, or the erosion was accomplished in several intermittent short-lived events.
145
Sediments
The sedimentation rate (in cores NR-1, NR-35, SK226 and SS657) decreases from north to south with corresponding increase in the accumulation of biogenic silica (Table 4.9). The increase in biogenic silica and its accumulation rate towards the south reflect decrease in terrigenous input and increase in siliceous microfossil concentration. For example, intensified monsoon in Asia and the Indian Subcontinent between 125 and 75 ka (Prell and Kutzbach, 1987) appears to have been responsible for discharge of considerable amounts of sediment by rivers like the Ganges and the Bramhaputra resulting in least concentration of biogenic silica to the north of the IONF (Pattan et al., 1992). The effect of bottom water mass on depositional condition can also be shown by the distribution of radiolarians. As the majority of radiolarians inhabit in the upper 100 m of the oceanic water column, their surface distribution in the CIOB indicates three well-defined water masses in the basin (Gupta, 1996). The water masses are (1) high salinity (>34.5/psu), cooler (<27 C) highly productive (>0.2 mgC/m3/h) water mass in the south-western part during the southwest monsoon, represented by high Pyloniids assemblage; (2) comparatively low saline, warmer, low-productive water mass in the north-western part during the southwest monsoon, characterised by high Euchitoniids assemblage; and (3) a transitional one in the remaining areas. Moreover, down-core fluctuation of the ratio of Pyloniids to Spongodiscids could be a function of monsoon. Because of the CIOB’s proximity to the equator, these oceanographic variations can be strongly related to long-range transfer function of monsoon intensities (Prell and Kutzbach, 1987). Sedimentation in this basin is further influenced by a pronounced front of the bottom water at 10 S latitude, with the characteristic hydro-chemical structure separating the reversing monsoon and the subtropical gyres. Seasonal reversal of winds and currents during the monsoon is also noteworthy due to its influence on the distribution of nutrients. Spectacular monsoon reversal in the northwest Indian Ocean involves northeastward flow of the Somali Current between April and September, which supports strong upwelling and high biological productivity.
Table 4.9 Concentration and accumulation rate of biogenic silica, rate of sedimentation and sediment density in different sectors of the IONF Biogenic silica
Sector
A B C D
Water depth (m)
Core length (cm)
Concentration (%)
Accumulation rate (g/cm2/ year)
Sedimentation rate (mm/ka)
Density (g/cm3)
5250 5325 5450 5270 5050
41 35 28 32 36
24.71 33.54 30.41 26.76 22.59
1.14 105 – 1.69 105 4.05 105 1.45 105
1.6 4.6 2.2 2.0 2.0
0.287 0.287 0.254 0.757 0.321
Sources: Banakar et al. (1991), Pattan et al. (1992) and references therein. IONF: Indian Ocean Nodule Field.
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In contrast, during October–March, water current flows towards the southwest, without any upwelling activities. The record of biogenous sedimentation suggests that the monsoon has been active since the Mid Miocene (10–12 Ma). Upwelling and consequently monsoon was stronger during glacial and weaker during interglacial periods, such as at present. Also, the monsoon and related atmospheric circulation in the CIOB depend on the uplift of the Himalayas (more rapid during the last 3 Ma), as this huge mountain chain acts as a barrier between high atmospheric pressure during winter and low atmospheric pressure during summer in the Indian Subcontinent ( Johnson and Nigrini, 1982; Kolla and Kidd, 1982).
2.3. Depositional environment The distribution of rare earth elements (REE) in sediment reflects the environment of deposition, and varies as a function of lithology, sediment type and bottom water condition (Fig. 4.13). As an example, the REE concentration in a sediment core from the IONF is furnished in Table 4.10. The total REE concentration in the carbonate sediment in the western part of the IONF is minimum (average 80 ppm), increases to a maximum in detrital clay in the northern part (average 183 ppm) and shows moderate concentration in siliceous sediment (average 169 ppm). The REE were carried to the IONF sediments in two phases—Ti phase (detrital indicator) and Mn phase (oxide indicator). The REE concentration pattern in the Ti and Mn phases has been contrasting—in the detrital phase REE concentration decreases from La towards Lu, while in the oxide phase concentration records a reverse trend (Nath, 2001; Pattan and Banakar, 1997). Distribution of REE, and in particular cerium (Ce) anomalies, in marine sediments are considered as indicators of depositional environment, such as, the limit of the spreading of marine anoxia (Liu et al., 1988), spread of the AABW (Glasby et al., 1987) and variations in surface productivity. In the IONF, a southward evolution of a Ce anomaly in sediment from negative to positive is evident. There is also a consistent hump across the middle rare earth elements (MREE—Sm, Eu, Gd and Tb) and a noticeable Eu anomaly (1.2). The observed Eu anomaly suggests considerable aeolian input of detrital component to the deep-sea sediment of the IONF (Banakar et al., 1998). In the calcareous sediment, the Laxs, Cexs and Ybxs proportions are 35, 13 and 44% of their respective bulk component and in siliceous sediment Laxs remains unchanged, while Ybxs and redox-sensitive Cexs increase to 54 and 40%, respectively. A positive association among opal, Mnxs and Ybxs might suggest a major role of Mn-oxide coating on biogenic-settling particles in fractionating heavy REE (HREE) and Ce. The detrital clay zone has no excess La, Ce or Yb, as expected. Laxs does not yield any systematic latitudinal trend. In siliceous sediment, the Ce anomaly becomes prominent (up to 1.5) and the enrichment of HREE over LREE (Lan/Ybn ¼ 0.6) becomes more significant than in the calcareous zone (0.8). LREE is more prone to the adsorptive removal from the water and should lead to LREE-enriched patterns in the sediments (Elderfield, 1988). On the contrary, an HREE-enriched distribution might indicate a mixed mechanism. The regeneration associated with the release of carbonyl ions to the deep water might provide large scope for the deep-water complexation of
147
Sediments
2
Terrigenous sediments (type-I)
1
2533 2532, 2535 2501 2483, 2513 2537
0.4 2
Siliceous clays (nod. free reg.) (type-II)
1
101, 150 F156, F157 153
0.5 2
Siliceous clays (overlain by nods) (type-III)
1
NR120D NR124, 2528 NR129
0.4
Sample/shale
1
Calcareous sed. (type IV)
CaC
O : 3 47 %
CaC
O : 3 62%
CaCO
3
0.1
: 87%
S121 S124 2494 S127
0.05 6
Pelagic red/clays (type V) 210 164, 144 132 148 160 136 150, 231
6 0.6 1
MORB
0.07 La Ce Nd Sm Eu Gd Dy
Yb Lu
reg = region, nod = nodules, sed = sediment, MORB = Mid Ocean Ridge Basalt
Figure 4.13 Shale-normalised characteristic pattern of rare earth elements (REE) in various sediment types in the Indian Ocean Nodule Field (IONF). Flat pattern holds no signature of fractionation between light rare earth element (LREE) and heavy rare earth elements (HREE). Absence of Ce anomalies may indicate a mixed continental source (Nath, 2001).
regenerated REE (Murray et al., 1991; Piper, 1974). The cumulative effect of this, in addition to precipitation of colloidal ferromanganese oxide particles, might have left HREE-enriched patterns in the sediments along with the more pronounced Ce anomaly. Remineralisation of oxide coating at depth has been understood to balance
Table 4.10
REE concentration (ppm) in a sediment core from the IONF
Core depth (cm)
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Total REE
000–004 020–022 032–034 048–050 060–062 076–078 096–098 108–110 124–126 136–138 142–144 148–150 165–170 180–185 195–200 210–215 230–235 255–260 285–290 320–325 335–340 350–355 365–370 400–405 425–430 450–455 475–480 490–495
31.75 26.19 29.81 31.11 35.91 38.86 37.40 37.52 36.61 44.42 44.64 40.20 38.62 47.49 44.33 42.15 46.40 43.51 38.10 46.95 44.28 43.47 44.01 49.94 53.82 63.71 58.02 60.33
82.45 72.50 75.70 81.53 93.97 93.64 93.24 88.63 86.15 102.37 103.41 92.62 86.24 103.98 102.55 97.99 108.45 108.08 95.69 101.77 101.18 103.98 105.39 110.66 108.65 113.46 112.65 112.47
8.07 6.64 7.38 7.76 9.23 9.97 9.91 9.88 9.42 11.05 11.20 10.21 9.82 11.89 10.98 10.63 11.66 11.11 9.58 12.26 11.09 10.93 11.16 12.79 13.36 15.6 13.93 14.54
35.44 28.86 32.23 34.47 41.55 44.39 44.50 43.55 42.32 49.09 49.90 45.87 44.17 53.11 49.06 48.14 51.29 49.11 41.79 55.30 49.87 48.16 49.98 57.97 60.20 69.24 62.92 64.54
9.28 7.69 8.39 9.47 11.28 12.33 11.92 11.61 11.79 13.11 13.36 12.34 11.60 14.27 13.58 12.89 13.16 12.61 11.02 13.99 13.51 12.64 13.02 15.83 15.94 17.79 16.12 16.67
2.38 1.96 2.05 2.14 2.80 3.06 3.16 2.99 2.81 3.16 3.30 3.15 2.86 3.50 3.31 3.15 3.35 3.21 2.77 3.37 2.85 2.99 2.97 3.32 3.47 3.99 3.63 3.53
8.49 6.77 7.36 8.19 9.65 10.45 10.46 10.21 9.86 11.68 11.56 10.79 10.36 12.53 11.52 11.11 11.90 11.11 9.63 12.19 11.54 10.90 11.32 13.02 13.43 15.54 14.25 13.92
1.37 1.13 1.24 1.32 1.67 1.76 1.82 1.74 1.68 1.94 1.93 1.76 1.75 2.09 1.90 1.86 1.93 1.82 1.58 2.12 1.88 1.83 1.91 2.26 2.35 2.72 2.39 2.49
8.19 6.87 7.27 8.47 9.93 10.77 10.72 9.97 10.58 11.7 11.82 11.17 10.95 13.01 12.09 11.62 11.54 10.88 9.57 12.02 11.73 10.94 11.77 14.12 14.55 16.51 14.82 15.23
1.74 1.36 1.55 1.68 1.96 2.13 2.09 2.09 2.09 2.41 2.48 2.17 2.16 2.60 2.40 2.33 2.37 2.22 1.98 2.53 2.38 2.19 2.29 2.75 2.99 3.52 3.07 3.11
4.30 3.59 3.91 4.40 4.97 5.29 5.18 5.08 5.10 6.21 6.08 5.86 5.62 6.73 6.20 6.06 6.10 5.55 4.88 6.45 6.21 5.57 5.77 7.07 7.73 9.00 7.87 8.01
0.57 0.45 0.54 0.59 0.63 0.67 0.72 0.66 0.67 0.82 0.81 0.71 0.71 0.86 0.78 0.77 0.81 0.72 0.62 0.87 0.78 0.70 0.75 0.90 1.01 1.20 1.02 1.03
3.76 3.14 3.56 3.79 4.29 4.62 4.54 4.44 4.58 5.60 5.42 5.04 4.98 5.72 550 5.20 5.32 4.76 4.36 5.55 5.42 5.06 4.96 6.18 6.62 7.93 7.00 6.72
0.54 0.44 0.52 0.54 0.62 0.66 0.66 0.67 0.63 0.81 0.79 0.71 0.69 0.82 0.76 0.71 0.77 0.71 0.60 0.81 0.72 0.67 0.68 0.83 0.95 1.16 0.99 1.00
198 167 182 196 229 239 236 229 224 264 267 243 251 279 265 255 275 265 232 276 266 263 266 297 305 341 319 324
Source: Pattan et al. (2005). Note: IONF, Indian Ocean Nodule Field; REE, rare earth elements.
Sediments
149
the preferential removal of LREE in the upper water column (Sholkovitz et al., 1994). The very strong association of Ce with Mn in both bulk and excess element correlation matrices and the southward positive evolution of the Ce anomaly are suggestive of burial of Ce via the Mn-oxide phase (Banakar et al., 1998). REE concentration, which is generally higher in the upper oxic zones, suggests its upward diffusion through the sediment column and subsequent incorporation in the oxyhydroxide phase (Pattan and Banakar, 1997). REE fractionations are found to be characteristic for each sediment type—for example, flat shale-normalised patterns for terrigenous sediment, positive Ce anomaly in siliceous sediment, negative Ce anomaly in calcareous sediments and LREE-depleted patterns in pelagic red clay. Based on this, Nath et al. (1992) suggested that REE signatures and fractionations in the IONF are indicative of depositional setting, lithological variations and surficial diagenetic processes. The concentration and accumulation rates of 10Be in different sediments of the IONF also suggest extensive removal of the isotope along the continental margins towards regulating its distribution in deep sea sediments (Nath et al., 2007).
2.4. Diagenesis The sediments in the IONF are overlain in most areas by ferromanganese nodules and crusts. Although the seawater largely contributes metals for the formation of these manganese oxides through chemical precipitation (hydrogenous method), a significant contribution of metals comes through diagenetic remobilisation. Pore water plays an important role in such diagenetic reactions. Consequently, detailed pore-water studies were made to determine nutrients, estimate diffusive fluxes and geochemical balances, understand the nature and sequence of oxidation and oxidants and the role of these in diagenesis of the organic matter. Some of the organic matters, CaCO3 and opaline silica, which form in the surface water due to biological productivity, settle to the bottom where they undergo decomposition and dissolution. Organic matter decomposition follows a sequence of processes depending upon the availability of the oxidant, involving successive utilisation of O2, NO3, MnO2, Fe(OH)3 and SO4 (Berner, 1982). The initial increase in nitrate at the surface sediment reflects oxygen respiration coupled with nitrification. At least two sediment cores (both from sector C) show mottling/bioturbation in the intermediate layer between 10 and 25 cm. This intermediate layer records a sharp fall in the nitrate values and along with increased levels of organic carbon in the solid phase indicates the effect of bioturbation and consumption of oxygen. This suggests that oxic and anoxic processes are active at different intervals. The subsurface layer between 10 and 18 cm in the studied cores shows oxic diagenesis, while the layers beyond these depths show the existence of anoxic conditions and hence favour denitrification. Thus, it appears that the process is being controlled by the reaction rates and bioturbation of sediments (Nath and Mudholkar, 1989). The relation between early diagenetic process and the nutrient levels in the pore water are examined from three sediment cores spread over different sedimentary regimes (Fig. 4.14). Interesting results with no diffusion gradient in nitrate
150
Mukhopadhyay, Ghosh and Iyer
Nitrate 20 25 30 35 70E
0
Organic carbon 2 4 6 8
80 INDIA Chennai
N
SRI LANKA
Nitrate 20 30 40 50
247
0
Organic carbon 2 4 6 8
0
Organic carbon 2 4 6 8
m
CIOB 246
5000
226
0
Nitrate 10 20 30 40
Core 226
10S
C EN TRAL I NDIAN
RID G
E
0
E C HA G OS LA CC ADI VE RID G
185
10N
Core 247
200 km
Core 246
0
Goa
Figure 4.14 Pore-water chemistry: concentration of nitrate and organic carbon in three sediment cores (Nath, 2001). Location of samples: 247 ¼ IAPB region, 246 ¼ sector A and 226 ¼ sector C.
concentration are found in terrigenous sediment regime (sample 247 in India-Australia Plate Boundary (IAPB) zone), whereas sediments from the southern siliceous area (sample 246, sector A) and pelagic clay area (sample 226, sector C) do show such gradient in diffusion of nitrate. This probably indicates a strong relationship between
Sediments
151
nature of pore-water chemistry and pulses of turbiditic sedimentation (Nath, 2001). The fact that early diagenetic processes control the pore-water nutrient levels has also been demonstrated in a study of another two cores from the IONF. The cores—one each from sectors B (NR-21) and C (NR-35)—show nitrification in the lithologically more uniform upper layers. At intermediate depths, the bioturbated zones contain organic carbon, which was probably not fully decomposed and showed signs of denitrification. Core NR-21 showed reworking and thorough mixing of the sediment throughout its length, as is evident from the nearly horizontal decay profile and the highly variable distribution of 230Th (Table 4.8). Core NR-35, on the other hand, provides typical transition metal profiles. The minimum solid-phase Mn is encountered at intermediate depths without any spikes in the transition metals and an increase of Mn, Ni and Cu towards the surface indicates anoxic diagenesis (Dymond et al., 1984), which facilitates the diagenetic growth of nodules. In sector A also, the oxidation of remobilised Mn2þ appears to have taken place down core (NR-1). The formation of micronodules below the surface of the sediments and the down-core oxidation of remobilised Mn2þ probably act favourably for transition metal movement to the surface to form ferromanganese nodules (Banakar et al., 1991; Mudholkar et al., 1993). Additional evidence for sediment mixing comes from recrystallised biogenic opal, identified by a peak of 3.31–3.34 A˚ in X-ray diffraction (XRD) of the bulk sediments. The intensity of the peaks was normalised to one for each core. In core NR-21, the normalised intensity varies within 16%, while in the other two cores it varies within about 25%. Low variation in the intensity of the biogenic opal in core NR-21 (in sector B) might therefore be due to mixing of the older and younger sediments, leading to the nearly uniform distribution of biogenic opal with depth, unlike the random distribution pattern in the other two cores. It appears from the solid-phase Mn distribution in NR-21 that the diagenetic reactions and remobilisation of Mn were established subsequent to the redistribution of the sediment, which otherwise would have resulted in uniform distribution of solid-phase Mn within the core. However, later diagenetic reactions in this core have not influenced the 230Th distribution, which preserved the evidence for mixing and support the validity of the 230Th method of dating.
2.5. Sediment consolidation The physical processes associated with sedimentation and its consolidation is expected to have a profound bearing on the engineering characteristics of the sediment. Any information about those physical processes helps to reveal the degree of consolidation, including nature of compaction and plasticity of sediment. Knowledge of these geotechnical properties is also considered essential in order to understand the behaviour of sediments under dynamic loading, which is required for designing any seafloor mineral mining system. To ascertain the post-precipitation characteristics of sediments in the IONF, several sediment cores were examined (Khadge, 2000, 2002). The cores were recovered from three sectors in the IONF—1 gravity box core (at 9 S, 77 E) and 15 spade cores (between 10 S and 10 100 S, 75 E, and 76 E) from sector A, and 6 spade cores from sector C (between 11 550 S and 12 150 S, 75 450 E, and 76 E).
152 Table 4.11
Mukhopadhyay, Ghosh and Iyer
Physical properties of sediments from the central region of the IONF Maximum
Minimum
Average
Sector B Specific gravity Porosity Wet bulk density Void ratio Water content Shear strength Liquid limit Plastic limit
– (%) (g/cm3) – (%) (kPa) (%) (%)
2.86 94.4 1.21 16.9 515 4.79 280 156
2.15 84.8 1.10 5.6 410 1.63 160 48
2.32 91.1 1.15 10.6 460 3.12 228 108
Sector C Specific gravity Porosity Wet bulk density Water content Shear strength Liquid limit Plastic limit
– (%) (g/cm3) (%) (kPa) (%) (%)
2.60 94 1.27 553 10.1 280 166
1.88 88 1.12 380 1.6 206 103
2.16 90.5 1.13 441 3.77 233 127
Source: Khadge (2000, 2002). Note: IONF, Indian Ocean Nodule Field.
Incidentally, all these cores are bottomed by siliceous clay/ooze, at depths well below the CCD. Table 4.11 shows variation in physical properties of seafloor sediments. The sediments at the northern part of sector A contains abundant radiolarian tests and four clay minerals—montmorillonite (in predominate quantity), illite, kaolinite and chlorite. Water content, which varies with the textural change and clay mineralogy, decreases with depth (Fig. 4.15). To a depth of 470 cm, water content is consistent up to a value of 370% and then a decrease occurs down core. This water content is calculated on dry-weight basis that corresponds to about 81% on wet-weight basis. Overburden pressure of the sediments might have squeezed the water out. In contrast, sediments in the southern parts of sector A contain either smectite or illite as the dominant mineral phase. The water content calculated in dry-weight basis ranges from 369 to 577% in the north and from 380 to 553% in the south. The high water content is probably due to the abundant presence of smectite (including montmorillonite). Both these minerals have an expandable lattice structure and consequently have the highest water-holding capacity. This capacity was further augmented by the presence of about 60% microfossil/radiolarian tests, which are hollow and could therefore be filled with water. The Attenberg limits show consistency in depth-wise variation with water content. The liquid limit (LL) and plastic limit (PL) of the sediment, which reflect the capacity to withstand load, show higher values in the southern parts of sectors A and C (LL ¼ 223–239% and PL ¼ 105–136%) than in the northern areas of
PL, LL, WC (%) 0
100 200 300 400
Sp. Gr.
Wet Porosity (%) Density (gm/cc)
2.0 2.5 70
80
90
1.1 1.3 1.5
Shear strength (kPa) SD/ST/CL (%) 2
6
10
14
0
0
50 100
CL
SD
*
M/I/K + C (%)
50 100
3.1 3.9 4.7
PL
M
LL
Illite
K+C
WC Silt
Depth (m)
2.3
5.5 6.3 7.1
PL = Plastic Limit, LL = Liquid Limit, WC = Water Content, M = Montmorillonite, I = Illites, K = Kaolinite, C = Chlorite, SD = Sand, ST = Silt, CL = Clay, * = Disturbed sediment, not studied
Figure 4.15 Variation in physical properties down the sediment column. Note drastic change at 470-cm depth corresponding to Pliocene^ Pleistocene boundary (Khadge,1998).
154
Mukhopadhyay, Ghosh and Iyer
sector A (average LL ¼ 20%, average PL ¼ 109%; Table 4.11). The shear strength of the IONF sediment shows a depth-wise increase from the surface to the bottom and from the north to the south (2.2–13 kPa at 9 S, 1.6–7.2 kPa at sector A, and 1.6–5.4 kPa at sector C). The surface and bottom sediments are highly bioturbated and show medium to high plasticity. The value of shear strength of the IONF sediments is, however, lower than those reported from the nodule-rich areas of the Pacific Ocean (10–19 kPa; Hirst and Richards, 1975). The sediment column in the IONF shows an unconformity at a depth of about 470 cm (Fig. 4.15). The sediment at this depth is characterised by high-specific gravity, high density and low porosity compared wih lower values for sediments above and below this level. For example, the sediments of the sectors A and C are in general characterised by these average values: specific gravity 2.18, wet bulk density 1.14 g/cm3 and porosity 90.2%; these abruptly changes to 2.5, 1.54 g/cm3 and 71%, respectively, at 470-cm depth. Such an abrupt change may have been caused by variation in mineralogy (such as increase in montmorillonite from 40 to 67% at this depth) and increased degree of bioturbation in the sediment (Khadge, 2000, 2002). Again, the shear strength data, which reflect on the bearing capacity and settlement characteristics of sediment, spiked to 12.8 from 10 kPa on the top and lower layers. The unconformity at 470 cm, described earlier, spans about 16 cm in thickness and can be calibrated to the depositional history at the Pliocene–Pleistocene boundary. The clay mineralogy and physical properties show drastic changes at this layer, indicating the change in sedimentation characteristics. The drastic increase in montmorillonite content, spike in shear strength values and the presence of pumice at this depth imply change in mineral composition caused by increased input from ridge-weathered rock-sediment during the Early Pliocene. The total absence of radiolarian fossils at this depth also indicates either climatic change or volcanic activity that precluded their preservation. In summary, the IONF encompasses three major sediment types—siliceous (80% area, having positive Ce anomaly), pelagic/red clay (10%, LREE depleted) and terrigenous (10%, flat shale-normalised REE pattern). Mineralogically, while illite, chlorite and kaolinite show increase in abundance from south to north (sectors D to A), smectite and montmorillonite show an opposite trend. About half of the silica is contributed to the CIOB biogenically. Secondary volcanic material may have been sourced from both local basin eruptions and IVA. The process of sedimentation in the IONF essentially depends on dissolution of carbonate, circulation of bottom water mass and depositional mechanism (diagenesis and consolidation).
C H A P T E R
F I V E
Ferromanganese Deposits
Contents 157 157 159 161 163 165 167 174 176 178 182 184 185 188 189 192 193 194 194 195 196 202 205 205 208 211 214 220
1. Nodule Characteristics 1.1. Distribution 1.2. Physical constitution 1.3. External morphology 1.4. Nodule nucleus 1.5. Mineralogy 1.6. Chemistry 1.7. Internal structure and growth 1.8. Age of IONF nodules 1.9. Buried nodules 1.10. Micronodules 2. Factors Influencing Nodule Formation 2.1. Topography and nodule distribution 2.2. Morphology and nodule chemistry 2.3. Acoustically transparent sediment layer 2.4. Role of secondary eruption 2.5. Sedimentation, bottom currents and environment 3. Dynamics of Nodule Formation 3.1. Source of elements 3.2. Nodule at the sediment–water interface: A paradox 3.3. Processes of nodule formation 3.4. Model of nodule formation 4. Ferromanganese Encrustation 4.1. Occurrence and characteristics 4.2. Crust and paleoceanography 5. The World Oceans Scenario 5.1. Ferromanganese resources from the world oceans 5.2. Inter-basin model for nodule growth
Ferromanganese deposits occur both as nodular and as encrustation forms, and have attracted intense focus over the decades for their commercial potentialities and academic significance. From a commercial angle, the nodular varieties (nodules) are considered more rewarding than the encrusted forms (encrustations). Ferromanganese deposits cover about 46 million km2 of the ocean floor, at water depths ranging Handbook of Exploration and Environmental Geochemistry, Volume 10 ISSN 1874-2734, DOI: 10.1016/S1874-2734(07)10005-X
#
2008 Elsevier B.V. All rights reserved.
155
156
Mukhopadhyay, Ghosh and Iyer
from 3 to 6 km (Fig. 5.1). The nodule deposits are typical two-dimensional bodies, most commonly occur at the sediment–water interface and vary in abundance from being thinly scattered in some places to densely populated in other areas, often exceeding 10 kg/m2. Compositionally, although about one-third of the nodule is composed of manganese and iron, studies on deep-sea nodules, however, gained considerable momentum when it was realised that the nodules contain smaller amounts of a number of more valuable metals like, nickel, copper, cobalt, molybdenum, vanadium, lead and titanium. These metals are in short supply from the fast depleting resources on land. Hence, all the three major oceans have been extensively investigated to locate nodule deposits of economic grade (Table 5.1). In addition, advancement in technology for deep-sea exploration, mining and metal extraction processes is making deep-sea ventures feasible. A conservative estimate puts the metal potentiality of ferromanganese deposits in the Pioneer Area within the Indian Ocean Nodule Field (IONF) at 759 MMT (million metric tons), comprising of 144 MMT of manganese, 7 MMT of nickel, 6.5 MMT of copper and 0.85 MMT of cobalt (Survey for Polymetallic Nodules Project Report) among others. The area retained by India in the IONF after surrendering 50% of the allotted area to the United Nations Convention on Law of the Sea (UNCLOS) is estimated to contain about 382 MMT of metals, of which 95, 4.5, 4.4 and 0.42 MMT, respectively, of manganese, nickel, copper and cobalt are present. Presently available mining technology is capable of recovering a little more than 1000 tons of nodules per day, which means that the resources of the IONF would last for many hundreds of years.
North America
Europe Asia
Africa South America Australia
Nodule present
Nodule abundant
Figure 5.1 Distribution of ferromanganese deposits in the world oceans (Cronan, 1980; Rawson and Ryan,1978).
157
Ferromanganese Deposits
Table 5.1
Ocean
Distribution of manganese nodules in the world oceans Northern Hemisphere ( )
Atlantic 18–35 Indian
00–11
Pacific
4–30
Southern Hemisphere ( )
Stations covered
Number of analysis
Ni þ Cu (%)
30–40 50–60 10–20 32–40 20–46 55–63
307
555
0.45
2391
1926
0.66
2082
5144
1.29
Sources: Rawson and Ryan (1978), Andreev et al. (1984), Data Banks of Scripps Institution of Oceanography [USA] and Survey for Polymetallic Nodules [India]. Note: Stations covered and follow up analyses represent the entire oceans.
1. Nodule Characteristics 1.1. Distribution In several areas of the world oceans including the Indian Ocean, a congenial environment for nodule formation exists (Fig. 5.1). Broadly, an area in excess of 4000 m water depth, siliceous bottom sediment, availability of sufficient seeds, considerable supply of metals and low rate of detrital sedimentation are the essential conditions for nodule formation. Abyssal hill topography and the slopes of small seamounts add favourably to these conditions. In the Indian Ocean, manganese nodules cover about 10–15 million km2 of the deeper regions, with ferromanganese nodules and crusts occurring in several basins. A majority of these basins contain sub-marginal grade of nodules (Cu þ Ni þ Co < 2%) and may not encourage the metal-market for the time being. However, the areas located to the south of the equator in the Central Indian Ocean Basin (CIOB) appear to have the highest metal grade (three-metal grade ¼ 2.0–2.4%) and the best abundance (>5 kg/m2) of nodules. Such abundance and metal concentration represent para-marginal grade. In the CIOB, the largest basin in the Indian Ocean, nodules are largely brown to black in colour with variable morphology (Fig. 5.2), and are mostly located to the south of 10 S. As mentioned in previous chapters, the area with the highest nodule abundance and grade in the CIOB comprises the IONF. This field is divided into four sectors—A, B, C and D—and roughly bordered by 10 S to 16 300 S and 72 E to 80 E (see Fig. 1.2). Nodules are most abundant where the bottom sediment is predominantly siliceous clay and as the IONF is largely covered with siliceous sediment, the seafloor coverage of nodules at many localities in this field often exceeds 75%. In contrast, pelagic–red clay dominates in the southern part of the CIOB (south of the IONF) where nodule abundance is moderate and distribution patchy. In terms of primary productivity (100–150 wgC/m2/day, Parsons et al., 1977) and sedimentation rate (3 mm/1000 years, Udintsev, 1975), the IONF appears to be the ideal site to form enriched nodules. The IONF has an average
158
Mukhopadhyay, Ghosh and Iyer
Figure 5.2 Morphological variations in ferromanganese nodules of the Indian Ocean Nodule Field (IONF). Note shark tooth acting as nucleus (bottom).
water depth of 5000 m and possibly holds the only mineable area for ferromanganese deposits in the Indian Ocean. Since the early 1980s, the CIOB was explored to delineate economically workable manganese nodule-bearing areas. Extensive investigations over an area of 4 million km2 later helped demarcate a Pioneer Area (300,000 km2) consisting of two Application Areas of about 150,000 km2 each. The two Application Areas record a combined estimated nodule resource of about 1.3 billion metric tons. Further, close-grid exploration since 1987 focused on three parameters: high nodule abundance, rich metal grade and comparatively smooth unobstructed topography with a slope preferably <3. The relative weights of these three parameters (abundance, grade and topography) were assessed to identify the mineable potential of the
Ferromanganese Deposits
159
nodule deposits by dividing the total Pioneer Area into several smaller blocks. The combined values of the three parameters were determined for each block to help decide which half of the Pioneer Area is to be relinquished to the International Seabed Authority (ISA). This study also indicated the presence of a highly probable First Generation Mine Site within the IONF. This is the site wherein the nodule abundance is >5 kg/m2 and the grade (Ni þ Cu þ Co) is >2%. Resource estimates have shown that the IONF (square block in Fig. 1.2) represents the second largest manganese nodule-rich area in the world oceans, after the Equatorial North Pacific (ENP) nodule belt (Mukhopadhyay et al., 2002).
1.2. Physical constitution Acoustic and physical properties of IONF nodules were studied in some detail (in both dry and wet conditions). The non-destructive sound (acoustic) propagation method provided useful information on the nature of nodules. A pair of longitudinally and transversely oscillating transducers was used to measure compressional (Vp) and transverse (Vs) wave speeds through the nodules, respectively, following pulse transmit time method. The coefficient of anisotropism was calculated by dividing the maximum velocity by minimum velocity in different directions. Attenuation can be measured by determining the loss of acoustic pulse per unit length during propagation through the nodules. IONF nodules are porous and fragile, their density commonly varying between 1.8 and 2.4 g/cm3 (Table 5.2). The nodules contain water up to half their weight. The Indian Ocean nodules have an average density of 2 g/cm3, 56% porosity and 46% water content. IONF nodules are anisotropic, and smaller nodules have been found to be more anisotropic than the larger ones. Usually less dense nodules, due to their low internal coherence, show greater energy loss during sound propagation. In comparison to nodules from other sediment domains, those occurring on siliceous sediments appear denser and more porous, and with higher Mn/Fe ratios, they allow P waves to propagate at higher speeds. A comparative study of the Indian, Atlantic and Pacific ocean nodules showed a gradual decrease in S velocity from the Indian Ocean nodules to those of the Atlantic and the Pacific oceans, and an increase in P velocity (and porosity) along the same oceanic sequence (Table 5.2). A mutually dependent relation between acoustic properties and physical properties such as size, density and porosity possibly exists, but the role of chemical composition on the acoustic character of nodules is not yet clearly known. The study also revealed a direct proportional relationship between porosity and density, which along with anisotropy tend to decrease with nodule size. Acoustic propagation (P-wave velocity) through IONF nodules increases by about 18%, and density by 31% with increase in water content. The P-wave travels faster in larger nodules. Attenuation also follows the same trend (Mukhopadhyay and Ramana, 1990). Nodules are very commonly ornamented on their external surface by mammillae of different size and relief. Mammillae are localised tumorous growths of oxide precipitation. To constrain the influence of topographic variations on surface texture and physical properties of nodules, a part of the acoustical, morphological and physical data was treated statistically. One-way analysis of variance (ANOVA)
160 Table 5.2
Mukhopadhyay, Ghosh and Iyer
Physical properties of manganese nodules of the IONF
Density (g/cm3)
Nodule size class Small 2.04 (20–40 mm) Medium 1.98 (40–60 mm) Large 1.90 (60–80 mm) Very large 1.84 (>80 mm) Manganese 2.66 crust Nodule surface texture Smooth 1.99 Rough 1.98 Nodule bottom sediment types Calcareous 1.68 Siliceous 1.96 Pelagic 2.00 Nodules from different oceans Pacific 1.95 Atlantic 1.94 Indian 1.98
Porosity (%)
Water content (%)
P-velocity (m/sec)
S-velocity (m/sec)
63.03
44.89
2081
1076
63.01
46.45
2126
1046
61.50
52.10
2160
1182
57.90
45.90
2192
1107
16.60
6.70
5140
2904
61.26 63.48
46.56 47.36
1696 2179
1049 1114
54.67 62.98 58.92
48.25 45.73 41.92
1926 2144 1896
0952 1101 1075
38.96 55.06 62.44
2363 2425 2589
2173 1728 1093
Sources: Sundkvist (1983), Ma et al. (1986), Mukhopadhyay (1988) and Ghosh and Mukhopadhyay (1991).
and multiple analysis of variance (MANOVA) of variance-covariance matrix were conducted to test variations in nodules from sector C. A highly significant variation was observed, except for density, between topographically elevated locale and abyssal plain area at a ¼ 0.05. Nodules from elevated location along a seamount profile showed the highest density (2.17 g/cm3) and porosity compared to nodules from the abyssal areas (1.92 g/cm3; Mukhopadhyay and Banerjee, 1989). This may be because the nucleus of nodules occurring at elevated areas is composed largely of basaltic fragments. Magnetic properties of nodules are examined by using different techniques such as natural remnant magnetisation (NRM), susceptibility and high-field hysteresis. Magnetic susceptibility and high-field hysteresis studies on about 50 nodules from the IONF showed that, apart from the bulk paramagnetic material, very little magnetic material occurs in super-paramagnetic (SP) state, and the relative susceptibility (RS) falls with increase in Fe and Ti content in nodules (Valsangkar, 1994). Thermal properties of ferromanganese deposit in the IONF also offer interesting information. Nodule and crust behave differently, and even within various types of
161
Ferromanganese Deposits
nodules. When heated up from 120 C to 1000 C, the IONF nodule loses mass weight from about 16 to 31% and ferromanganese crust loses mass weight from about 22 to 28%. Similarly, with increase in temperature, the surface area of nodules changes. The surface area increases from 39 m2/g for nodules and 55 m2/g for crusts at the room temperature to 59 m2/g for nodules and 118 m2/g for crusts at about 400 C. Such increase in surface area, however, drastically comes down with any further increase in temperature above 400 C (Table 5.3).
1.3. External morphology The principal external morphological parameters of nodules are size, shape and surface texture. A field classification of nodules involving these three parameters has been suggested, for example, l(D)r ¼ large discoid nodule with rough surface or s(E)s ¼ small ellipsoidal nodule with smooth surface (Raab and Meylan, 1977; Halbach and Oz¨kara 1979). However, this classification did not find wide application because of the frequent local variability of nodule morphology. Perhaps, a straightforward detailing of the morphological parameters is a better procedure in the large-scale characterisation of nodules. The diameter of nodules in the IONF varies from <10 mm to several centimetres but is more commonly between 20 and 60 mm. Based on their size, nodules are classified into five types (Table 5.4): very small (<20 mm), small (20–40 mm), medium (40–60 mm), large (60–80 mm) and very large (>80 mm). Depending on the environment and depth of formation, larger or smaller nodules may locally dominate, giving rise to mixed nodule populations (see also Fig. 5.2). Nodules in the IONF show variable shapes. Akin to the earlier descriptions from other oceans (Friedrich et al., 1983; Glasby, 2000; Glasby et al., 1982, 1983), smaller nodules in the IONF are generally spheroid to sub-spheroid, whereas larger nodules tend to be elongated, discoid, flattened or irregular. An increase in size appears to lead to an increase in elongation or irregularity of the nodules and hence the degree of nodule sphericity is inversely proportional to the nodule size (cf. Ghosh and Table 5.3
Thermal properties of IONF manganese nodules
Loss of mass (wt%) on heating for 4 hrs Type @120 C @400 C Mononodule 16.50 21.53 Polynodule 15.80 20.96 Mn Crust 22.15 33.36 2 Change in surface area (m /g) with temperature Type @ Room @120 C temperature Mononodule 39 39.3 Polynodule 40 103.8 Mn Crust 55 52 Source: Parida et al. (1997).
@600 C 27.83 23.54 37.35
@800 C 30.56 27.52 41.57
@1000 C 30.92 28.49 42.20
@300 C @400 C @500 C
@600 C
52.2 115 146
7.2 13 05
59 118.7 71.1
22.2 59.4 27.4
162 Table 5.4
Mukhopadhyay, Ghosh and Iyer
Size distribution of nodules in the IONF in terms of morphological types
Nodule size (mm)
Surface texture Rough Smooth Shape Spheroidal Ellipsoidal Discoidal Elongate Irregular Polynucleate
Very small (<20)
Small (20^40)
Medium (40^60)
Large (60^80)
Very large (>80)
87.50 12.50
68.62 30.00
68.64 26.69
69.23 26.92
66.67 33.33
69.09 28.04
4.50 4.50 – – – –
51.35 22.97 5.86 1.80 18.02 17.84
32.05 38.46 1.92 2.56 25.00 25.08
13.32 13.33 6.67 33.33 33.34 25.71
– – 75.00 100.00 – 25.00
40.95 28.33 4.52 5.00 21.19 21.38
Average
Source: Valsangkar et al. (1992).
Mukhopadhyay, 1999). The lowering of sphericity with the increase in nodule size is caused when the nodules attaining a mature stage resist bottom currents to make them rotate on the seafloor. The nodules then tend to grow along the equatorial belt, slowly taking an elongated and discoidal form. Some larger nodules are characterised by a distinct bulging shown by a knobby band along the equatorial rim. These are called hamburger-shaped nodules, whose equatorial bands mark the sediment–water interface at the seafloor. As mentioned earlier, nodules are very commonly ornamented on their external surface by localised tumorous growths (mammillae) of oxide precipitation. These mammillae are of different size and relief. Mammilla size is the length ratio between the base of a mammilla and the diagonal diameter of the whole nodule, while mammilla relief is the ratio of height to base length of the mammilla itself. These protrusions vary in size and relief, and also in their abundance on the top and bottom sides of nodules. The surface texture of nodules is either smooth or rough. Many nodules do not have the same surface texture on both sides. Finely granulated smooth surface, often exhibiting pits and cracks, constitute the top surface of nodules, which remains exposed to the overlying water column and grow by metal precipitation from the overlying water through the hydrogenetic process. Microscopic examination of the top surface reveals fine scratches or shining streaks caused probably by the action of bottom water current. In contrast, the bottom surface of nodules, in many cases, represents a rough, mammillated, coarsely granulated feature. Usually, larger the nodule, greater the possibility of variation in surface texture between the top and bottom sides. Many hamburger-shaped nodules, for example, notably display such variations. These nodules are often characterised by a very rough bottom side with intense biological activities (broken worm tubes). This surface remains in contact with the bottom sediment and grow by metal accumulation from the sediment’s interstitial water through sub-oxic diagenetic remobilisation (Glasby et al., 1983; Mukhopadhyay, 1988). Tumourous
Ferromanganese Deposits
163
mammillated growths also frequent the equatorial zones of these nodules, and such growths sometimes point towards the bottom, indicating the bottom sediment as the possible source of elements for nodule growth. It is now agreed that the external morphology of nodules is largely controlled by local as well as regional variability of oceanic environment. For example, a nodule field proximal to seamounts and other elevated volcanic topographic features is likely to receive plenty of seed materials in the form of rock fragment, and would present a local facies variation in nodule morphology (Iyer and Sharma, 1990). It has been observed that nodules from elevated regions are larger and have smooth external surface while nodules found at the foothills or in basins are smaller with rough outer surface (Mukhopadhyay and Nath, 1988; Mukhopadhyay et al., 2002). As is discussed later, the various morphological parameters of nodules appear to be interlinked (see Section 2 for details).
1.4. Nodule nucleus To initiate nodule growth, an essential component is the nucleus around which the concentric layers of ferromanganese oxide grow to form the nodule. Any solid object available on the ocean floor may serve as potential nucleating agent onto which ferromanganese oxide precipitation begins. The nucleus in majority of cases is rock fragment derived from seamounts, abyssal hills and fracture zones (FZs), palagonite, clay, shark teeth and older nodules. The chance of a single crystal of any mineral forming the nucleus is very rare. Lalou and Brichet (1976) found an elongated forsterite crystal, about 5-mm long and 1-mm wide, forming the nucleus of a nodule. Ghosh and Mukhopadhyay (1995) reported the unusual occurrence of a phillipsite crystal of large dimension (21 10 8 mm) as the nucleus of a nodule from the IONF (Fig 5.3). The paleoceanographic significance of this volcanic debris-derived phillipsite crystal has been discussed earlier (see Chapter 3). Interestingly, thrown-away objects such as iron bolts and sparkplugs showing ferromanganese oxide accretion have also been recovered. The size of the nucleus varies, often showing no relationship with the nodule size. A large nodule can have a tiny core, whereas smaller nodules with large core are also encountered. Some nodules do not have any distinct nucleus. For these nodules, it is possible that the primitive nucleus, formed of some volcanic material, has been totally replaced by ferromanganese oxides. This possibility is supported by the fact that fracturing of the cores and inter-penetration of oxides leading to fragmentation and replacement of core material has been observed in larger nodules of the ENP (Glasby et al., 1982). However, the shape, internal constitution and chemistry of nodules in the IONF appear to have been influenced by the nature and morphology of the nucleating agents (core). Many factors, which include seafloor topography, sediment type, bottom current activity and basinal volcanism, control the nature of the core material. The external shape of nodules often reflects the shape of the nucleus, as the shape of the nodule core would guide the configuration of initial oxide layering. This is particularly noticeable in young nodules with thinner oxide crusts, where the shape of these nodules resembles the shape of the core. Again, the abundance of
164
Mukhopadhyay, Ghosh and Iyer
Figure 5.3 The largest single phillipsite crystal (21108 mm) constituting nucleus of a diagenetically formed nodule from the Indian Ocean Nodule Field (IONF) (Ghosh and Mukhopadhyay, 1995).
mononucleate (single nucleus) or polynucleate (having several nuclei) types in a population is largely influenced by the seafloor morphology. Mononucleate types occur principally in abyssal plains, whereas polynucleate nodules commonly form on the flanks and topographic steps of abyssal hills. Rolling of polynucleate nodules down the slope may occur, and mixing of these nodules with mononucleate types occurring in the basal plains may form a mixed population. An interesting study on oxide:nucleus (O:N) thickness ratio was conducted on nodules of the IONF recovered from a seamount environment (seamount height about 1000 m, Mukhopadhyay and Nath, 1988). The nodules from the upper slope close to the summit of the seamount showed a low O:N ratio compared to high ratio in those occurring on the plain. Close to the summit, nuclei were largely composed of indurated sediment and volcanic rock fragments. This is in sharp contrast to the nodules found at the foothills and deep basinal regions, where both mono- and polynucleate nodules occur and the nucleus is largely made up of altered rock fragment, indurated clay and old nodules. These differences are reflected in surface texture and composition. For example, the nodules from the elevated region are extremely smooth textured and contain high concentrations of Fe and Co compared to the nodules on the plain, which are predominantly rough textured and have high concentrations of Mn, Ni and Cu. With the increase in nodule size, the O:N ratio increases, suggesting a diagenetic mode of metal accumulation in these nodules.
Ferromanganese Deposits
165
Size and composition of nucleus thus influence the metal value of nodules. Again, a nodule having a large rock nucleus is likely to show bulk metal content lower than that for another nodule of the same size but having a smaller nucleus of similar composition. If the nucleus were an older nodule fragment, the bulk metal content would have been different. From an economic point of view, therefore, a systematic study of nucleus size, mononucleate/polynucleate character and composition of nucleus appears important for a better understanding of the bulk chemical composition of nodules. Nodules formed around earlier nodule fragments would also indicate two periods of ferromanganese accumulation.
1.5. Mineralogy Ferromanganese nodules are composed of a complex mixture of materials that include various minerals of manganese and iron, amorphous components and a variety of accessory silicate phases. The most common manganese phases in nodules ˚ manganite), birnessite (7 A ˚ manganite) and d-MnO2 (Burns are todorokite (10 A and Burns, 1977; Buser and Gru¨tter, 1956). Pyrolusite and psilomelane, which are found abundantly in terrestrial manganese deposits, and some other manganese minerals (such as cryptomelane and lithiophorite) have also been reported to occur in nodules, but are rather less common (Table 5.5). Mo¨ssbauer spectroscopy has confirmed that iron in nodules occurs essentially as ferric oxyhydroxide, probably representing a mixture of goethite (a-FeOOH) and lepidocrocite (g-FeOOH). Other reported minerals include hematite (a-Fe2O3), maghemite (g-Fe2O3) and akaganeite (b-FeOOH). Ferrous oxyhydroxide, in contrast, may occur only in negligible amounts. The iron phases are either very fine grained or amorphous: the amorphous phase is a hydrated ferric oxyhydroxide polymer (FeOOH. xH2O; Burns and Burns, 1977; Johnson and Glasby, 1969). Commonly reported accessory materials in nodules are rock fragment, volcanic glass, fossil test and diverse silicate minerals such as quartz, feldspar, pyroxenes, amphiboles, clay minerals, zeolites, biotite, olivine, rutile, magnetite, ilmenite, barite, sphene, calcite, aragonite and apatite. Concentration of detrital quartz in some manganese oxide-rich layers of the Indian Ocean nodules has been suggested to be indicative of sporadic influxes of clastics at certain intervals (Ghosh, 1982). Minerals such as the zeolites and the clay minerals (commonly phillipsite and montmorillonite-chlorite, respectively) seem to represent alteration products of submarine volcanic material, and therefore appear to be of authigenic origin (Ghosh and Mukhopadhyay, 1995; Iyer and Sudhakar, 1993a). Regional variability in nodule mineralogy has been observed in a number of studies. For example, todorokite is reported to be the principal mineral in nodules from deeper basinal areas of oceans occurring in not so oxidising condition. d-MnO2, a more oxidised phase, is common in nodules at shallower depths on elevated areas, such as on the top (and slope) of seamounts and on mid-oceanic ridges. Birnessite probably takes a position in between the two above-mentioned phases (Table 5.5). However, deviations from this rule have been noted in ENP (Glasby and Thijssen, 1982), in some places of the IONF (Mukhopadhyay, 1988), Madagascar Basin and Blake Plateau (Cronan, 1975). Nodule mineralogy is, therefore, not strictly
Table 5.5 Principal manganese minerals in IONF nodules
Mineral
Diagnostic characters
Todorokite (Na,Ca, 10 A˚ manganite, d-spacing 9.6, [3MnO2Mn(OH)2 xH2O], Mn2þ)2Mn5O12 and buserite 3H2O ˚ manganite, d-spacing 7.2 7A Birnessite (Ca,Na) 2þ 4þ [4MnO2Mn(OH)22H2O] (Mn ,Mn )7 O143H2O Disordered birnessite (vernadite) d-MnO2 ˚. d-spacings 2.4 A˚, 1.4 A
Mn (%)
Fe (%)
Mn/ Fe
Ni (%)
Cu (%)
Co (%)
Pb (%)
Zn (%)
20.0
9.50
2.10
0.84
0.69
0.13
0.08
0.10
14.7
19.3
0.76
0.25
0.08
0.36
0.14
0.05
Sources: Burns and Burns (1977), Cronan (1980), Rao (1987) and Mukhopadhyay (1988).
Ferromanganese Deposits
167
depth-dependent and probably guided by specific formational environment. Among the factors influencing such environment are the prevailing degree of oxidation caused by bottom water currents [such as Antarctic Bottom Water (AABW) or other bottom water mass], relative rates of diagenetic metal supply (manganese enrichment), and phase transformation (post-depositional conversion of d-MnO2/ vernadite to todorokite) by intra-nodule diagenesis. Because of probable isostructural character, epitaxial intergrowths of FeOOH. xH2O and d-MnO2 may account for the close association of d-MnO2 and todorokite (Burns and Burns, 1977; Rao and Nath, 1988). The fact that mononucleate nodules from the ENP have more todorokite than d-MnO2, whereas polynucleate nodules of the same area contain relatively higher abundances of d-MnO2 than todorokite, may indicate that nodule mineralogy may also be related to nodule morphology (Glasby et al., 1982). It appears that the uptake of transition metals into nodules is controlled by the ˚ manganite, i.e., atomic ratio of the incorporated metals relative to manganese (10 A todorokite). In the IONF, todorokite is found to be associated with siliceous sediment that characterises high-to-moderate biological productivity. These todorokites occur at greater depths and allow easy substitution for Mn2þ by Ni and Cu. In contrast, d-MnO2, birnessite and FeOOH occur at relatively shallow water depth, largely in association with pelagic sediment showing low biological activity. Such a situation facilitates incorporation of Co and Pb in greater amount into the Mn4þ phase in birnessite, and Fe3þ phase in FeOOH (Burns and Burns, 1977). Similar situation from the ENP suggests that increased biological productivity can facilitate increased release of divalent metal ions to the sediment column and the same would influence increased formation and stabilisation of todorokite in nodules, resulting in the formation of a high-grade nodule deposit (Glasby and Thijssen, 1982). Therefore, biological productivity, mineralogy and chemical composition appear to be interlinked and account for the noted variations in the nodules.
1.6. Chemistry Deep-sea nodules are enriched in a large number of elements relative to their average abundance in the earth’s crust. Mn, Mo, Co, Ni and Cu are found strongly enriched in nodules while other elements show poor to feeble enrichment (Table 5.6). Again, Mn:Fe ratios as well as the contents of Ni, Cu, Co and Zn in these nodules display large variations from basin to basin in the Indian Ocean (Table 5.7). IONF nodules have the following average composition: Mn 24.58%, Fe 7.89%, Co 0.14%, Ni 1.16% and Cu 1.12%. The nodules from the central part of the IONF are relatively richer in Mn, Cu and Ni, while the nodules from the southern part of the same field and beyond are richer in Fe and Co. From the genetic point of view, diagenetic and mixed-type nodules of this field are distinctly richer in Mn, Cu and Ni in comparison to the contents of these elements in hydrogenous nodules, which contain more Fe and Co ( Jauhari and Pattan, 2000). A major population of IONF nodules is characterised by high metal-grade (Ni þ Cu þ Co 2.0%). These nodules cover about 75% of the seafloor and are
168
Mukhopadhyay, Ghosh and Iyer
Table 5.6
Element enrichment in nodules compared to continental crust
Element (%)
Abundance in oceanic nodules
Crustal abundance
Enrichment factor
Mn Fe Ni Co Cu Mo V Ti
16.02 15.55 0.480 0.284 0.259 0.0412 0.0558 0.647
0.095 5.6 0.0075 0.0025 0.0055 0.00015 0.0135 0.570
168.6 2.76 64.0 113.6 47.09 274.66 4.13 1.14
Sources: Taylor (1964), Cronan (1980), Nath et al. (1992) and Pattan et al. (1994).
Table 5.7
Composition (wt%) of manganese nodules from various basins in the Indian Ocean
CIOB
Mn 20.64 Fe 10.82 Ni 0.68 Cu 0.54 Co 0.15 Zn 0.12 Pb 0.05
Wharton Basin
Crozet Basin
West Australian Basin
Madagascar Basin
Somali Basin
15.97 11.67 0.39 0.22 0.16 0.07 0.08
13.00 15.76 0.30 0.14 0.17 0.04 0.08
21.40 – 0.86 0.55 0.19 – –
13.31 18.10 0.20 0.11 0.29 0.05 0.11
17.57 14.73 0.50 0.16 0.20 0.05 0.07
Sources: Cronan (1980), Andreev et al. (1984) and Siddiquie et al. (1984).
predominantly underlain by siliceous sediment. The nodules rich in Mn, Ni and Cu are more common in areas characterised by abyssal hill topography and associated relief features. This is also corroborated by about 2000 chemical analyses of nodules from little less than 1000 stations in the IONF and neighbouring locations ( Jauhari and Pattan, 2000; Valsangkar and Khadge, 1989; Table 5.8). A generalised interelement relationship in nodules is given in Fig. 5.4. In general, nodules from the abyssal plains have high Mn/Fe ratio, high Cu and Ni, whereas those from the summit of large seamounts contain high Fe and Co and low Mn/Fe ratio. It is possible that topography influenced sedimentation rates and oxidation state at local facies level, which, in turn, may have affected the distribution, morphology and chemistry of nodules (Ghosh and Mukhopadhyay, 1999). As indicated above, the nodule chemistry is related to depositional environment and to its mineralogy. For example, it is generally found that todorokite-rich nodules are enriched in Mn, Ni and Cu, while Fe and Co is found more in d-MnO2-rich nodules (Table 5.5). Crystallochemical considerations suggest that the common
169
Ferromanganese Deposits
Table 5.8
Composition of manganese nodules from the IONF
Si Al Fe Mn Ti Ca Mg Na K P Cu Ni Zn Co Pb Mo Ba Ce Y Nd Sr
Maximum
Minimum
Mean
No. of sample
18.80 4.90 20.5 48.60 0.83 2.10 2.46 2.64 2.27 0.29 2.73 2.21 1.14 0.43 1090 871 2854 1296 30 257 1032
5.90 1.40 2.4 6.5 0.18 0.74 0.98 0.51 0.32 0.07 0.13 0.18 0.02 0.07 382 160 434 178 05 49 232
9.20 2.8 7.10 24.40 0.43 1.63 1.90 1.80 1.10 0.17 1.04 1.10 0.12 0.11 712 570 1570 528 13 147 679
23 33 1119 1119 37 37 37 22 33 33 1108 1108 676 1108 22 22 26 37 33 37 26
REE Concentrations in sector C nodules of the IONF Sample
La
Ce
Sm
Lu
Ceþ
SREE
Ba
Sr
Mn/Fe
S-139 S-183 S-122 F-81
193 156 181 152
784 713 745 460
51.4 43.9 47.0 43.3
2.9 2.6 2.6 2.5
0.27 0.32 0.27 0.12
1305 1145 1234 889
1885 1834 1555 1758
919 799 965 670
2.75 3.13 2.45 2.93
Sources: Nath et al. (1992), Pattan et al. (1994), Banakar and Jauhari (1994) and Mukhopadhyay et al. (2002). Note: Lead to Strontium and REE in ppm, rest in wt%. Data for sector C are specially given as this sector is potentially most important.
cations in nodules are Mn4þ, Fe3þ, Co3þ, Ni2þ, Cu2þ and Zn2þ, with smaller amounts of Mn2þ, Mn3þ and Co2þ and traces of Fe2þ (Burns and Burns, 1977). The positive correlation among Mn, Ni, Cu and Zn suggests that cations of Ni2þ, Cu2þ and Zn2þ (also possibly some Co2þ) substitute for Mn2þ in todorokite. For example, nodules with more than 3.5% Ni þ Cu content are not known. Availability of only minor amount of Mn2þ for substitution in todorokite by Ni2þand Cu2þ may have constrained the Ni þ Cu limit at 3.5%. Cobalt enrichment in d-MnO2˚ ) substituting for Mn4þ rich nodules is possibly due to the ability of Co3þ (0.525 A ˚ ) ions in the disordered d-MnO2 phase in a highly oxidising environment. (0.54 A
170
Mukhopadhyay, Ghosh and Iyer
˚ ) by Co3þ in the It has been suggested that the substitution of Fe3þ (0.645 A associated and intergrown ferric oxyhydroxide phase may not be feasible, as the ionic radius of Fe3þ is much larger than that of Co3þ (Burns and Burns, 1977). Adsorption is an important mechanism that may also help enrichment of metals in nodules. Experimental data show that manganese and iron oxides are capable of removing many metals from seawater by adsorption, and in this regard enrichment of cobalt by adsorption of Co2þ on hydrous manganese oxide in nodules has been emphasised (Murray and Brewer, 1977). The enrichment of Pb in d-MnO2-rich nodules may be a good case in support of the role of adsorption, because the ionic A 2.0
r = 0.79
Co
1.5 1.0 0.5 0.0 0
3
6
9
12
15
30
40
50
30
40
50
Fe 2.0 r = 0.80
Ni
1.5 1.0 0.5 0.0 0
10
20 Mn
2.0 r = 0.90
Cu
1.5 1.0 0.5 0.0 0
10
20 Mn
Figure 5.4 (Continued )
171
Ferromanganese Deposits
B 3.20
Ni/Cu
2.53 1.87 1.20 0.53 0.94
2.46
3.90 5.50 Mn/Fe
7.03
8.55
8
10
8
10
Cu/Ni
1.0 0.8 0.6 0.4
0
2
4
6
Mn/Fe
Tod/δ-MnO2
4.0 3.2 2.4 1.6 0.8
0
2
4
6
Mn/Fe
Figure 5.4 (A) Inter-element relation in nodules and (B) relation between chemistry and mineralogy and elemental ratios (Banerjee et al.,1999).
˚ ) is considerably larger than that of Mn4þ (0.54 A ˚ ), which radius of Pb4þ (0.775 A may prohibit easy lattice substitution. However, the possibility of some Pb substituting for Mn4þ cannot be ruled out. The enrichment trends of trace elements in nodules are interesting. A sympathetic relationship exists between Cd and Mn as also between As and Fe. Uranium contents in bulk nodule samples, particularly from the ENP nodule belt in the Pacific, have been found to vary between 3 and 5 ppm (Kunzendorf et al., 1982). A clear association of U with Fe in these nodules appears to exist, suggesting that U occurs mostly in the iron-rich d-MnO2 phase. Exponential depth decay profiles for 230Thxs activity and the 230Thxs/232Th activity ratio were obtained from two oriented nodules from the southern part of the sector C in the IONF (Banakar, 1990). Such analyses suggest that while the sample SS-657 has been growing with a fixed orientation since the start of ferromanganese oxide accretion about 1 million years before present, the other nodule sample SK-176 offers indications of rotation several times over this period (Fig. 5.5).
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Mukhopadhyay, Ghosh and Iyer
1000 SK - 176 Bottom
SS - 657 Bottom
SS - 657 Top
mm 3.2
/my 1.2 mm
1.9 m
10
m/my
1.3 mm
/my
/my
100
230 Th (xs)
dpm /(
); 230 Th(xs)/232Th activity ratio (
)
SK - 176 Top
0 0.2
0.6
1.0
0.2
0.6
1.0
0.2
0.6
0.2
0.6
1.0
1.4
Depth (mm)
Figure 5.5 Isotope composition in nodules. Note the variation of top from bottom of nodules (Banakar,1990).
The abundance of the 14 naturally occurring rare earth elements (REE) in relation to the distribution of Ca, P and Fe in nodules from different topographic and sedimentary domains of the Indian Ocean was determined. The REE in IONF nodules occur primarily in the iron oxyhydroxide and phosphatic phases, or in places comprising of Fe, P and Ti. The concentration of Ce in nodules has been found to be very high (1000 ppm and above), that of La and Nd greater than 100 ppm, while the rest of the REE have concentrations much lower than 100 ppm, with Lu showing the lowest value, below 3 ppm (Table 5.8). For coexisting macronodules (nodules >1 cm) and micronodules from the IONF, the average REE abundance is higher in macronodules than in micronodules (Pattan et al., 1994). Otherwise, according to these authors, both categories of nodules from the same sedimentary environment show similar Ce anomalies, fractionation pattern and REE partition coefficients. Positive Ce anomalies in nodules are known from all the oceans, but negative Ce anomalies have also been reported in a few cases (Elderfield and Greaves, 1982). In the Indian Ocean, the Ce anomaly variations are possibly controlled by FeOOH.H2O phase, epitaxially intergrown with d-MnO2.
173
Ferromanganese Deposits
The REE concentration in IONF nodules has been found to be higher compared to their concentration in surrounding sediments. The Ce anomaly is moderately positive (0.25) in these nodules, suggesting accretion under cold, nutrient-rich, oxidising bottom environment resulting from the movement of the AABW. This moderately positive Ce anomaly in nodules is associated with negative Ce anomaly in bottom sediments, indicating preferential removal of oxidised Ce to the nodules from the water column via Fe-hydroxide colloidal flocks (cf. Glasby et al., 1987). Compared to the North American shale composite (NASC–reference material), IONF nodules are found enriched in middle REE (MREE) (Sm) over light REE (LREE) (La) and farther over high REE (HREE) (Lu), suggesting preferential uptake of Sm over La and Lu both in sediment (by about 20%) and in nodules (by about 35%; Table 5.8, Fig. 5.6).
A
0.7
Ce-anomaly
0.5
x
0.3
x
x
0.1 x 5
0
10 Fe %
15
20
0.7 Crusts WIO nodules CIOB nodules x Nuclei
Ce-anomaly
0.5
x 0.3 xx 0.1 x 0
1
2
3 Mn/Fe
Figure 5.6
(Continued )
4
5
174
Mukhopadhyay, Ghosh and Iyer
Macronodule Micronodule Sediment
Log (Partition Coefficient)
B 6 5 4 3 La Ce
Nd
Sm Eu Gd
Dy
Er
Yb Lu
Figure 5.6 (A) A generally positive relation between Ce-anomaly and Fe, and a diffused relation between Ce-anomaly and Mn/Fe (Nath et al., 1994). (B) Average rare earth elements (REE) partition coefficient for macronodule, micronodule and sediment (Pattan et al.,1994).
1.7. Internal structure and growth The internal structural features preserved by the ferromanganese oxide materials from the inner to the outer parts essentially record the complex history of nodule growth. The concentric banding of ferromanganese oxides shows five typical textural patterns in the world oceans, illustrating distinct compositional variations (Sorem and Fewkes, 1977). The patterns are (1) massive (dense layers, high Mn/Fe ratio), (2) mottled (discontinuous and chaotic layers), (3) compact (dense layers but very low Mn/Fe ratio), (4) columnar (radial pattern) and (5) laminated (short, dense and columnar). Details of the textural patterns are best observed in polished sections, examined microscopically using reflected light (Fig. 5.7). The two principal internal growth features of IONF nodules are parallel layers and dendritic, colloformic features (columnar/cuspate patterns). In most of the nodules, columnar structure and parallel layer are often found stacked in close repetition, from the inner to the outer parts. The columnar features display a colloform fabric and are composed of sub-concentric laminae (arcuate cusps) and lenticular patches of highly reflecting light coloured manganese-rich material. The cuspate columns sometimes show outward radial pattern and bifurcate. They do not always maintain the same geometry and spacing everywhere. These layers in general contain todorokite as the major manganese mineral, show enrichment in Mn, Ni and Cu and may have been formed by diagenetic remobilisation of metals. Columnar features are indicative of a higher growth rate. Parallel oxide layers, on the other hand, which generally occur over and underlain by columnar sets, may display wavy patterns with finer crenulations. These iron-rich or iron–manganese-rich layers are of much lower reflectivity (dark layers). These layers contain phillipsite, quartz and feldspar, are rich in Fe and Co and may have been precipitated largely from the water column. Parallel layers suggest slow growth under unfavourable conditions, or interruptions in growth (Banakar, 1990; Banerjee et al., 1991; Heye, 1975; Pattan, 1988). The importance of relative rates of nucleation towards the development of a particular growth pattern in preference to another has been suggested. It seems that
175
Ferromanganese Deposits
A
0.1mm
B
D
0.1mm
C
E
F
Figure 5.7 Growth patterns in nodules (Banerjee et al., 1999). Note: (A) growth hiatus between light gray todorokite and dark gray vernadite, (B) laminated microstructures at the contact with core shown as (C) and grading outwards into botroyds, (C) undissolved radiolaria and frustules, (D) globular todorokite and vernadite around biogenic tests, matrix is recrystallised todorokite, (E) intelaminated growth structure merging into random, dendritic microstructure, (F) darkgray detritus mineral on top disrupts dendritic microstructures. Photographs taken by Ore microscope (reflected light, oil immersion).
abundant nucleation favours the formation of continuous parallel layers, whereas columnar structures grow in conditions where nucleation is not forthcoming (Ghosh, 1982). Scanning electron microscopy of these two major structurally distinguishable growth zones in the Indian Ocean nodules revealed that the inner
176
Mukhopadhyay, Ghosh and Iyer
zones of these nodules are characterised by widely spaced, isolate columns and abundant chaotically folded microlaminations. The solution-reprecipitation process under fluctuating bottom current velocity might produce such chaotic layers (Cronan and Tooms, 1968). The outer zones, in contrast, show no chaotic folding, are well organised and have compact columns grading into colloform layering. Similar fabric differences between the inner and the outer zones of nodules have also been reported elsewhere (Siddiquie et al., 1978; Sorem and Fewkes, 1977), and these differences suggest a change in the environment from the inner to the outer zones of growth. Nodule growth rates have been measured through radiometric and isotopic methods. The techniques applied so far utilise Th230, U234, Pa231, Be10, K-Ar, hydration rind dating and fission track dating of nodule nucleus. Ku (1977) discussed various techniques and indicated the limitations of the different methods used. Lyle (1982) proposed that the growth rates of manganese nodules may be calculated by using the equation: R ¼ 16.0 [E(Mn)/E(Fe)2] þ 0.448, where R is growth rate in mm/Ma and E(Mn) and E(Fe) are concentrations of Mn and Fe, respectively, in nodules. According to Lyle (1982), the accumulation rate of Mn is proportional to the square of that of iron. Sharma and Somayajulu (1987) have however suggested a different equation for calculating nodule growth rate (R), where R ¼ 8.33 [EMn/(EFe)2] þ 2.16. The EMn and EFe represent Mn and Fe concentrations, respectively, in the nodule. It is to be noted that radiometric dating usually gives lower growth rate values than those determined by using the above equations. Published growth rate data (Table 5.9) show a slow rate of growth of the order of a few millimetres in a million years for deep-sea nodules (1–10 mm/Ma). Based on the exponential depth decay profile of 230Th(xs) and 230Th(xs)/232Th(ac), the average accretion rate of Indian Ocean nodules was obtained as 1.2–1.3 mm/Ma for top layers and 1.9–3.2 mm/Ma for bottom layers (Banakar and Borole, 1991). Heye (1975) determined the growth rates of ENP nodules as 4–9 mm/Ma by using the Ionium method. The study recognised certain zones of rapid growth (>50 mm/ Ma) and also interruptions in growth. Of interest is also the suggestion that iron-rich zones of nodules grow slower than those rich in manganese, so that nodules with lower Mn/Fe ratios may be taken to have grown at a slower rate than those with higher Mn/Fe ratios (Heye and Marchig, 1977). As already indicated, this variability is likely to be influenced by the variable supply of metals from the diagenetic (pore water in sediment) and the hydrogenous (overlying water column) sources, affecting the growth rates in areas of nodule formation. Nodule growth also depends on such factors as the nature of the bottom sediment, metal flux rate in the overlying water column and biological activity. These are discussed in a later section.
1.8. Age of IONF nodules The estimated growth rate of IONF nodules, as available from radiochemical studies and radiometric age dating, varies between 2 and 3 mm/106year (Table 5.9). This growth rate, when observed in relation to the thickness of the accumulated ferromanganese layers, suggests that the formation of the nodule deposits in the IONF probably began during the Late Miocene–Early Pliocene time (between 8 and 3 Ma).
177
Ferromanganese Deposits
Table 5.9
Average growth rates (mm/106years) of manganese nodules
At different water depths (m) and different oceansa Ocean
3000^4000
4000^5000
5000^6000
North Pacific South Pacific Atlantic Indian
5.7 1.75 6.50 –
4.74 3.47 3.16 2.11
4.4 4.50 – 3.1
Along different layersb Ocean
Pacific Pacific Pacific Indian
Area
9N 9 N 11 N IONF
Top layers
Bottom layers
Method of determination
4.3–4.6 1.9–2.4 1.4–1.9 1.2–1.3
2.5–3.3 1.5–2.5 4.0–5.2 1.9–3.2
230
Th/232Th Th 10 Be 230 Th (excess)/232Th (activity) 230
Occurring on different sediment typesc Sediment type
Average
Range
Terrigenous
4.45 4.22 5.37 4.72 20.89 12.81
3.00–6.20 3.49–5.07 1.60–17.20 2.74–10.90 7.70–45.30 5.96–25.51
Terrigenous–Siliceous Siliceous
(a) (b) (a) (b) (a) (b)
a,b
Ku and Broecker (1967); Somayajulu et al. (1971), Krishnaswami and Cochran (1978), Lalou et al. (1979), Krishnaswami et al. (1982); Sharma and Somayajulu (1987) and Banakar and Borole (1991). Banerjee and Mukhopadhyay (1991). (a) Following Lyle (1982) equation: Growth rate ¼ 16.0 [EMn/(EFe)2] þ 0.448; (b) Following Sharma and Somayajulu (1987) equation: Growth rate ¼ 8.33 [EMn/(EFe)2] þ 2.16. See text for explanation.
c
Such assessment is also supported by the results from paleoenvironmental investigations (Ghosh and Mukhopadhyay, 1995). The ENP nodules, whose formation was initiated during the Lower Miocene (15 Ma), are therefore much older than IONF nodules. Southwest Pacific (SWP) nodules, however, started forming around Pliocene (3.5 Ma, Glasby et al., 1982; Martin-Barajas et al., 1991). Thus, IONF nodules seem to be older than SWP nodules but younger than ENP nodules. Based on size and nucleus characteristics, it is further suggested that the majority of nodules in the IONF are first-generation nodules, whereas those of the ENP are of second generation and comparatively mature. In the IONF, hydrogenous nodules appear older to their diagenetic counterpart, indicating that the process of diagenesis started late in the IONF (Martin-Barajas and Lallier-Verges, 1993).
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1.9. Buried nodules Although the bulk of macronodules (1 cm) occurs at the sediment–water interface, these nodules are often found buried at different depths within sediments (Figs. 5.8 and 5.9). Recovered only by sediment cores, buried nodules show low abundance with depth in the sediment column. Apparently, it is difficult to estimate whether all buried nodules were formed at the sediment surface prior to their burial. In some areas in the Pacific Ocean, nodules buried within the top 3 m of sediment show about 35% of total nodule abundance (Horn et al., 1972). In the ENP belt of this ocean, about half of the nodules are found at the sediment surface, the remainder being buried (Glasby et al., 1982). Roy (1981) suggested that buried nodules in modern oceans are concentrated at stratigraphic hiatuses. Goodell et al. (1971) reported the occurrence of 3.4-Ma-old buried nodules at sediment depth of 1.6–1.7 m from the Albatross cordillera. Taking into account the crustal subsidence, attempts were made to correlate the depth of nodule burial with the rate of ocean floor spreading (Glasby, 1977). Sediments from more than 60 spade and gravity cores (water depth 4700–5800 m) from the IONF were analysed to study buried nodules. Of these, about 21% cores contain nodules at various depths. Interestingly, no sediment cores recovered north of 8 S contain buried nodules, akin to the near-complete absence of surface-nodules in these latitudes. The reported maximum depth at which buried nodules are found has been 545 cm. Similar to the surface nodules occurring at the sediment–water interface, the buried nodules are largely elliptical, discoidal, irregular and polynucleated, and have both smooth and rough surface textures. A detailed examination of the internal structure and growth features of the buried nodules (from sector A) reveals a nearly alternate recrystallised todorokite and thick d-MnO2rich layers both showing dendritic texture, with intercalations of clay-rich zones. The first layer over the nucleus is in fact formed of clay (Banerjee et al., 1991). Five buried ferromanganese nodules were recovered at depths of 166–168 cm, 172–174 cm, 228–230 cm, 328–330 cm and 418–420 cm from the siliceous sediment domain of the IONF (9 S, 76 E, sector A). Detailed major, trace and REE studies of these buried nodules revealed that nodules of top three levels were formed through early diagenetic process (Mn/Fe 9.3–15.1). While the fourth level of nodules appears to have formed by hydrogenetic method (Mn/Fe 1.6), the nodules from the lowest level were the product of diagenetic process (Mn/Fe 3.0). The REE concentration, normally attributed to seawater as source, ranges between 164 and 497 ppm. These nodules show a moderate middle and heavy REE enrichment, despite being 2–3-fold lower than the surface nodules. There is hardly any evidence to suggest that nodules grow or dissolve after burial (Pattan and Parthiban, 2007). However, a clear decrease in the size of nodules from 12.5 to 1.5 cm2, has been recorded with depth (Pattan and Parthiban, 2006). Buried nodules have also been recovered in a core from depths of 330, 525 and 545 cm below the sediment surface near Chagos Trench (11 07.50 S, 72 30.10 E, sector C). This core was collected from an uneven seabed of water depth of 5710 m, which has been bottomed by calcareous ooze. Besides contributing new information on the carbonate compensation depth (CCD) level in the Indian Ocean, these
Normalised concentration of elements (max = 1)
Buried depth (cm) of nodules in core
2.0
0.6 0.8 1.0 La (1), Ce (2), Nd (3) Sm (4)
0.6 0.8 1.0
0.6 0.8 1.0
Eu, Gd, Dy Ho
Er(1), Yb(2) Lu(3)
0.6 0.8 1.0 Y
1.0
Ce
Ba
0.6 0.8 Li
0.6 0.8 1.0 Mo Pb Zn
Nl Co
0.6 0.8 1.0 Cu
Co
0.8 1.0 Mg
P
0.6 0.8 1.0 Na
0.8 1.0 Ti
0.6 Al
0.2 0.4 K
Mn
0.6 0.8 1.0 Fe
0.4
8.7 10.0
15.2
19.0 20.0
1 2 3 4
22.5 23.5 3
2 1
27.5 28.0 0.07 92.5
Figure 5.8 Element distribution in buried nodules from the Indian Ocean Nodule Field (IONF) (Pattan and Banakar, 1993).Vertical axis indicates depth of nodules buried in sediment core, while normalised values of elements are given along horizontal axis.
180
Mukhopadhyay, Ghosh and Iyer
A
B
0
10 AAS - 40 GC - 03
167-169 m
1
0 100
10 172-174 m 1
0 10 Sample/shale
Core depth (cm)
200
226-228 m 1
0 300
10 328-330 m 1
400
0 10 418-420 m
Yb
Lu
Tm
Er
Ho
Tb
Dy
Gd
Eu
Sm
Nd
Pr
0 La
500
Ce
1
Figure 5.9 Shale normalised (NASC) pattern of rare earth elements (REEs) of buried nodules (Pattan and Parthiban, 2006). Occurrence depth of buried nodules is shown on the left.
nodules are probably important markers of paleoenvironment (Mukhopadhyay et al., 1994). In addition, a 5-m long gravity core recovered from a water depth of 5099 m in siliceous ooze sediment (13 030 S, 75 440 E, sector C) holds buried manganese nodules at 4 levels: at the core top, at 30–36 cm, at 310–315 cm and between 325 and 330 cm. A substantial amount of aeolian dust-input has been recorded to the top layers up to a depth of 3 m in this sediment core, and the nodules are considered to have formed largely through hydrogenetic method. Eight
181
Ferromanganese Deposits
nodules buried at different depths within the top 1 m of a box core from siliceous clay domain at 14 S and 74 E (water depth 5240 m, sector D) of the IONF have been examined. Seven of the buried nodules, ranging in diameter from 3 to 6 cm, occur within the top 30 cm of the core. The maximum content of Mn and Fe in these nodules is 29.1 and 14.1%, respectively, which compares well with the contents of these elements in many nodules occurring at the surface (Table 5.10). In view of the presence of d-MnO2 as the major manganese phase and Mn/Fe ratio in the range of 1.9–3.1, these buried nodules may be considered to have formed mostly by hydrogenous accretion. Based on 230Th excess decay profiles, an average growth rate of about 4 mm/Ma has been estimated for the buried nodules in the IONF. These nodules appear to have formed through precipitation from seawater (hydrogenesis) prior to their burial (and termination of growth) at about 225 ka (Pattan and Banakar, 1993). It was also suggested that the accretion rates of buried nodules and those occurring on the surface from the same area of IONF are similar (Somayajulu et al., 1971). In summary, buried nodules from siliceous ooze/clay areas are rich in manganese, copper and nickel (Table 5.11), are comparable to macronodules occurring above at Table 5.10 Composition of buried nodules in comparison to micro-macro/surface nodules and sediment a from the IONF Elements
Buried nodules
Sediment
Micronodules
Surface nodules
Mn Fe Ni Cu Co Zn Ce La Nd Sm Eu Yb Mn/Fe
22.7–29.1 8.16–14.11 5620–11906 4977–9725 1263–1787 716–1457 323–812 80–171 86–182 19.1–37.8 4.5–9.1 8.0–15.3 2.78–2.06
0.60 2.02 184 256 59 119 94 35 42 11.3 2.7 4.8 0.29
35.02 1.80 1.41 1.62 00.16 00.37 261 44.9 70 43.1 11.1 14.5 19.4
26.20 9.30 10320 9456 1585 1183 455 119 139 30.0 7.2 11.0 2.82
Rare earth element enrichment in micronodulesb
a
Sediment type
Micronodule/ sediment
Macronodule/ sediment
Macronodule/ micronodule
Siliceous Calcareous Red Clay
2.9–5.6 13.3 1.60
15.2 – 1.50
2.9 – 1.00
Pattan and Banakar (1993) and Pattan and Parthiban (2006). Pattan et al. (1994). Note: For micronodules, Cerium to Ytterbium in ppm, rest in wt%. For buried nodules, surface nodules and sediment all elements in ppm except manganese and iron (wt%). b
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Mukhopadhyay, Ghosh and Iyer
the sediment–water interface and may have been formed through processes ranging from hydrogenetic to diagenetic. In contrast, buried nodules from pelagic clay are rich in iron and cobalt and are likely to have formed by hydrogenetic method. It seems that when the sedimentation rate in an area becomes much greater than the upwards force of buoyancy following reworking of sediments by benthic organisms, burial of nodules takes place.
1.10. Micronodules As mentioned previously, manganese micronodules, up to about 2 mm in diameter, but commonly <1 mm, occur in both surface and subsurface sediments of all the major oceans. The morphology, surface texture and internal features of micronodules are in general similar to those of macronodules, and are found often buried at different depths within sediments showing varying element contents, for example, Mn, Ni, Co and Cu decrease, while Fe follows a reverse trend with depth (Table 5.12); its abundance decreases with depth in siliceous clay areas but may show a reverse pattern in pelagic clay (Fig. 5.10; Mukhopadhyay et al., 1988). The micronodules in the IONF contain on average 35% manganese, 1.8% iron and about 3% copper and nickel combined (Table 5.10). Micronodules from the ENP belt show comparable composition (in percentage): Mn, 35.2–36.4; Fe, 2.2–8.0; Ni þ Cu, 1.7–2.4 (Friedrich et al., 1977). Compared to the nodules of normal size (macronodules) from the Pacific and the Indian oceans, the micronodules have distinctly higher contents of manganese. It is further observed that micronodules in the IONF, when compared to surface and buried macronodules, show very low contents of REEs and high Mn/Fe ratios (5–101). The average REE abundance is highest in macronodules (1164 ppm), moderate in micronodules (650 ppm) and least in sediment (210 ppm). Variations in the distribution of micronodules in different sediment types and their internal growth structures are shown in Figs. 5.10 and 5.11, respectively. Table 5.11 Transition elements (wt%) in macronodules and buried nodules from different sediment domains Siliceous clay
Mn Fe Ni Cu Co Mn/Fe
Pelagic clay
Macronodule
Buried nodule
Macronodule
Buried nodule
27.34 5.06 1.05 1.00 0.11 5.40
24.57 8.08 0.87 0.87 0.128 3.04
21.58 8.39 0.71 0.53 0.175 2.57
19.11 9.54 0.79 0.41 0.187 2.00
Sources: Jauhari (1990) and Pattan and Parthiban (2006).
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Ferromanganese Deposits
Table 5.12 Depth in sediment core from surface (mm)
20–40
40–60
80–100
140–150
160–170 170–180 192–200
Composition of micronodules along the sediment depth in the IONF
Description
Mn
Fe
Ni
Cu
Co
Mn/Fe
Coarse MRN with diatoms, todorokite MRN growth on diatom/ radiolarians, as cavity fillings of todorokite crystallites Coarse lumpy MRN, at times needle shaped, bacterial cells in the Mn-Fe groundmass Mn-Fe crystallites in cavity, Nannobotroyids adjacent to phillipsite crystal Boxwork of Fe-rich minerals Mn-rich crystallites with diatoms. Sheaf-like, neddlelike, Fe-rich crystallites, Ferich botryoids
83.8
8.20
1.97
5.36
0.70
10.22
84.3
8.78
2.58
3.84
0.58
09.60
95.5
0.95
1.12
1.57
0.67
83.4
8.50
3.60
3.65
0.62
9.81
0.8
97.2
0.5
0.90
0.60
0.08
96.1
1.56
1.07
1.00
0.26
2.45
92.6
0.28
0.30
0.38
100.5
61.6 0.02
Source: Banerjee and Iyer (1991). Note: Abundance of Mn, Fe, Ni, Cu, Co together equals to 100 volumes. MRN ¼ micronodules.
It is likely that the redox state of sedimentary environments regulates the characteristics of micronodules: their size, abundance, composition and formation, as well as their dissolution in the sediments (Stoffers et al., 1981; Sugisaki et al., 1987). In this scenario, micronodules can be used tentatively as indicator of the sedimentary environment. Again, dissolution of micronodules within the sediments has been considered important to supply essential elements for the formation of macronodules at the sediment–water interface in the Equatorial Pacific (Stoffers et al., 1981).
184 Percentage of micronodule per unit volume
Mukhopadhyay, Ghosh and Iyer
20 Red clay Siliceous ooze
16 12 8 4
20
40
60 80 Depth (cm)
100
120
140
Figure 5.10 Abundance of micronodules in two major sediment types in the Indian Ocean Nodule Field (IONF) (Mukhopadhyay et al.,1988).
Macro- and micronodules from the same sedimentary environment appear to have similar distribution trends of transition metals, Ce anomalies, fractionation pattern and partition coefficients, thereby suggesting that a similar process may control the enrichment of these elements. Mukhopadhyay et al. (1988), however, suggested that micronodules might not significantly contribute towards the growth of macronodules in the IONF. Often close association of micronodules with biological debris (such as radiolarian fragments, diatom tests and bacterial cells) may indicate a genetic link between the dissolution of these bio-debris and the growth of manganese micronodules (Banerjee and Iyer, 1991).
2. Factors Influencing Nodule Formation It appears that the formation, distribution and grade of ferromanganese nodules vary from ocean to ocean, basin to basin and even from place to place within a basin. For example, various sectors within the IONF host nodules of variable characters. Again, manganese content in nodules increases with the degree of oxidation and with water depth, though nodules with high manganese concentration are formed also from hydrothermal contribution. It would then certainly be interesting to examine the various factors that might influence the characters of nodules. Some of these factors could be topography, availability of nucleating material, sedimentation rate, biological productivity, bottom sediment type, supply of metals, influence of bottom currents, physico-chemical (oxidising) environment, etc. Depending much on local and regional factors, the different nodule parameters appear interlinked. We discuss each of the factors and resulting situations below.
Ferromanganese Deposits
185
A
B
Figure 5.11 Micronodules under Scanning Electron Microscope (Mukhopadhyay et al., 1988): (A) polynucleated with botryoidal structure and (B) rosettes of d-MnO2.
2.1. Topography and nodule distribution Seafloor topographic variations play an important role in the distribution of nodules. High abundance of nodules in the Pacific and the Indian oceans is more common in areas characterised by abyssal hill topography and associated relief features. Abundant occurrence is found on the slopes and flanks of topographic highs (seamounts and abyssal hills), where large numbers of nuclei (‘seeds’) are easily available from exposed rock outcrops. The distribution of nodules may be patchy in areas of high relief and uniform on plains. Nodule occurrences in the North Pacific are more evenly distributed in flat, low relief areas; the abundance may although be less than that in hills and high relief areas. A similar situation exists in the Indian Ocean. Recent surveys in the IONF have shown that areas with local rugged relief showing heights up to a few hundred metres have higher abundances, even exceeding 15 kg/m2 at places. Nodules occurring on seamount summits and slopes are more iron-rich. However, the distribution of nodules in these areas has been patchy.
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Mukhopadhyay, Ghosh and Iyer
In contrast, areas with low topographic relief have lesser abundance but a regular distribution of nodules (cf. Mukhopadhyay et al., 2002; Tables 5.13 and 5.14). Detailed study in the four sectors of the IONF has been able to trace an interesting relationship between nodule abundance and topographic variations. The most tectonically affected and compressed sector D shows a very high abundance of nodules (average 5.82 kg/m2, the highest value at one location being 13.73 kg/m2). This sector has been infested with a large concentration of seamounts (population as high as 19,350 per million km2) but very low occurrence (10%) of taller and larger seamounts. In contrast, the tectonically least stressed sector C, with majority of the taller and larger seamounts (64%), shows substantial abundance (5.34 kg/m2) of nodules though the overall seamount population has been less than that in sector D (10,200 per million km2). A moderately stressed sector B displays moderate abundance (4.12 kg/m2) of nodules. The seamount population in this sector is 11,800 seamounts per million km2, of which only 19% are taller and larger. In contrast, sector A, which shows signs of tensional stress, records least seamount population (9400 per million km2) and nodule abundance (1.83 kg/m2). The above data probably point to the positive relationship between the population of large seamounts and the consequent availability of volcanic fragments to act as nuclei (Mukhopadhyay et al., 2002). Table 5.13 Variations in the distribution and morphology of manganese nodules from different topographic domains in the IONF
Parametres
Water depth (m) Nodule abundance (kg/m2) Predominant size (mm) Predominant shape Predominant surface texture Nature of nucleation Type of nucleus Nucleus: Oxide thickness Nodule: Crust abundance
Seamount summit
Seamount foothill
Abyssal hill
Abyssal borderland
Abyssal plain
4691 5.53
5269 6.71
4874 5.52
5300 6.30
5100 3.85
40–60
40–60
20–40
40–60
20–40
SS, D, E
D, SS, E
SS, D
SS, D, O
E, Sq
G, CG, SM
SM, G, CG
G
G, CG
G
MþP
MþP
M
MþP
M
HM, VRF 1.75
ARF, ON 0.66
VRF 1.22
HM, ON 0.59
HM 0.52
5.46
8.42
5.23
9.46
12.33
Sources: Mukhopadhyay (1987), Mukhopadhyay and Nath (1988) and Mukhopadhyay and Banerjee (1989). Note: SS ¼ Subspheroidal, D ¼ Discoidal, E ¼ Elongated, O ¼ Oblong, Sq ¼ Squarish; G ¼ Granular, CG ¼ Coarse Granular, SM ¼ Smooth; M ¼ Mononucleate, P ¼ Polynucleate, HM ¼ Hard mud, VRF ¼ Volcanic rock fragment, ARF ¼ Altered rock fragment, ON ¼ Older nodules.
187
Ferromanganese Deposits
Table 5.14 Variations in composition of nodules (wt%) in differing topographic and tectonic regimes
Mn Fe Ni Cu Co Zn Pb Mn/Fe
Seamounts
Ridges
Indian Pacific
Indian Pacific
Seamount summit
Abyssal Seamount hills foothill
Abyssal plain
15.23 16.21 00.30 00.08 00.31 00.04 00.08 00.93
15.03 18.70 00.18 00.10 00.21 00.06 00.10 00.85
23.71 9.40 0.98 0.93 0.13 – – 2.2
21.36 6.13 1.24 1.34 0.12 – – 3.48
23.62 6.09 1.43 1.41 0.11 – – 3.88
18.39 15.01 00.44 00.13 00.61 00.07 00.18 01.18
18.74 13.42 00.66 00.48 00.26 – – 01.31
Indian Ocean Nodule Field (IONF)
28.76 6.57 1.34 1.35 0.12 – – 4.36
Sources: Skornikova and Andrushenko (1974), Dymond et al. (1984), Ghosh and Mukhopadhyay (1999) and Mukhopadhyay et al. (2002).
In addition to the contribution from the seamounts, seafloor faults (both normal and reverse types) contribute nucleating material for nodule formation. For example, several of these faults in the IONF experienced large tectonic movements of high scarps with throw >100 m (see Chapter 2). Friction associated with such large tectonic movements along these faults may have produced broken and crushed rock fragments, which acted as additional seeds for nodule formation and in higher abundance of nodules. For example, in tectonically stretched sector C, the number of major faults with high throws is more (66%), though their abundance (750 per million km2) is less than those in highly compressed sector D (1530 per million km2) and relatively less compressed sector B (1150 per million km2). This density distribution of major faults probably compensates for the poor presence of taller seamounts in sectors B and D, in contributing rock fragments leading to relatively high abundance of nodules (average 5.82 and 4.12 kg/m2, respectively). As explained in Chapter 2, the seafloor faults occurring in the IONF were formed during Paleocene–Eocene time from the then roughly east-west-trending Mid-Indian Ocean Ridge located to the south of the present IONF. A majority of the south-facing reverse faults in the IONF (see Table 2.3) occur in sectors B and D. It is possible that these faults may in fact be the original north-facing normal faults, transformed now to become south-facing reverse faults due to compressional stress acted on the underlying crust. For example, north-facing normal faults were expectedly formed on a crust that was stretching under tensile stress during the rapid northward movement of the Indian Plate before 58 Ma (represented by sector A), and between 56 and 51 Ma (sector C). In contrast, the crust encountered compressional stress due to reduction in speed of the Indian Plate at 58 Ma (represented by sector B) and subsequent touch between the Indian and the Eurasian plates at about 51 Ma (sector D). Accordingly, the throw direction of several faults in sectors B and D underwent a change from north-facing normal faults to south-facing
188
Mukhopadhyay, Ghosh and Iyer
reverse faults corresponding to a change in stress pattern: tensile to compressional. High percentages of reverse faults have been recorded in sectors B (43%) and D (49%), against a moderate 26% in sector C and almost nil in sector A. The data suggest that the crust of sectors B and D underwent severe abrasion between the blocks along the fault planes during tectonic reorientation and adjustment from normal to reverse faults. The abrasion produced ample rock fragments as seeds to act as nuclei, and ultimately enhanced the nodule abundance (Mukhopadhyay et al., 2002).
2.2. Morphology and nodule chemistry The bearing of morphology on the composition of nodules is immensely significant not only from academic point of view but also from the angle of resource mining. Mononucleate nodules in the IONF are found principally at more than 5000 m water depths. They are larger in size and characterised by higher Mn/Fe ratio and high Ni and Cu contents. For example, the Mn/Fe ratio and Ni þ Cu percentage increase from 2.46 and 1.73 in smaller nodules (size 20–40 mm) to 4.46 and 1.80 in larger nodules (size >80 mm), respectively. The concentrations of Fe and Co, however, decrease along the same size sequence from 9.33 and 0.16 to 6.56 and 0.10%, respectively (Table 5.15; Mukhopadhyay and Ramana, 1990). Similar relations between nodule morphology and chemistry are observed in area G of the ENP (Friedrich et al., 1983; Glasby et al., 1982; Halbach et al., 1981). Additionally, an increasing trend of Mn/Fe ratios down the slope of a seamount was observed in the IONF. The ratio increases from about 1.5 or 2 at the summit or Table 5.15
Influence of morphology on the composition of manganese nodules (wt%) Manganese
Nodule size Very small Small Medium Large Very large NE Pacific# Surface texture Top layer* Bottom Layer* Top layer Bottom layer
Iron
Nickel
Copper
Cobalt
Mn/Fe
1
2
1
2
1
2
1
2
1
2
1
27.70 25.00 23.80 21.90 21.30 22.40
20.07 24.29 22.95 21.82 20.04 –
6.60 6.30 6.90 7.60 9.45 8.10
6.18 6.09 6.62 7.03 6.94 –
1.14 1.19 1.13 0.95 0.77 1.16
0.94 1.20 1.16 1.07 1.06 –
1.31 1.16 1.04 0.91 0.68 1.02
0.80 1.14 1.03 0.98 1.08 –
0.11 0.11 0.10 0.12 0.10 0.25
0.12 0.10 0.10 0.12 0.09 –
4.19 3.97 3.45 2.88 2.25 2.76
27.12 31.20
– –
7.76 4.47
– –
1.30 1.77
– –
0.78 1.27
– –
0.36 0.24
– –
3.49 6.98
14.70 20.00
– –
19.30 9.50
– –
0.25 0.84
– –
0.08 0.69
– –
0.36 0.13
– –
0.76 2.10
Sources: 1 ¼ Dymond et al. (1984), Valsangkar and Khadge (1989), Jauhari (1990), Jauhari and Pattan (2000); 2 ¼ Banakar et al. (1989), # ¼ Glasby et al. (1986). Note: # and * indicate Pacific Ocean nodules. Rest from the Indian Ocean.
Ferromanganese Deposits
189
close to the summit areas to almost 4 in basinal areas away from the seamount. In a similar style, the concentrations of Mn and Ni also increase in nodules occurring at greater depths. Nodule compositional variations therefore appear to have been guided by the combined action of a variety of geochemical processes operating in different bathymetric settings. Interestingly, analyses of nodules from 24 locations in the IONF show antithetic relation between size and composition of nodules. Here, higher metal content (Cu þ Ni þ Co ¼ 2.47%) is found in smaller size class (40 mm), which decreases with increase in nodule size. Iron and moisture content however increase gradually with nodule size. Working on little <900 samples, Jauhari and Pattan (2000) reported that Mn/Fe ratio and the concentration of Ni þ Cu þ Co% decrease sequentially from 4.20 and 2.56% in <20 mm size nodules to 2.28 and 1.54% in >100 mm size nodules, respectively (Table 5.15). Similar relations are observed in area C of the ENP, where small size nodules (<60 mm) contain high Mn/Fe ratio and high Ni þ Cu values than larger nodules (Friedrich et al., 1983). It is generally accepted that nodules tend to become flat, discoidal and ellipsoidal with increase in size. The larger nodules are most stable on the seafloor and are expected to enrich by receiving metals both from seawater (through hydrogenesis) and from interstitial pore water (through diagenesis). In the IONF however, no consistent relation has been found between size and composition of nodules. This may indicate that the process of diagenetic remobilisation of elements in the IONF is infrequent and limited to local scale only.
2.3. Acoustically transparent sediment layer As mentioned earlier, nodules are formed in three basic ways: with elements contributed by seawater through hydrogenous precipitation, by bottom sediment through pore-water diagenesis and by hydrothermal precipitation at local scale. However, the element enrichment in nodules seems to be largely controlled by the process of diagenesis (Glasby, 2000). For diagenesis, the viscosity and thickness of the thin top sediment layer, also known as acoustically transparent sediment layer (ATSL), occurring between seawater and semi-consolidated sediment at the seafloor play important roles (Mukhopadhyay and Nath, 1988; Usui et al., 1987). The degree of diagenetic mobilisation of elements essentially depends on the rate of sedimentation and the upward flux of dissolved elements within the sediment column to reach the peneliquid sediment–water interface of ATSL above. The ATSL is a soft mixture of sediment, seawater and dissolved elements. The upwards flux of metal-leached warm solution from the sub-surface depth to the ATSL through diffusion has been of paramount importance in the diagenetic process. This flux again basically depends on the seafloor volcanism and sub-surface igneous activities in the neighbourhood. The temperature gradient within the ATSL, degree of remobilisation of metals from sediment, rate of upwards diffusion within the sediment and oxidation potential of the overlying water column are some of the key aspects which appear to control the diagenetic growth of nodules. A large seamount (located approximately at 12 360 S and 76 170 E, sector C), several smaller abyssal hills and the neighbouring seafloor bottomed by siliceous
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Mukhopadhyay, Ghosh and Iyer
Table 5.16 Influence of bottom sediment types on the composition of manganese nodules (wt%) Sediment typesa Calcareous
Mn Fe Ni Cu Co Zn Pb Mn/Fe
Siliceous
Red clay
Indian
Pacific
Indian
Pacific
Indian
Pacific
15.84 14.48 00.40 00.14 00.21 00.05 00.10 01.09
08.15 12.94 00.11 00.09 00.08 – – 00.48
24.60 07.10 01.10 01.04 00.11 00.12 0712* 03.46
29.14 05.67 01.53 01.08 00.27 00.11 00.06 05.14
21.70 09.90 00.66 00.52 00.18 00.07 00.12 02.19
20.17 07.97 00.81 00.49 00.29 00.08 00.12 02.53
Thickness of peneliquid/ATSL layer (m)b Seamount summit
Parametres
Average 08 Range 0.4–19.5 Nodule composition: Mn/Fe 2.52 Nickel þ 1.91 Copper (%) Cobalt (%) 0.13
Seamount foothill
Abyssal hill
Abyssal borderland
Abyssal plain
10 4.5–28.2
07 0.5–11.2
14 3.0–16.2
17 8.2–29.7
4.36 2.69
3.48 2.58
3.88 2.84
3.74 3.00
0.12
0.12
0.11
0.09
Concentration of lithic elements in nodules compared to sediment c
K Na Mg Ca Ba Si Al Ti P Sulfur a
Manganese nodule
Bottom sediment
Concentration coefficient
00.73 01.97 01.57 02.23 00.23 07.69 02.70 00.69 00.37 00.51
01.33 01.82 01.42 13.60 00.26 19.65 05.35 00.26 00.11 00.30
0.5 1.1 1.1 0.16 0.9 0.4 0.5 2.6 3.3 1.7
Skornikova and Andrushenko (1974), Dymond et al. (1984), Jauhari and Pattan (2000), Ghosh and Mukhopadhyay (1999), *in ppm. b Mukhopadhyay et al. (2002). c Lisitzin (1996), McKelvey (1986), Baturin (1988). Note: ATSL ¼ Acoustically Transparent Sediment Layer.
Ferromanganese Deposits
191
sediment in the IONF were chosen to investigate the influence of the ATSL (Table 5.16). The summit of the seamount is <4300 m from a surrounding depth exceeding 5200 m. Of the five locations examined for this purpose, four were distributed in sector C, with one close to the summit of the seamount, the others from a nearby abyssal hill, from the plain close to the foothill of the seamount and from plain area far away from any topographic elevation. The fifth sampling station occurs at the boundary between sectors A and B representing a typical abyssal environment. The soft, peneliquid ATSL was delineated using a 3.5 kHz echo-sounder, the thickness varying from <1 m near the summit to about 30 m in the abyssal plain. In addition, the composition of nodules was also determined to obtain a fair picture of nodule dependence on the ATSL characteristics vis-a`-vis topography. It is found that close to the seamount summit (water depth 4691 m) where the ATSL is thin (8 m), the nodules are extremely smooth, less spherical (average sphericity 34%), with nuclei to oxide thickness ratio 1.75. The nodules predominantly are of medium size (40–60 mm, population 67%), with hard mud and volcanic rock fragment forming most of the nuclei. Here, nodules are 5.46 times more abundant than the encrustations. The abundance of nodules is 5.53 kg/m2 and is characterised by low Mn/Fe ratio (2.52) and low grade (2.04%). The nodules from another abyssal hill (water depth 4874 m) are predominantly small in size (about 51%), smooth to rough textured, sub-spherical to discoid-shaped and occur on a 7-m thick ATSL. Registering an abundance of 5.52 kg/m2, the nodules here mostly had volcanic rock fragments as nucleus, and thereby increasing the nucleus to oxide thickness ratio to 1.22. The nodules here show moderate Mn/Fe ratio (3.48) and grade (Ni þ Cu þ Co ¼ 2.70%). In contrast, nodules from plain area in siliceous clay domain (water depth 5269 and 5300 m, respectively), where the ATSL is relatively thick (average 10 and 14 m), are rough textured, relatively more spherical (average sphericity 39%) and show low oxide to nucleus ratio of 0.66 to 0.59. These locations show nodule abundance of 6.71 and 6.30 kg/m2, respectively, and are populated largely by nodules (8.42 and 9.46 times more than encrustations). Commonly nucleated by altered rock fragment and older nodule, the nodules here display a mixed assemblage of mono- and polynucleation. About 42 to 47% of the nodules in these locations are of medium size, with a high Mn/Fe ratio (4.36 and 3.88) and high grade (2.81 and 2.95%, respectively). The area occurring at the boundary between sectors A and B records the influence of detrital sedimentation (Nath et al., 1989) and associated high biological productivity. Consequently this area, though bottomed by siliceous clay, shows a clear departure in nodule properties from those of other locations in terms of highest thickness of the ATSL (average 17 m), and hosting small-sized mononucleate nodules (about 72% are from 20–40 mm class) of atypical shape. An increase in oxide to nucleus ratio, roughness, sphericity, Mn/Fe ratio and Ni þ Cu percentage in nodules from the plain area are thus found directly proportional to the ATSL thickness (Table 5.16; Mukhopadhyay and Nath, 1988). This suggests a high degree of metal contribution from bottom sediment through the process of diagenesis. Hence, variations in nodule composition reflect the combined effect of geochemical processes influenced largely by the nature of ATSL and different bathymetric settings. Economic deposits of ore grade nodules in both the ENP and the CIOB occur in abyssal environment characterised by biosiliceous
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bottom sediment. The importance of biosiliceous substrate is elaborated later when origin of nodules is discussed. But it is important to note here that systematic mapping of the deep-sea sediments to delineate siliceous substrate cannot be ignored for ore grade nodule search.
2.4. Role of secondary eruption The size, shape, composition of the nucleus and the thickness of nucleus to oxide layer in a nodule influence to a large extent the morphology and the bulk chemical composition of nodules. IONF nodules mostly comprise altered basalts, shark teeth, broken pieces of old nodules and volcanic rock fragments as nucleus. In this regard, the reported presence of a large volcanic province in the IONF could have significantly contributed both nuclei and metals for the growth of the ferromanganese deposits (see Chapter 3). In the IONF, a majority of the nucleating materials were available through Late Miocene volcanism around which precipitation of ferromanganese oxides was initiated between 8 and 3 million years (Ghosh and Mukhopadhyay, 1995). If we agree that about 90% of manganese introduced into the ocean has a hydrothermal origin, there is likely to be a strong relationship between submarine hydrothermal activity and composition and grade of nodules (Glasby, 1988; Manheim and Lane-Bostwick, 1988). However, a majority of such hydrothermal activities are mostly found close to the mid-oceanic ridges, the nearest of such spreading ridge being located at a distance of about 1000 km from the IONF. Considering the slow rate of accretion (growth) of the deep-sea nodules (few milimetres in million years), the rapid mixing time of manganese in oceans (c. 1000 year) and short half-life of Fe2 precipitation (c. 2–3 min), it is unlikely that the metals derived through hydrothermal activities from the far-away ridge crest will have considerable impact to influence the grade of IONF nodules. By contrast, there are indications that the IONF experienced several in situ localised magma eruptions during the last 50–60 Ma (see Chapter 3). It may be plausibly inferred that flux from these local eruptions contributed metals to the water column and subsequently enriched the ferromanganese deposits in the IONF. The eruptions are mostly found near the ancient seamounts or along the major faults. In fact, about 71% of secondary eruptions occur in sector C, followed by 19% of eruptions in sector B and 10% in sector D (Table 5.17), while no secondary eruptions occur in sector A. If this distribution data of secondary eruptions are compared with the grade of the nodules, we find an interesting relation: the areas with more secondary eruptions show a high grade (sum of Ni, Cu and Co). This direct relation suggests that secondary eruptions had influenced the grade of nodules, by supplying metals both to the water column for hydrogenous precipitation and to the bottom sediment for later accretion through diagenesis. The only exception to this relation is seen in sector A, which hosts high-grade nodules (2.89%), despite the absence of any recognisable secondary eruption (Table 5.17). While the exact reason for such departure is unknown, it seems possible that relatively high biological productivity influenced the enrichment of nodules in sector A (Mukhopadhyay et al., 2002). Increased sedimentation of lithogenic detritus from continents in this sector is likely to have covered a large
193
Ferromanganese Deposits
Table 5.17
Abundance and composition of IONF nodules Sector A
Sector B
Sector C
Sector D
8.72 1.83
12.69 4.12
18.79 5.34
13.73 5.82
24.50 1.39 1.41 0.09 2.89
24.20 1.18 1.11 0.11 2.40
25.33 1.22 1.29 0.13 2.64
24.10 0.99 1.19 0.13 2.31
2
Abundance (kg/m ) Maximum Average Composition (%) Mn Ni Cu Co NiþCuþCo Mid-plate Volcanism Abundance/Mkm2 Seamounts >500 m/Mkm2
00 00
2403 449
1263 276
900 509
Sources: Jauhari and Pattan (2000) and Mukhopadhyay et al. (2002). Note: Mkm2 ¼ Million square kilometre.
portion of the seeds occurring on the seafloor. This is exemplified by low abundance of nodules in sector A. However, high biological productivity in this region (being close to equatorial productivity zone) allowed bottom sediment to become enriched in elements. This happens when larger number of organisms dies and decomposes at the bottom releasing elements to the sediment. Subsequently through sub-oxic diagenesis within the sediment, these elements get deposited on the available seeds. Precipitation of elements on fewer seeds available on the seafloor may have caused high grade but the low abundance of nodules in this sector. While seamount population thus can be related to nodule abundance, presence of secondary eruptions (Iyer et al., 1997a,b; Mukhopadhyay, 1998) are found to influence both the chemical grade and abundance of nodules.
2.5. Sedimentation, bottom currents and environment A low sedimentation rate supports the formation of nodules. High nodule abundance occurs preferentially in those regions where sedimentation rate is extremely low vis-a`-vis in areas of higher sedimentation rate where quick burial of growing nodules occurs. The broad distribution of nodules in the world oceans reveals that the Atlantic Ocean nodules are poor in abundance and inferior in composition, than the Pacific and the Indian oceans nodules, probably due to relatively high sedimentation rate in that ocean. Note that the equatorial part of the CIOB is practically devoid of nodules, which may well be due to higher rate of terrigenous sedimentation here inhibiting nodule growth. An inverse relationship between nodule abundance and rate of sedimentation is therefore emphasised. Further, the observation that high Mn/Fe ratio in nodules is related to low sedimentation rate, and that mononucleate nodules form under lower sedimentation rate than polynucleate nodules would suggest that sedimentation rates do influence the nodule distribution,
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Mukhopadhyay, Ghosh and Iyer
morphology and chemistry (Glasby et al., 1982; Piper and Williamson, 1977; Von Stackelberg, 1984). The role of bottom currents in the context of variations in nodule grade is also considered crucial. High bottom current generally hinders accumulation of sediment, and thereby creates environments favourable for nodule growth. Such a higher bottom current velocity caused by the entry of AABW in the ENP lowers sediment accumulation rate and favours abundant formation of nodules there (Glasby et al., 1982). In the IONF, radiochemical analyses and radiolarian studies indicate that about 175 ka of sediment record is missing. A sufficiently strong bottom current could have caused this (see Chapter 4; Banakar et al., 1991). As suggested earlier, an oxidising environment, represented by a highly positive redox potential, is primarily required for economic grade nodule formation. Manganese enrichment increases with the degree of oxidation. Also the viscosity and the thickness of ATSL play important roles in nodule formation and element enrichment. Thick ATSL favours diagenesis, which is reflected in high Mn/Fe ratio and higher concentration of Ni and Cu in nodules occurring on thick peneliquid layers. The role of increased biological productivity to contribute metals to the sediment on their decomposition and later diffusion of such metals through sediment pores to the nodules cushioned on the ATSL has been of prime importance to chemical-enrichment of manganese nodules in the IONF. The upward diffusion of metals is enhanced in the locale of any sub-surface warmer region, caused probably by igneous activities facilitating secondary eruptions. Investigations on the role of bacteria in creating an environment conducive (or non-conducive) to the formation of ferromanganese deposit have been carried out (Das et al., 2005; Sorokin, 1972). The results suggest that abundance, and aerobic/ anaerobic viability of bacteria, their autotrophic potential, coupled with geochemical, sedimentological and physiographic nature of the substratum appear to be extremely important parameters, contributing to the process of incorporation of metals into the nodules through scavenging followed by diffusion and/or precipitation.
3. Dynamics of Nodule Formation The formation and growth of manganese nodules involve several complex aspects many of which are not very well documented. Over the last two decades attempts have been made by various workers to understand the dynamics of nodule formation in the IONF, involving the source, age and the processes of the formation. A model for the formation of nodules in this field has also been suggested.
3.1. Source of elements Identifying the source of elements for the formation of nodules in the IONF has been one of the most difficult tasks. The source may be more than one: (1) continental weathering (elements transported by rivers and other drainage systems),
Ferromanganese Deposits
195
(2) submarine volcanism (low-temperature hydrothermal solutions or volcanic exhalations), (3) weathering (halmyrolysis, i.e. elements released through alteration of basalts and transported in dissolved state) and (4) cosmic material (increased sedimentation of cosmic spherules). Although the sources listed above may be appreciated generally, uncertainty remains as to what extent the distributions of nodule deposits depend on these sources. For example, terrigenous supply of metals cannot be directly evaluated because of the apparent lack of correlation between drainage basins and nodule deposits. Also, the distribution of nodule deposits does not always show relation to that of the mid-oceanic ridges and volcanoes. Submarine alteration (halmyrolysis) could be an important source of elements to be transported as dissolved as well as particulate phases, but the extent of contribution is again difficult to estimate. Increased fall of cosmic materials may account for an enhanced supply of elements, notably Ni, but the details of the supply potential are not known (Ghosh and Mukhopadhyay, 1999). It is evident from the above that the elements may be derived in a variety of ways from more than one ultimate source. Long-distance transport of elements from these sources by advection, diffusion processes (Martin and Knawer, 1985), and diffusion loss of Mn from highly reduced sediments to bottom water (Dymond et al., 1984) may be considered important in this context. The role of the sources to enrich the oceanic system by additional fluxes, circulation and mixing should not be underestimated. The metals supplied from above sources are drawn into the nodules through complex chemical processes of adsorption, absorption and precipitation. These are described later.
3.2. Nodule at the sediment–water interface: A paradox The process of accumulation of elements in nodules has been a complex phenomenon that deservedly has drawn major attention over the decades. The preferential abundance of nodules at the sediment–water interface appears to be a paradox. This is because sedimentation normally represents a continuous process even in abyssal regions, and the nodules are then likely to be found gradually buried. Also, the growth rate of nodules in the order of few mm/106 years in deep seas is much lower than the sedimentation rate in the order of few mm/103 years, which would suggest a rapid nodule burial. However, rather surprisingly, the nodules are found to occur at the sediment surface. Studies have been carried out to identify the mechanism, which promotes occurrence and the growth of nodules at the sediment–water interface by avoiding sediment accumulation on them. Nodule occurrence at the seafloor could be due to erosion or non-deposition of sediment in a particular area. Nodule morphology, described previously, provides some interesting clues to resolve the question of maintenance of nodules at the sediment–water interface. The removal of sediment by the action of bottom currents over long periods could help nodules remain at this interface. Smaller nodules being spheroidal are susceptible to movement, rolling and easy overturn on the sediment surface. These nodules with smooth surface are more mobile on the seafloor, the bottom current velocity dictating the extent of nodule movement. As a result, these
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Mukhopadhyay, Ghosh and Iyer
nodules keep themselves free of sediments and remain at the sediment–water interface. The role of benthic organisms in this regard is also very important. Deep-sea bottom photographs have shown that these organisms remove sediments from the upper surfaces of nodules (Paul, 1976). Moreover, the burrowing and pumping activities further help the process of maintaining nodules at the sediment surface. The bottom current energy expended by benthic organisms is significantly higher by many orders of magnitude than the energy required to maintain nodules at the sediment surface. Following Archimedes’ principle, the gravitational difference between nodules and sediment may also favour nodule occurrence at the sediment surface. It is also shown that to maintain nodules at the ATSL, the force required is approximately proportional to the third power of nodule diameter. It is estimated that over a thousand year period (assuming a sedimentation rate of 1 mm/1000 years), an amount of 6.4 10–6 joules and 1.9 10–3 joules would be required to raise 1 mm every year to maintain the 1 and 8 cm diameter nodules, respectively, at the sediment–water interface (Glasby, 1977). It is reported from some areas that nodules tend to break by themselves after reaching a critical age due to internal stresses—very large nodules are seldom found in ancient seafloor or in regions of slow nodule growth (Heye, 1975). However, larger nodules may be maintained at the sediment–water interface by bioturbation as observed in the ENP (Glasby et al., 1982). The movement of sediments (along slopes) can also help derive the required energy to maintain nodules at the interface. With increase in size, the nodules become ellipsoidal-discoidal and take a hamburger shape. Such nodules show a very rough bottom surface (partly buried within sediments) and smooth top surface with an equatorial bulge around the centre marking the sediment–water interface. These nodules are obviously less mobile (i.e. more static) on the seafloor. It is possible that oxides from the bottom side of these larger nodules fixed within sediment may dissolve in a relatively reducing condition and diffuse upwards in solution and reprecipitate at the comparatively more oxygenated sediment–water interface (Kent, 1980). If so, this mechanism will lead to some sort of a continuous ‘auto-lifting’ to maintain consistent growth of nodules at the sediment–water interface. Hence, metal-rich rough lower surface of larger nodules, formation of the equatorial bulge possibly due to enhanced reprecipitation at the sediment–water interface and the presence of high relief mammillae along this bulge (Mukhopadhyay, 1988) are some of the possible indications in support of the ‘dissolution ! diffusion ! reprecipitation’ mechanism in nodule growth (Friedrich et al., 1977). A greater degree of investigation would be required in the future to understand the nature of various forcing parameters responsible to maintain nodules at the sediment–water interface. These parameters among others are friction at benthic boundary layer, viscosity of ATSL, bottom water current, nodule morphology, specific gravity of nodules and burial susceptibility.
3.3. Processes of nodule formation The occurrence of nodules at the sediment–water interface would readily suggest that the bottom water and the sediment pore water (i.e. interstitial water) are the two very likely sources of elements for accumulation around a nucleus to form nodule.
197
Ferromanganese Deposits
Three distinct nodule growth processes have been identified (Bonatti et al., 1972; Halbach et al., 1981): (1) Early diagenetic—elements are supplied from interstitial pore water due to diagenetic remobilisation in the sediment column, (2) Hydrogenetic—nodules formed by the slow precipitation of colloidally bound elements from near-bottom sea water and (3) a mixed genetic process—elements are supplied both by sediment pore water and by near-bottom sea water (Table 5.18). A majority of the large nodules in the IONF (and in other oceans as well) reflects characteristics of the mixed type. This is because, being larger, such nodule would be more static on the seafloor and will, therefore, draw elements both from water column on the upper surface and from pore water on the lower surface. Such growth develops morphological differences on the two surfaces. Several studies have confirmed the role played by the organisms (including bacteria) in enriching nodule deposits. For example, both the nucleus and the overlying crust of nodules are often found dented and burrowed by different organisms (Fig. 5.12). Also the metabolic activities of a microbial population may stimulate or inhibit by the composition and condition of the substratum. Depending on the composition of the substratum (siliceous or pelagic), interrelations between bacterial community and physico-chemical parameters leading to nodule formation have been found considerable (Table 5.19). One can obtain an idea on the distribution of bacteria in nodules, associated sediments and the overlying water column from Table 5.20. In fact, the prime nodule belts, such as the ENP in the Pacific Ocean (between 6 N and 20 N) and the IONF in the Indian Ocean (between 10 S and 16 300 S), occur in areas bordering the equatorial zone of high biological productivity. The association between economic grade nodules in these two major belts and abundant surface productivity strongly suggests biological enrichment of the Table 5.18 Composition (wt%) and genesis of manganese nodules in the IONF
Co
Ni
Cu
Mn/ Fe
5.70
0.12
1.31
1.39
4.77
20.97
11.48
0.19
1.21
1.13
1.83
24.16
7.69
0.14
1.21
1.13
3.14
Type Distinctive character
Mn
A
27.22
B
AB
Metals supplied from interstitial water (early digenetic growth) Metals supplied from near bottom seawater (hydrogenous growth) Metals supplied from both the sources— interstitial water and near-bottom seawater
Fe
Sources: Halbach and Ozkara (1979), Sudhakar (1995) and Ghosh and Mukhopadhyay (1999).
198
Figure 5.12
Mukhopadhyay, Ghosh and Iyer
Burrows and borings of various organisms in manganese nodules (Banerjee, 2000).
transition elements. Additionally, interaction of bacteria with seabed considerably influences the process of nodule formation. For example, the size, abundance and metabolic activities of microbes may be encouraged or restrained by the composition of the seabed. For instance, pelagic sediments in the south of the IONF have low total organic carbon (TOC), and bacterial community become nutritionally more flexible
199
Ferromanganese Deposits
Table 5.19
Bacteriological response in different sediment domains in IONF
Water depth (m) Dominant nodule type Nodule chemistry Seafloor bottom condition Temperature of bottom water ( C) Continental flux Major influencing current Dominant clay type Geochemistry SiO2/Al2O3 Excess silica (%) Biogenic silica (%) Dissolved oxygen (ml/L) Carbon/Nitrogen ratio Organic carbon (%) CaCO3 (%) Direct bacterial counts (in per gram of sediment) Total Naturally viable Aerobic viable Anaerobic viable
Siliceous clay/ ooze
Pelagic/red clay
5325 Diagenetic High Mn/Fe
5201 Hydrogenetic Low Mn/Fe
0.9–1.03 High AABW Illite
> 1.03 Low NADW Montmorillonite
6.7 22.97 10–35 4.2–4.3 3–6 0.25–0.35 0–0.5
4.5 12.27 5–10 4.1–4.2 3–6 0.15–0.20 0.5–1.5
7.2108 4.3108 9.1107 3.6107 2.3108 1.8108 1.3108 7.9107
1.4109 4.4108 7.2107 4.3107 2.6108 1.3108 3.9107 1.3107
Heterotrophic retrievability (per gram of sediment) 100% concentration of ZoBell marine Agar 4.4103 1.2102 50% concentration of ZoBell marine Agar 4.7103 4.4103 Autotrophic retrievability (per gram of sediment) Ammonia oxidiser (NI) bacteria 1.2103 1.3102 Nitrite oxidisers (NII) bacteria 8.9103 6.6102 Denitrifiers 2.2104 2.2103 14 Autotrophic uptake of C by sediment 174 (DPM)
6.8104 5.7103 3.3104 3.4103 3.0105 2.8104 1.7104 2.3103 5.7103 3.8103 478395
Source: Das et al. (2005). Note: Direct bacterial counts done by epifluoroscence microscopy. DPM ¼ Disintegration per minute.
200
Mukhopadhyay, Ghosh and Iyer
Table 5.20 Population of bacteria in manganese nodules, associated sediment and seawater Manganese nodule Population
Sediment
Top
Bottom
Nucleus
Seawater
Total number of bacteria Aerobic bacteria/cm3 Denitrifying bacteria Anerobic bacteria/cm3 Iron depositing bacteria Manganese depositing bacteria
14–37106
31–37
15–21
–
–
1.6–2.9108 0.5–3.1
0.9–5.4
0.1–5.4
00
0.1–5.4
0.2–3.0103 0.1–5.4103 0–6105
00
1.2–54103 0.1–12103 0.5–31103
4–9
–
92–7.6105 0–120
0–31
0–120
0.9–40103
103–107
104
103–106
105–106
104–105
Sources: Sorokin (1972), and Baturin (1988). Note: Populations in million, if not indicated otherwise.
here as compared to microbes occurring on siliceous sediment domain in the north (Das et al., 2005). Organisms normally scavenge trace quantities of metals from water column during their life and get settled on the seafloor after death. During decomposition and subsequent dissolution of their tests at the seafloor, these dead organisms release various metals (such as Mn, Ni, Cu and Zn) to the sediment, which gets continuously covered with next settling sediment. However, the porous nature of the siliceous sediment, in particular, supports greater mobility of the transition elements within the sediment. Characteristically, bio-siliceous sediment (such as siliceous ooze) having a loose porous texture is the predominant substrate in both the ENP and the IONF. The interstitial waters carry these elements upwards to the relatively oxidising environment at sediment–water interface causing enhanced supply of the elements for the nodules. Hence, a favourable biogeochemical cycle, high productivity and porous bottom sediment appear responsible for early diagenetic contribution of elements in the genesis of nodules of these belts. A majority of IONF nodules however shows higher contribution of metals through hydrogenetic precipitation from the overlying seawater. The possible mechanism for such hydrogenous precipitation of metals is flocculation and deposition of colloidal iron oxides under high pH condition. This is followed by deposition of manganese minerals through autocatalytic partial oxidation of dissolved Mn2þ ions by precipitated Fe3þ oxide (Burns and Burns, 1977). But, at places in the IONF, there has been substantial diagenetic contribution for the enrichment of metals in the nodules. In such cases, upwards remobilisation of metals is favoured by porous to semi-porous siliceous bottom sediment. During diagenesis, Mn2þ is first adsorbed in
Ferromanganese Deposits
201
Mn4þ and then oxidised by microbes to become Mn4þ, which in turn again adsorbs Mn2þ, and the process repeats. Based on the amount of accumulated organic matter at the sediment–water interface (where oxidising potential, i.e. Eh ¼ 0), a boundary exists between the top oxidising and the bottom-reducing environment (Horn et al., 1972). For example, high concentration of organic matter at the seafloor consumes more oxygen for their decomposition to make the nearby environment reducing, and elevating the Eh boundary much above the sediment–water interface, while a moderate concentration of organic matter reduces the Eh to the level as at sediment– water interface. A low concentration of organic matter helps place the Eh boundary beneath the interface within the sediment. Hence, depending on the concentration (and subsequent decomposition) of organic matter at the bottom environment, ideal situations for diagenetic remobilisation are created within the ATSL. During diagenesis the processes of upward and lateral diffusion of metals, particularly of manganese, copper and nickel, through the sediment are significant. The average element concentrations in various types of nodules (i.e. buried, as well as macro- and micro-nodules on the surface) in the IONF reveal that micronodules have the highest Mn/Fe ratio (19.45) and grade (Co þ Cu þ Ni ¼ 1.9%), followed by macronodules (2.82 and 2.14%, respectively) and buried nodules (2.33 and 1.76%, respectively). Such concentrations in the associated bottom sediment are poor (0.30 and 0.50%, respectively), suggesting transfer of metals from sediment to the nodules resting at the sediment–water interface, in preference to the nodules buried within the sediment column ( Jauhari and Pattan, 2000). Using a three-component diagram, the geochemistry (contents of Mn, Fe and Ni þ Cu þ Co) can be used to identify the process of metal enrichment in nodules (Halbach et al., 1981). The Mn/Fe ratio at 2.5 separates the hydrogenetically formed nodules from those of the early diagenetically formed nodules (Mn/Fe > 2.5 ¼ diagenetic). Again, contents of Ni and Cu in nodules increase with increase in Mn/ Fe till the ratio reaches 5. Beyond this ratio a further increase in Mn/Fe values heralds in fact a decrease in Ni and Cu contents. Therefore, the Mn/Fe ratio of about 5 appears to represent not only nodules best in terms of economically valuable metal contents, but would also represent the critical point of concentration reversal. The geochemistry suggests that element concentration in IONF nodules was largely accomplished through hydrogenetic process, though an Mn/Fe ratio ranging between 0.96 and 8.76 indicates the contribution both from water column (through hydrogenesis) and from sediment pore water (through diagenesis). In the IONF and the ENP, enhanced supply of transition elements such as Ni, Cu, and Zn with higher Mn/Fe ratio probably led to the stabilisation of todorokite as the principal manganese oxide phase (Glasby et al., 1982; Halbach et al., 1981). By contrast, low-productivity regions, such as the red/pelagic clay sediment domain in the southern part of the CIOB and that in the SWP Ocean, essentially show a greater influence of hydrogenetic activity. The nodules of these areas are largely formed through precipitation from seawater and are characterised by a lower Mn/Fe ratio and lower Ni, Cu and Zn contents (but relatively higher Fe and Co contents). This might have favoured the formation of d-MnO2 rather than todorokite as the principal manganese oxide phase in these nodules. In summary, it is evident now that variable contributions of different accretionary processes are
202
Mukhopadhyay, Ghosh and Iyer
important for nodule growth on the ocean floor. Elements may be seawater-derived as well as sediment-derived, the relative dominance of a particular source being indicated by the Mn/Fe ratios. Hence, contribution of seawater and interstitial water to a large extent influences the trend of metal enrichment in nodule as well as nodule mineralogy (Aplin and Cronan, 1985; Halbach and Ozkara, 1979).
3.4. Model of nodule formation It is well recognised that the local and regional oceanic environments to a large extent influence the various characteristics of IONF nodules. The three most influencing parameters for the formation of nodules in the IONF are (1) availability of seed (2) seafloor characteristics and (3) source of metals. Various combinations of these three parameters to hypothesise the nature of nodules expected to grow under a given environment are shown in the form of a model of nodule genesis in the IONF (Fig. 5.13). The model is essentially based on the fact that seamounts, large faults and FZs are generally responsible for supply of rock fragment as seed for the formation of nodule while the nature of bottom sediment (ATSL), sediment type, biological productivity, secondary eruption and low rate of sedimentation play significant roles in influencing the nodule composition and grade (Mukhopadhyay et al., 2002). The growth and enrichment of nodules are generally facilitated by enhanced metal supply to the water column and a congenial substratum favouring diagenetic process of element incorporation. The enrichment sequence for divalent metals in the nodules through diagenetic process compared to the hydrogenetic one is Cu > Ni > Zn > Mo Mn > Ti Ba (Friedrich et al., 1983). A study of the conditions that existed during the last 10 Ma in the IONF (age of IONF nodules is less than 10 Ma) has been made. While siliceous clay and ooze comprise the floors of all sectors within the IONF (except perhaps the southernmost part of sector D), the influence of detrital sedimentation, and consequently the rate of sedimentation increase towards north (sectors A and B). The sedimentation rate in the IONF decreases from 9 mm/ka in the north to 1 mm/ka to the south (Borole, 1993a). In addition, the degree of biological productivity is also enhanced towards lower latitudes. However, the availability of seeds to form nodules is not that promising in these two sectors (A and B), as most of the tectonic activities (which supplies seeds) and intra-plate secondary volcanism (which supplies both metal þ seed) are concentrated in the south (sectors C and D). In sector A, the thick oxide layer (nucleus:oxide ratio [N:O] ¼ 0.57) in nodules has high Mn/Fe ratio and high grade (2.89%). A relatively thick ATSL (>30 m) and comparatively high biological productivity in this sector ensure contribution of metals to the nodules through both hydrogenetic and diagenetic processes. This sector incidentally records high sedimentation rate, many times more than the growth rate of nodules (1.2–3.2 mm/Ma; Banakar, 1990), and hosts very low abundance of tectonic (fault and fold) or volcanic (seamount and secondary eruption) activities. These may have been the reason for an extremely low abundance of nodules in the sector. In fact, the underlying crust of this area was formed during the
BLP +++ SDR +++ SMT 06% FLT 02% RFT 00% RMS 65 m SEP 00% SDP >90% ATSL Thick
A
A = 1.83 G = 2.89
Siliceous + terrigenous clay BLP +++ SDR ++ SMT 10% FLT 13% RFT 43% RMS 194 m SEP 19% SDP 87% ATSL Mode
C
B
A = 4.12 G = 2.40
Siliceous clay
BLP ++ SDR + SMT 61% FLT 68% RFT 26% RMS 90% SEP 71% SDP 90% ATSL Thick
A = 5.34 G = 2.64
SMT FLT RMS RFT SEP BLP SDP SDR
Seed
Metal
Siliceous clay D
A = 5.82 G = 2.31
BLP + SDR + SMT 23% FLT 17% RFT 49% RMS 154 m SEP 10% SDP <80% ATSL Thin
Siliceous + red clay
Figure 5.13 Probable model of formation of nodules in four sectors of the Indian Ocean Nodule Field (IONF) (Mukhopadhyay et al., 2002). BLP ¼ biological productivity, SDR ¼ sedimentation rate, SMT ¼seamount population, FLT ¼ fault population, RFT ¼ reverse faults, RMS ¼ root mean square, measure of seafloor crumpling, SDP ¼ sediment porosity, ATSL ¼ acoustically transparent sediment layer, SEP ¼ secondary eruption.While
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Mukhopadhyay, Ghosh and Iyer
period (before 58 Ma) involving high rate of spreading (90 mm/year, half rate), extreme stretching and least compression. Sectors B and C represent a unique situation where moderate-to-low biological productivity, low-sedimentation rate but fairly substantial concentrations of intraplate volcanic activities are reflected in an increased abundance of nodules. The presence of many closely spaced south-facing reverse faults in sectors B and C suggests crustal deformation and release of stress through the deformed fault planes. These may have opened up several sealed conduits for the sub-surface trapped magma to ascend. The resulting secondary eruption (some as young as 10 ka) was likely to contribute rock fragments and elements to the seawater. Such sources of metals had certainly enriched the grade of the nodules through both hydrogenetic and diagenetic processes. The formation of nodules in sectors B and C and their enrichment in Mn, Cu and Ni, in particular, appear to be partially contributed by elements from sediment interstitial water. Leaching of elements from the bottom sediment by interstitial water, warmed by sub-surface igneous activities caused diffusion of elements (largely Mn, Cu and Ni) to the sediment–water interface. These elements, in turn, may ultimately get incorporated into the nodules through diagenesis. As discussed, the upwards diffusion of element-laden interstitial water and the discharge of elements ultimately at the sediment–water interface are facilitated by various physico-chemical conditions. When the concentration and temperature gradient at this interface become steep, and when the oxidation potential of overlying seawater is high relative to the interstitial water, rapid precipitation of Mn, Cu and Ni occurs. The igneous activities/intrusives within the sediment in sectors B and C could provide the required energy to extract and transport metals from the interstitial pore spaces to the seafloor. So, the diagenetic contribution of metals to the nodules of sectors B and C takes place in addition to that obtained predominantly through hydrogenesis. In sector D, an extremely low-sedimentation rate (1 mm/ka) and increased occurrence of south-facing reverse faults (51%) could be responsible for high abundance of nodules (5.82 kg/m2; Table 5.17). The faults may have originally been north-facing normal faults, which were transformed to reverse faults under large-scale tectonic compression. There have been very few mid-plate volcanic activities in this sector. The biological productivity as well as the thickness of the ATSL is extremely low in sector D. The absence of igneous activities in the subsurface layers of this sector, coupled with other unfavourable seafloor conditions for metal accumulation, as described above, may account for reasonably low diagenetic contribution to the nodules in this sector. Such a situation is responsible for the formation of nodule deposits of relatively low grade.
SMT, FLT, RFT, RMS and SEP are responsible to supply seed material, the SDR, SEP, ATSL, BLP and SDP control hydrogenetic and diagenetic contribution of metals to the nodules. Higher values (þ) of BLP, ATSL, SDP and lower values ( ) of SMT, FLT, RFT help sector A to have high grade^ low abundance of nodules.The opposite is true in sector D. An optimum balance of these parameters added with high (þ) SEP makes sectors B and C ideal for the formation of nodules of high grade and abundance.
205
Ferromanganese Deposits
4. Ferromanganese Encrustation Ferromanganese encrustations have regularly been recovered from the deep sea along with nodules, but in view of the more widespread nature of occurrence and the economic significance of nodules, the crusts have been less intensely examined. Encrusted samples have often been studied in assemblages mixed with nodules, and even at times the chemical grade of nodules and crusts have been reported together, to represent deep-sea oxide deposits.
4.1. Occurrence and characteristics As in other oceans, ferromanganese encrustations have been described from the CIOB, where they occur in different physiographic settings (Table 5.21). They represent ferromanganese oxide accumulation in the form of slabs, pavements and coatings on basaltic substrates at different depths. These blanket-like, largely smooth-surfaced, brownish black to black accumulations occur in different dimension and thickness (20–60 mm; Figs. 5.14 and 5.15; Banakar and Borole, 1991; Iyer, 1991; Nath et al., 1992, 1997). The substrates of these encrustations (like nucleus in nodules) show a wide variety, for example, hardgrounds, altered rocks/pumice, indurated claystones and broken parts of animal body. Hardgrounds are formed by syn-sedimentary lithification of pelagic zeolitic clay following diagenesis of palagonite, an alteration product of volcanic materials (Gupta, 1995). The substrates of the encrustations in the IONF are mostly Paleogene in age, and often show traces of ichthyoliths (phosphatic microscopic skeletal debris of fishes) and variable degree of
Table 5.21
Composition (wt%) of manganese crusts from different oceanic settings Continental margin
World oceans Mn Fe Ni Co Cu Mn/Fe
38.69 01.34 00.12 00.01 00.08 28.90
Indian Ocean Mn Fe Ni Cu Co
– – – – –
Seamount
Mid-ocean ridge
Deep sea
14.62 15.68 00.35 01.15 00.06 00.92
15.51 19.15 00.31 00.40 00.08 00.81
17.99 17.25 00.51 00.32 00.23 01.04
12.37–23.89 09.20–21.50 00.29–00.43 00.05–00.59 00.22–00.47
07.79–14.41 14.70–29.50 00.10–00.11 00.03–00.05 00.04–00.09
16.89–22.73 12.66–18.16 00.31–00.46 00.15–00.39 00.29–00.42
Sources: Rao and Pattan (1989), The Open University (1995), Rao (1992) and Nath et al. (1997).
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Figure 5.14 Ferromanganese crust recovered from Afanasy Nikitin seamount (left and centre, depth 1700 m) and from the Indian Ocean Nodule Field (IONF) (right, depth 5250 m) (courtesy VK Banakar, Goa, India).
Surface texture of the crust
Smooth
Botryoidal to smooth 0 1
Mixed layer (Fe-Mn layer and aluminosilicates)
2
Weathered layer (aluminosilicates from basalt weathering)
Glass Pillow basalt
Depth across layers (mm)
Fe-Mn oxide layer
Fe-Mn oxide layer
3 4 5 Mixed layer (Fe-Mn oxide and aluminosilicates)
Pillow basalt 10
Figure 5.15 Vertical cross section of crust having variable surface texture (after Iyer,1991).
Ferromanganese Deposits
207
bioturbation. Mobile epibenthic megafauna are the main constituents of bioturbating organisms (Banerjee, 2000). Ear-bone (Tympanic Bulla) of a minke-whale (Balaenoptera acutorostrata) acting as a substratum to thick oxide encrustation was reported from a water depth of 5200 m in sector C (Banakar, 1987). It is found that encrustations occurring close to topographic highs have thick oxide coating with smooth surface texture, probably suggesting rapid precipitation from seawater. The encrustations recovered from higher elevation on the seamount and those from the abyssal plain from water depths below the CCD in the IONF do not show much compositional variation. For example, the Mn flux of 60–140 Mg 2cm 2ka 1 for a seamount encrustation is closely comparable to the flux of 40–150 Mg 2cm 2ka 1 in nodules in the abyssal plain, which indicates similar removal mechanism of Mn from seawater. Even the Co and Ni fluxes are of similar order and appreciably higher than the Cu flux. The oxide layers of the encrustations in the IONF show a precipitation rate varying between 1.7 and 3.4 mm/Ma (Banakar and Borole, 1991; Banakar and Hein, 2000). The crusts are mostly formed in the hydrogenetic way by precipitation of dissolved materials from the water column. Metallic elements, in turn, may have been contributed to the water column by chemical interaction, outpourings of fractionated melt and hydrothermal seepages. Such volcanic exhalations in the local scale were encountered in the basin (represented by volcanic-hydrothermal materials, Iyer et al., 1997a), at raised topographic expressions (represented by secondary eruptions at the foothills of large seamounts, Mukhopadhyay, 1998) and near structural lineaments (e.g. trace of the triple junction on the Indian Plate, reactivated FZs, Mukhopadhyay et al., 2002). Hence, a combined contribution from hydrothermal and hydrogenetic components has been proposed for the formation of IONF crusts. Incidentally, the microstructural and elemental composition of various layers within the crust helps suggest the possible source of metals and the accretionary process. Several IONF encrustations show botryoids, cusps and laminations as the dominant microstructures; the botryoids formed of d-MnO2 are enriched in Co in the uppermost layers, whereas the cusps and laminations with todorokite are enriched in Cu. In view of the fact that many such encrustations are potentially important due to their higher contents of cobalt (and platinum) than that in the nodules (Banakar and Borole, 1991; Banakar et al., 1998; Hein et al., 1988, 1990; Iyer, 1991; Nath et al., 1997; Rao, 1992), examination of all types of ferromanganese encrustations from the IONF and from other world oceans is being carried out. Again, the mammilated structure of the botryoids that are convex upwards, enrichment in Co, and an Mn/Fe ratio of about unity, suggest a hydrogenous origin for most of the encrustations. Some crusts in the IONF, however, also show smooth and fine laminations with todorokite as the main mineral phase. These may suggest their formation during a period of very restricted early diagenesis (Iyer, 1991). In contrast, crusts of hydrothermal origin show low metal contents, as well as todorokite–birnessite mineralogy and very fast growth rate (tens of millimetres per million years). Such manganese encrustations have recently been recovered from the walls of the rift valley north of the Indian Ocean triple junction (IOTJ; Nath et al., 1997). These crusts, having maximum thickness of 20 mm only, are found to be
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highly porous and are characterised by very low Mn/Fe ratio, low transitional metal content, low REE contents and negative Ce anomalies (Tables 5.21, 5.22 and 5.23). The REE patterns further suggest that the hydrothermal contributions to the crusts are between 60 and 90%, the rest being contributed by the ambient seawater. Substantial quantity of hydrothermal input to the formation of manganese encrustations (Mn/Fe ¼ 0.63–0.79) has also been reported from the intersection area between the Vitiyaz FZ and the Central Indian Ridge (Mukhopadhyay et al., 1998). In the IONF, there have been reports of sporadic occurrence of ferromanganese encrustations formed in part by hydrothermal input (Iyer, 1991). Increased carbonate dissolution during Middle Miocene (10–13 Ma) has been suggested to be the possible reason for the formation of the encrustations in the IONF. This process of formation may have been caused by enhanced AABW activity during Middle Miocene. The time gap between the age of the substrate (50–60 Ma) and the initiation of encrustation formation (10–13 Ma) may be accounted for as the period of weathering and alteration of the basalt substrate. Initial alteration and replacement of 50–60-Ma-old basaltic materials followed by precipitation of oxide layers from ambient seawater could represent the process of growth of the IONF encrustations. The generally low Mn/Fe ratio (<2.5) and the presence of d-MnO2 as the main manganese mineral in the IONF encrustations suggest to their hydrogenetic origin from seawater.
4.2. Crust and paleoceanography Paleoceanographic events have been chronicled through the smooth exponential decay pattern of 230Thxs, and 230Thxs/232Thac in depth profiles of two encrusted samples from the IONF (xs ¼ excess, ac ¼ activity, Table 5.24, Fig. 5.16). Surprisingly, in both the samples, signals of the last 400 ka are missing. This is unlike the Pacific and the Atlantic crusts that reveal complete records between 10 and 13 Ma. As mentioned earlier, IONF crusts grew at a rate of 1.7–3.4 mm/Ma and show the influence of enhanced AABW activity during Middle Miocene when increased carbonate dissolution possibly initiated the formation of these crusts (Banakar and Borole, 1991). The reason(s) for the loss of this record is under intense examination these days. Another encrustation with about 17-mm thick ferromanganese oxides was recovered from a depth of 5250 m at 10 300 S/79 000 E (sector B of the IONF). The encrustation occurs near to a seamount on a compacted, indurated sediment substrate that shows signs of bioturbation. The associated microtektites recovered from the substrate belong to the Australasian strewn field and are dated as about 0.77 Ma in age (Prasad, 1994). Based on this, the growth rate of the encrustation suggests a minimum accretion rate of 7.8–22.1 mm/Ma. This rate is much higher than the average growth rates of hydrogenous encrustations, thereby supporting additional input to the growth of the encrustation, suggestively either from localised hydrothermal activities or from secondary volcanic eruptions in the basin (Iyer et al., 1997a; Mukhopadhyay et al., 2002). Paleoceanographic studies have also been made on a 72-mm thick ferromanganese encrustation, obtained from the enlarged base of a seamount (sector C). The
Table 5.22
Element abundance (wt%) in manganese crusts of the Central Pacific and the IONF Central Pacific Hawaii Island
Mn 22.5 Fe 17.2 Ni 0.38 Cu 0.10 Co 0.59 Mn/Fe 1.31
Indian Ocean Nodule Field (IONF)
Marshall Island
International water
74C (D)
78C (D)
209D (C)
219D (D)
380F (C)
663X (C)
F8 (C)
SS11 (C)
20.6 12.4 0.40 0.04 0.89 1.66
26.2 14.9 0.52 0.06 1.00 1.76
20.30 7.89 0.55 0.53 0.19 2.57
18.56 9.21 0.89 0.42 0.12 2.01
29.54 3.95 1.46 1.58 0.08 7.48
26.81 6.58 1.14 1.03 0.11 4.07
19.27 13.81 0.29 0.08 0.27 1.39
17.80 13.16 0.32 0.10 0.22 1.35
16.25 16.41 0.33 0.056 0.30 0.99
18.83 16.00 0.32 0.096 0.30 1.17
Sources: Hein et al. (1992), Banakar and Borole (1991), Iyer (1991) and Pattan and Mudholkar (1991). Note: Sectors in parentheses.
210 Table 5.23
Mn Fe Ni Cu Co B C Na Mg Al Si P K Ca Ti V Cr Zn As Sr Y Zr Nb Mo Cd Sn Ba La Yb Pb
Mukhopadhyay, Ghosh and Iyer
Average composition (wt%) of manganese nodules from different oceans Atlantic
Indian
Pacific
World oceans mean
13.25 16.97 0.32 0.13 0.27 0.025 0.77 1.86 1.75 2.37 6.34 0.91 0.57 3.72 0.42 0.06 0.006 0.123 0.02 0.094 0.024 0.056 0.004 0.031 0.001 0.007 0.23 0.023 0.003 0.14
15.25 14.23 0.43 0.25 0.21 0.007 0.21 1.70 1.43 2.67 9.39 0.37 0.48 1.97 0.62 0.054 0.002 0.149 0.018 0.079 0.011 0.034 0.007 0.029 0.001 0.001 0.21 0.018 0.001 0.101
20.10 11.40 0.76 0.41 0.27 0.03 0.34 2.05 1.5 2.75 7.62 0.28 0.82 1.96 0.73 0.051 0.009 0.116 0.011 0.084 0.015 0.061 0.007 0.041 0.001 0.010 0.235 0.022 0.003 0.083
16.20 14.20 0.50 0.26 0.25 0.03 0.33 1.97 1.57 2.70 7.69 0.37 0.73 2.23 0.69 0.052 0.007 0.12 0.014 0.085 0.015 0.057 0.007 0.038 0.001 0.008 0.23 0.021 0.003 0.093
Source: McKelvey (1986).
crust grew at a uniform rate of 2.8 0.1 mm/Ma since 26 Ma on a rocky substrate of probably Eocene age. The internal older 30-mm thick zone has cuspate-type layering while the external younger part show pillar structure. Intense cooling at the polar regions and increased mixing of waters of deep and intermediate layers, leading to increased oxygenation of bottom water during Oligocene–Miocene period (age 25–15 Ma), could have resulted in such texture for the internal zones. The pillar structure at the outer part (formed later to 15 Ma) shows much higher content of oxide bound elements, and may have formed because of further increase in oxygen content in bottom waters and increased stability of seamount slopes (Banakar and Hein, 2000). Due to the change in paleoclimatic conditions during Mid-Miocene, a variation in element content in the IONF crust is encountered (Fig. 5.17).
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Ferromanganese Deposits
Table 5.24
Uranium and Thorium isotope concentrations in manganese crusts
Depth in crust (mm)
230 230
U (dpm/g)
Sample F8/380 0.00–0.22 8.01.5 0.22–0.74 7.31.4 0.74–1.08 9.151.4 Sample SS 11/663X 0.00–0.30 8.82.3 0.30–0.77 7.31.9 0.77–4.37 7.82.2
232
Th (dpm/g)
29.41.5 21.91.3 21.21.17 46.34.4 35.53.3 49.05.8
Thexcess (dpm/g)
7288.3 96.32.6 6.9 1.97 9105.5 197.63.6 34.94.3
230
Thexcess/232Th
23.01.36 4.160.2 0.320.1 21.72.5 5.720.8 0.750.11
Source: Banakar and Borole (1991). Note: 230Thexcess ¼ 230Thtotal – 238U.
Oxidised manganese ratio (O:Mn) determined by iodometric method has been studied on four nodules and two encrusted samples recovered from siliceous clay area. The O:Mn ratio of the encrustations is recorded as 1.81, which is higher than that of the associated nodules (1.73–1.75), indicating a higher oxidised state of the encrustations in comparison to the nodules. The O:Mn ratio also suggests that up to 81% of Mn occurs as Mn4þ (Pattan and Mudholkar, 1991), which is lower than that of the Pacific nodules (98% of Mn as Mn4þ; Murray et al., 1984; Piper et al., 1984). Higher O:Mn ratio in the Pacific nodules implies that these nodules are possibly more oxidised than those of the IONF (Broecker and Peng, 1982). Such variations of the O:Mn ratio in the Pacific and the Indian ocean nodules are probably due to variable oxygen availability, which depends on the rates of organic productivity (and decay), sedimentation and bottom water circulation (Pattan and Mudholkar, 1991).
5. The World Oceans Scenario Since the information was made known by Mero (1965) that several trillion tons of manganese nodules lay in the world oceans, extensive marine exploration activities were undertaken during the last four decades. The possibility of harvesting inexhaustible supply of metal-nodules from oceans helped formulate several national and international programmes. After about 40 years of study, a comprehensive assessment of manganese nodule resources remains overdue. Accordingly, in the following pages an effort is made for a purposeful assessment of nodule resources at ocean level. Since the characteristics of manganese nodule resources from the CIOB, which contains the IONF, have been fairly covered in earlier four sections of this chapter, we now briefly discuss the following:
Resources from the world oceans, and Inter-basin model for nodule growth
212 2 ⫻ 103 1.7 mm/Ma
103
2.3 mm/Ma
2.2 mm/Ma
2.5 mm/Ma
2.3 mm/Ma
3.1 mm/Ma
1.8 mm/Ma
2.5 mm/Ma
2.4 mm/Ma
2.7 mm/Ma
2.2 mm/Ma
3.4 mm/Ma
F-VIII-A
F-VIII-B
F-VIII-C
F-VIII-D
SS-XI-A
SS-XI-B
102
101
1
0
0.5
0
0.5
0
0.5
0
0.5
1.0
1.5
0.1
0.5
1.0 4.3 0.1 0.5
1.0
1.7
Depth in layers within crust (mm) 230Th exs
(dpm/g)
230Th exs
/ 232Th (ac)
Figure 5.16 Decay profiles of 230Thexcess and 230Thexcess/232Th activities in the Indian Ocean Nodule Field (IONF) crust. A probable break in growth (dotted line, 4th from left) corresponds to 150 ka BP. (Banakar and Borole, 1991). The inclined line at the bottom measures the ratio between 230 Thex and 232Th(ac), while the top line represents concentration of 230Thexcess.
3.6
10 A
7.1
20
10.7
30 B
14.3
40
17.8
50
21.4
60
25.0
70
C
3.0 0.5
Ni (%) 3.5 Co (%) Ba (%) Ce-Anomaly Cu (%) 1.0 0.20 0.25 0.30 0.20 0.25 0.30 0.10 0.15 0.200.10 0.15 0.3 0.4 0.5
Younger zone
Si/Al 2.5 Mn/Fe 0
Si Al
Mn Fe
Older zone
10Be/ 9Be extrapolated age
10 Be/ 9Be derived age
Age (Ma) 0
A = Slender compact pillar structure, B = Thick compact pillar structure, C = Randomly distributed isolated cusps increasing detritus towards base
Figure 5.17 Chemistry of ferromanganese crusts. Note sharp change in element contents during Middle Miocene(12 Ma). Age beyond 12 Ma is based on extrapolation of 10Be growth rate (Banakar and Hein, 2000).
213
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Mukhopadhyay, Ghosh and Iyer
5.1. Ferromanganese resources from the world oceans 5.1.1. The Atlantic Ocean The distribution of manganese nodules in this second largest ocean of the world has been found to be sporadic and scanty at places. This patchy coverage of nodules is caused by the probable impact of high rate of terrigenous sedimentation. Moreover, a large part of the Atlantic seafloor is covered by the Mid-Atlantic Ridge (MAR), which because of its elevation above the CCD does not offer a conducive environment for nodule formation. As a result, only few deeper, far-away basins occurring below the CCD on both sides of the MAR with low terrigenous sedimentation hold nodules of appreciable abundance. For example, widespread occurrence of nodules and encrustations has been reported from Drake Passage— Blake Plateau (Manheim, 1972) and Scotia Sea area (Cronan, 1975). In both the areas strong nutrient-rich bottom current activity helped the nodule formation process by reducing the effect of sediment accumulation and by supplying additional metals for precipitation. Ferromanganese encrustations have been recovered from the MAR and seamounts with greater frequency than in other oceans. The thickness of the ferromanganese layers in these crusts increases systematically away from the ridge axis, indicating time-dependent precipitation (Aumento et al., 1968). The greater differentiation in environment of deposition due to topographic complexities, presence of large shallow areas above CCD and intense bottom current activity made the nodule characteristics in the Atlantic Ocean different from that in the Pacific and the Indian oceans (Tables 5.24 and 5.25). Substantial input of metals from normal seawater, from terrigenous detritus and from volcanic eruptions appears to have enriched the nodules of this ocean, with limited contribution of metals through diagenetic method (cf. Cronan, 1975; Manheim, 1972). 5.1.1.1. Blake plateau Because of very little sedimentation and vigorous bottom water current, ferromanganese oxide precipitation in Blake Plateau is found mostly in the form of pavements rather than in nodular forms. However, nodules in Blake Plateau often contain todorokite as major manganese mineral, even though the environment of deposition must be rather similar to that of some seamounts where d-MnO2 normally occurs. The reason for such departure probably lies in the excessive influence of nutrient-rich AABW. It is seen that shallow areas in the Atlantic Ocean, frequented by nutrient-poor North Atlantic Deep Water (NADW), has d-MnO2 as the main manganese mineral. The Mn/Fe ratio in nodules from Blake Plateau region is greater than 1, in contrast to average ratio in the Atlantic Ocean nodules. This enrichment may have promoted formation of todorokite in these nodules. However, unlike other places, the manganese enrichment in nodules here cannot be attributed to diagenetic remobilisation of metals from the underlying sediments, simply because of strong bottom currents sweeping clear the sediments from this area. This means that not only the depth of occurrence and vis-a`-vis difference in redox potential (Eh), but also the nature of bottom water current influences the mineralogy and chemistry of nodules in the Blake Plateau (Manheim, 1972).
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Table 5.25 Element abundance (wt%) in manganese nodules from latitude-wise differing domains of the indian ocean
Mn Fe Ni Cu Co Mn/Fe
20 N^Equator (28)
Equator-20 S (73)
20 S-40 S (159)
40 S-60 S (42)
>60 S (2)
16.52 16.64 0.38 0.10 0.29 0.99
18.05 11.92 0.58 0.47 0.20 1.51
14.36 15.47 0.37 0.18 0.22 0.93
13.53 12.25 0.48 0.25 0.13 1.10
3.07 8.95 0.04 0.02 – 0.34
Source: McKelvey (1986). Note: Figures in parentheses indicate the number of samples analysed.
5.1.1.2. Drake Passage–Scotia Sea Widespread occurrence of nodules and encrustations is known from this area, located away from the MAR, below the CCD and far from land with only limited input of terrigenous material. The strong bottom current in this region also inhibits sediment accumulation. The nodules have both todorokite and d-MnO2 as the major manganese minerals. Iron is highest (maximum 40%) in samples particularly rich in d-MnO2 and moderate in those having todorokite. The concentration of Ni, Cu and Zn increases and Co decreases in samples from deeper levels. The Mn/Fe ratio in nodules from Drake Passage– Scotia Sea is higher than that in nodules occurring in other areas of the Atlantic Ocean, except Blake Plateau (Cronan, 1975).
5.1.2. The Indian Ocean The ferromanganese deposits cover an area of 10–15 million km2, and resources available are estimated to be 1.5 1011 tons. Observing an inverse relation between nodule abundance and sedimentation rate, nodules in some places cover more than 83% of the seafloor. Except high para-marginal grade in the IONF (Ni þ Cu þ Co ¼ >2%, abundance > 5 kg/m2), the resources in other basins of this ocean are largely of sub-marginal grade (grade <2% and abundance <5 kg/m2), and these are therefore not recommended for economic exploitation. Considering the fact that the economic value of any nodule deposit increases with high concentration of metals, higher abundance and largely even topography in the basin, nodules other than those in the IONF do not hold much economic significance. Nevertheless, nodule resources from various basins and areas of the Indian Ocean are briefly discussed below (Table 5.7). Such abundance in various latitudes in the Indian Ocean for example is furnished in Table 5.25. 5.1.2.1. Crozet Basin With an average water depth of 4570 m (range 4050– 5270 m), the sediments in this basin are siliceous ooze, pelagic clay and carbonate ooze. Nodules associated with each of these sediments show marked difference in composition and mineralogy. For example, the Mn:Fe ratio is the highest in nodules
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on siliceous sediment (1.51) and least on pelagic clay (0.78), with nodules in calcareous sediment showing intermediate values. The concentration of nickel ranges from 0.69% in nodules from siliceous ooze to 0.32% in pelagic clay and 0.38% in calcareous ooze sediment. The concentration of cobalt however shows a reverse order—highest (0.25%) in pelagic clay, moderate in calcareous clay (0.18%) and least in siliceous ooze (0.11%). Yet these values are lower than those in nodules from the IONF. Todorokite, birnessite and d-MnO2 have been found to be the principal manganese minerals in nodules occurring respectively in siliceous, calcareous and pelagic clay sediment domains. The average abundance of nodules in this basin varies from 5 to 10 kg/m2. 5.1.2.2. Madagascar Basin Shallow average depth (4620 m, range 3967–5209 m) of the basin, and low average values of Mn:Fe ¼ 0.75, Ni ¼ 0.20%, Cu ¼ 0.11% and high Co concentration (0.29%) characterise the manganese nodules of this basin. The concentration of Co and Pb is however higher in nodules from calcareous ooze than in those occurring in association with pelagic clay in the basin. d-MnO2 is the principal manganese mineral in the nodules of this basin, and nodule abundance vary widely between <5 and >40 kg/m2. The overall morphology and composition of nodules of this basin can be best compared with those occurring at the mid-oceanic ridges, aseismic ridges and seamounts. 5.1.2.3. Mozambique Basin The average water depth of the basin is 4880 m (range 4960–5450 m). Mainly terrigenous and calcareous ooze sediments cover the basin. Nodules occurring in association with the former sediment contain higher amounts of Cu, Ni, Pb, Zn, Co and Mn:Fe ratio (0.83) compared with nodules occurring in calcareous ooze (Mn:Fe 0.74). Within the basin there has been striking variations in manganese and iron contents in nodules. For example, Mn:Fe ratio in nodules ranges from 1.0 in Mozambique Channel to 1.21 along Mozambique Ridge to 0.83 on terrigenous sediment seafloor, and to 0.74 in calcareous ooze to 0.50 in other areas of the basin. 5.1.2.4. Seychelles–Somali Basin The average water depth of the basin is 4500 m (range 4190–4727 m), which is largely covered by calcareous ooze and pelagic clay. The nodules here contain relatively higher percentages of nickel and copper with high Mn:Fe ratio (1.17) compared to those occurring in Crozet, Madagascar and Mozambique basins. Nodules occurring in calcareous ooze areas show enrichment of elements (other than copper) than those found in pelagic clay. Todorokite is the major manganese mineral in the nodules. On the elevated areas (seamounts) in the northeastern part, the nodules show enrichment of cobalt (average 0.45%). 5.1.2.5. South Australian Basin The geomorphology of this basin shows greater variations in physiography with depth ranging between 4100 and 5967 m (average 4710 m). Two types of sediments—calcareous ooze and pelagic clay—essentially cover the basin. The nodules occurring on the former contain high percentage of manganese, copper, nickel, zinc and low content of iron, lead and cobalt, compared
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to those associated with pelagic clay. The economic potential of nodule resources in this basin is sub-marginal (Frazer and Wilson, 1980; Siddiquie et al., 1984). 5.1.2.6. Wharton Basin With an average depth (5260 m) more than that of the CIOB, this basin contains nodules rich in manganese, nickel, copper and poor in iron and cobalt. As in the CIOB, nodules associated with siliceous sediment in the Wharton Basin contain higher percentage of Mn, Ni and Cu (with todorokite as main manganese mineral), compared to those associated with pelagic clay. However, with large variations in both abundance (<5 to 15 kg/m2) and grade (Mn:Fe 1.8 to 1.4) within short distances, the occurrence of ore-grade nodules in the Wharton Basin has been far and few, and did not attract much attention of scientific community. 5.1.2.7. Other Occurrences Ferromanganese crusts of possible hydrothermal origin have recently been recovered from the walls of the rift valley north of the IOTJ (Nath et al., 1997). These crusts, having maximum thickness of 2 cm only, are highly porous. They occur on basalt substrate and are characterised by very low Mn/Fe ratio, low transitional metal, and low REE contents and negative Ce anomalies (Fig. 5.17, also see section 4.1). Therefore, it is of considerable interest to locate manganese crusts in hydrothermal settings, as these crusts may serve as indicators for sulphide mineralisation in the vicinity.
5.1.3. The Pacific Ocean The ferromanganese nodules and crusts occur extensively in the Pacific Ocean. There are five major areas in the ocean where nodules occur in large abundance: Central Pacific Basin (CPB), ENP, northeast Pacific-Musician seamount area, Southern Pacific Ocean around 60 S, and northern Peru Basin. We briefly discuss the first two areas where nodules of economic grade occur in large quantity. 5.1.3.1. Central Pacific Basin The major exploratory activities in this region, limited geographically within 15 N to 15 S and 180 W to 160 W, were carried out by the Geological Survey of Japan and China Ocean Mineral Resources Research and Development Association (COMRA), in addition to the efforts of other Euro-American countries. General distribution pattern of manganese nodules and related geological characteristics were made known through several studies in this basin (Exon, 1983; Glasby et al., 1982,1986; Mizuno, 1981; Piper et al., 1987; Rawson and Ryan, 1978; Usui et al., 1987). The CPB is bounded to the north by the mid-Pacific mountains, to the west by Marshall-Gilbert Islands, to the east by Line Islands chain and to the south by Manihiki Plateau and Penrhyn Basin. The CPB includes Magellan Trough, Magellan Rise and Nova-Canton Trough. The central part of the basin is a rather flat abyssal plain with a few isolated seamounts, whereas abyssal knolls trending WNW-ESE at a depth exceeding 5000 m characterise the northern part. A series of NW-trending island chains (Line, Gilbert, Marshall, etc.) occurring within the CPB appears to have been originated from hotspot activities (Usui and Moritani, 1992). The age of the CPB seafloor is early Cretaceous [from Deep Sea Drilling Project (DSDP) stratigraphy, Ewing et al., 1971], on which the Line Island originated at
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around 85 Ma (Late Cretaceous). Paleomagnetic evidence suggests that the CPB has mostly been below the CCD since Cretaceous, and calcareous sediments predominate only on local topographic highs. As the CPB is situated far from surrounding continents, pelagic processes, relating to surface organic productivity, control the sedimentation here. The presence of Neogene hiatus in the CPB indicates strong influence of bottom current (Nishimura, 1992). Two types of nodules are found in the CPB: nodules with gritty, granular, rough surface and the other with smooth surface. Generally, the nodules with rough surface are found in the northern and the central CPB. These nodules are very small in size, spherical in shape with concentric laminations internally, and are largely associated ˚ manganite (todorokite) as with and often buried in siliceous clay. They have 10 A 2 major mineral, and show low abundance (<5 kg/m ) but high concentration of Cu, Ni, Zn with high Mn:Fe ratio (>2) and appear to be diagenetic in origin. In contrast, nodules with smooth surface occur mostly in the northwestern CPB and Penrhyn Basin in association with pelagic clay, and are small in size, discoidal and irregular in shape. These nodules are hydrogenetic in origin, with densely laminated internal structure. They show a high abundance (10–20 kg/m2) but low Mn:Fe (1–2) and low concentration of Cu, Ni, Zn with d-MnO2 as the major manganese mineral. The nodules with rough surface require a moderate sedimentation rate and sufficient supply of organic matter through high biological productivity near the equator to keep the surface sediment reduced, whereas smooth nodules would need least or no sedimentation with bottom AABW current providing oxygen for the environment (Usui and Moritani, 1992). Nodules from Kiribati region in the eastern part of CPB also show two types of nodules: diagenetic, todorokite-rich, rough surface nodules having high Mn:Fe (3.37) and high contents of Cu þ Ni þ Zn (2.25%), and hydrogenetic, vernaditerich smooth surface nodules with concentric internal layering containing low Mn:Fe (1.37) and low Cu þ Ni þ Zn (1.02%; Bolton et al., 1992). The nodule growth rates have been 2.2–5.0 mm/million years in the northern CPB (Piper and Gibson, 1981) and 2.1–3.0 mm/million years in nodules from the central part of the CPB (Usui and Moritani, 1992). Ferromanganese crusts are often found in the shallower regions of this basin (water depth <1600 m) with maximum contents of Co and Ni reaching 1.92 and 1.26%, respectively (average 0.83 and 0.58%; De Carlo and Fraley, 1990). These crusts, based on mineralogy and chemistry, can be divided into five types: d-MnO2 (Co, Mn, Ni and Pb), Fe-oxyhydroxides and Fe silicate (Fe, Si and As), detrital aluminosilicate (Al, Cr, Si, Ti and K), biogenic phosphate (P and Ca) and biogenic non-phosphate (Cu, Ni, Ba, Zn and Cd). The d-MnO2 and biogenic nonphosphate rich types are most commonly found in areas of high productivity creating oxygen minimum zone. The abundance of detrital phase is in reverse order to the d-MnO2 distribution. Detrital input is apparently related to both aeolian and erosional bottom current processes (Hein et al., 1992). The REE concentration in the CPB crusts when normalised to the North American shale composition has been found lower than in the crusts of Hawaiian waters, but displays large positive Ce anomaly typical of hydrogenous ferromanganese deposits (De Carlo and
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McMurty, 1992). Based on geochemical criteria, the resource potential of the following areas is ranked in decreasing order: Marshall, Micronesia, Johnston, French Polynesia, Kiribati, Hawaiian and Tuvalu Islands. 5.1.3.2. Equatorial North Pacific As mentioned earlier, manganese nodules are most abundant in the ENP area of the Pacific Ocean with the world’s richest deposit located between 6 N and 20 N and 120 W and 160 W. Bounded by two major FZs—Clarion FZ to the north and Clipperton FZ to the south—the area produces nodules of highest grade and reasonably high abundance. Nodules from the ENP are most abundant at the sediment–water interface within the 2–12-cm thick acoustically transparent layer between water depths of 4300 and 5400 m. The highest abundance of nodules is found at the central part. Concentrations of nickel and cobalt are the highest in the smaller nodules of the northern part of the ENP. Nodules with high concentration of manganese occur in the southern part. The ENP seafloor is characterised by north-south trending long hills, ridges, intervening valleys and furrows. Several furrows, scattered seamounts and a number of faults mark the eastern part. In contrast, the western part of the ENP is characterised by five 1000-m-high seamounts separated by basins and ample volcanism. The abundance of nodules is high in the western part, while nodules from the eastern part show higher concentration of metals. It appears that the presence of nutrient and oxygen-rich AABW, availability of metals from in situ volcanisms and the presence of the CCD at 4900 m just above the zone of nodule occurrence have controlled the formation of the ENP nodules of the eastern part. Recently indications of hydrothermal vent activities, including sulphide crusts along the undefined fracture, have been reported at the central part of the ENP. Nodules in the ENP show exponential decay pattern for both 230Th (Thorium) and 231Pa (Protactinium) with depth. The corresponding accretion rate of the ENP nodules is 4 mm/million years. The ratio 234U/238U has only a narrow variation of 2% over the last few hundred thousand years (Ku and Broecker, 1967). An inverse relationship between cobalt content and growth rate of ferromanganese crust in the ENP suggests that the depletion of iron and manganese in bottom water creating oxygen minimum zone may be more responsible than hydrothermal input for cobalt enrichment. Rates of crust accumulation have been estimated to range from <1 mm/Ma in high-cobalt areas in absence of any hydrothermal activity to >100 mm/Ma in the cobalt-poor deposits located close to hydrothermal activity centres (Manheim and Lane-Bostwick, 1988). The abundance of the ENP nodules is largely determined by source and environment of formation and not sedimentation rate. For example, towards the equator rapid biogenic sedimentation associated with the equatorial high productivity limits nodules growth. Again, the pelagic sediments in the north accumulate at a slower rate (1 mm/thousand years) than the siliceous sediment in the south (sedimentation rate 3 mm/thousand years). Yet nodules are more abundant in the siliceous sediment area possibly due to increased biological source of metals and favourable diagenetic environment of nodule formation (Opdyke and Foster, 1970).
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5.2. Inter-basin model for nodule growth As is seen earlier, the essential requirements for the formation of nodules are nucleating materials, availability of metals in water column and sediment, seafloor tectonic and physiographic features, volcanic eruptions, favourable sediment–water interface, low rate of sedimentation, presence of nutrient-rich bottom water mass and oxidising environment. It would be interesting to discuss the inter-relationship among these parameters from different ocean basins and their sum effect on the formation of various combinations of nodule types under two headings: ▪ Tectonics and volcanisms ▪ Sediment, seawater and chemical environment 5.2.1. Tectonics and volcanisms As postulated in many studies over the past three decades, starting from Horn et al. (1972) to Mukhopadhyay et al. (2002), the abundance and grade of ferromanganese nodules and crusts do vary substantially with the given tectonic, volcanic and topographic regimes in a basin. The role of rock fragments to act as nuclei for nodules and crusts, and the contribution of volcanoes in creating topographic variations and releasing metals to the seawater and sediment have been substantial. The paleotectonic regime of the IONF included reactivation of structural lineaments, transformation of normal faults to reverse and alternately compressional and tensile stress conditions owing to plate collision and intra-plate deformation. Areas displaying intense crustal deformation, faults, folds and increased population of seamounts are found to contain nodules with high abundance. Similarly, areas infested with secondary eruptions are likely to influence both the grade and abundance of nodules (Iyer et al., 1997a,b; Mukhopadhyay, 1998). It is very likely that tectonic movement (friction) associated with high fault scarps with throw >100 m was responsible to contribute additional seeds in the form of broken and crushed rock fragment facilitating the formation of abundant ferromanganese nodules. It appears possible that with increase in intensity of compression on the crust, the chances of structural failures in the form of faults and friction augments. This contributes ultimately in high nodule abundance. Seed hypothesis is considered very important for nodule growth in the Clarion– Clipperton zone (CCZ, Horn et al., 1972). It appears that nodules are more abundant by 50–100% close to FZs, seamounts and explosive submarine volcanic areas than the areas devoid of such features. Of the 100 seamounts in Pacific, only 36 could be dated reliably through paleomagnetic method, and these 36 fall in four age groups: 36–45 Ma (Early Tertiary), 64–79 Ma (Late Cretaceous), 80–120 Ma (MidCretaceous) and older than 120 Ma (Early Cretaceous–Jurassic). It is estimated that a substantial number of these seamounts were formed mid-plate, while the rests were emplaced at the mid-oceanic ridge along with the underlying crust (Sager, 1992). Incidentally, the area allocated to China in the ENP (depth 4900–5400 m, CCD at about 4800 m) has been quite interesting, where direct influence of tectonics and volcanism on nodule characteristics can be seen. The detailed study (gravity, magnetic, swath bathymetry, deep-tow video) in this area by COMRA between 1991 and 1998
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revealed that volcanic activities (chain of seamounts) primarily dominated the western part, while north-south-oriented faults and long furrows mark the eastern part. As a possible direct consequence to these, the volcanic western part with a thicker ATSL (40–50 m) shows a higher abundance of nodules, while tectonically deformed eastern part with a relatively thinner ATSL (30–40 m) has higher-grade nodules of lower abundance (ISA, 2003). In the IONF however, thick ATSL (17–20 m) favours higher degree of diagenesis. This may indicate that upward diffusion of metals during diagenesis may be favoured by a limit to the thickness of ATSL, and the diffusion may, in fact, decrease with further increase in the ATSL. Recent evidences of hydrothermal activity in the centre of CCZ along an eastwest-oriented unnamed but topographically distinct FZ have been interesting, as nodules are especially abundant in this area. It is also seen that the tectonic lineaments (mostly faults and fissures) are aligned with high concentration of metalsrich nodules. These longitudinal features possibly help the nutrient and metal-rich bottom water mass to move easily, creating oxygenated condition for nodule formation and metal concentration (ISA, 2003). Again movement of the tectonic plates may have been responsible to drift nodules of one particular community to a situation where conducive environment of nodule formation does not exist, or to an area where volcanic activities and erupted rocks may supply additional metals to the growing nodules. The recent findings from the Indian Ocean add another dimension to the metal enrichment process. It is suggested that release of heat through fissures and faults from submerged volcanoes or deep-seated plume may have helped upward diffusion of metal ions through interstitial water in the sediment. Such diffusion of metals facilitates diagenetic growth of nodules at the sediment–water interface (Mukhopadhyay et al., 2002). Quantification of such input deserves detailed study in the future. As described in the sections 4.1 and 5.1.2.7, the contribution of tectonicsdependent hydrothermal fluid to the ferromanganese crusts has been reported from the walls of the rift valley north of the IOTJ (Nath et al., 1997) and from the intersection area between the Vityaz FZ and the Central Indian Ridge (Mukhopadhyay et al., 1998). In the Trans-Atlantic Geotraverse (TAG) field at the crest of the MAR, high Mn/Fe ratio, low trace metal contents as well as todorokite–birnessite mineralogy reflect largely a hydrothermal origin for the crusts. The crust here grew at rates of up to 200 mm/Ma. In the Pacific Ocean, crusts cover tops of Galapagos rift mounds. Again, manganese crusts were found from the flanks of the Tonga–Kermadec Ridge in the south-west Pacific Ocean. In the nearby Tonga and Lau Ridges crust occurs in association with strata-bound manganese oxides. The one-layered crust sample from the Tonga–Kermadec Ridge in the south-west Pacific Island arc recorded a growth rate of at least 500 mm/Ma. The crusts of the Tonga and Lau Ridges, which grew at rates of only tens of millimetres per million years, were formed by a combination of hydrogenetic and hydrothermal precipitation with the hydrothermal contribution averaging about 75% (Cronan et al., 1982; Hein et al., 1990). Hence, there appears to be a relation between tectonic and volcanic activities on the one hand, and abundance and metal enrichment on the other in the ferromanganese deposits of the world oceans.
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5.2.2. Sediment, seawater and chemical environment The direct effect of both tectonic and volcanic activities leads to the development of topographic variations in the seafloor, which influences among others the oxidising environment. We have seen that high abundance of nodules in the Pacific and the Indian oceans is more common in areas characterised by abyssal hill topography and associated relief features. Abundant occurrence of nodules is found on the slopes and flanks of topographic highs (seamounts and abyssal hills), where large numbers of nuclei (‘seeds’) are easily available from exposed rock outcrops (Horn et al., 1972, Mukhopadhyay and Nath, 1988). It is possible that nodules are more evenly distributed in flat, low relief areas, though the abundance may be less than that in hills and high relief areas, for example in the North Pacific and in the IONF, suggesting a positive relationship between nodule abundance and topographic variations (Mukhopadhyay et al., 2002). In the northern equatorial part of the CPB, smooth surfaced hydrogenetically formed nodules are abundant on locally isolated seamounts, abyssal hills and ridges. These are largely exposed to the water column, and occur on a thin ATSL, where the rate of sedimentation is comparatively high. In contrast, abundant rough surfaced diagenetically formed nodules are found in siliceous sediment in the central and the southern parts of the CPB. The exposed rough surfaced nodules are larger and abundant than the buried varieties. The abundance of nodules appears to be inversely proportional to the rate of accumulation of sediment (Usui and Moritani, 1992). Additionally, in both the ENP and the IONF, an increasing trend of Mn/Fe ratios down the slope of topographic highs was observed. The ratio varies from about 1.5 at locations close to summit to 2 or more at the base of the seamounts, with the ratio increasing up to 4 in basins away from seamounts (Glasby et al., 1982; Halbach et al., 1981; Mukhopadhyay and Nath, 1988). In a similar style, the concentration of Mn and Ni increases with depth. Nodule compositional variations are, therefore, likely to be dependent on the combined action of a variety of geochemical processes operating in different bathymetric settings. Compared to the Pacific and the Indian oceans, the Atlantic Ocean is characterised by less nodule coverage (Table 5.1, Fig. 5.1). Many factors contribute towards the development of unfavourable environment for the formation of nodules in the Atlantic Ocean. Among these disturbing factors, larger input of terrigenous material and high sedimentation rate are considered important. However, the deposits of the Blake Plateau, Drake Passage and Western Atlantic may be mentioned as examples where unfavourable conditions do not dominate. A detailed study of the internal textural features (micro-laminations) in nodules from the Pacific Ocean (the ENP and the CPB) and the IONF suggests that the nodule growth was interrupted several times, with erosion occurring at certain periods. On the other hand, turbidity in the southern part of the CPB affects the growth of nodules. Periods of rapid growth of nodules may be preceded or followed by periods of slow growth, or by periods of even no growth (Banakar, 1990; Dunham and Glasby, 1974; Heye, 1975; Sorem, 1967). Several interesting relations emerge when bottom sediments are considered as important factors in nodules formation. The Mn/Fe ratios in nodules occurring
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close to seamounts and at the spreading ridge axis in the Pacific Ocean are higher by 21 and 35%, respectively, compared to the nodules from similar physiographic setup in the Indian Ocean. At the abyssal plain, the Mn:Fe ratios in the bottom and the top layers of the Pacific Ocean nodules are also richer by 78 and 70%, than the corresponding layers in nodules from the Indian Ocean. In both the Pacific and the Indian oceans, nodules occurring on siliceous sediment show highest Mn:Fe ratios, followed by those occurring in red clay and calcareous clay sediments. Compared to corresponding condition in the Indian Ocean, the Mn/Fe ratio of the Pacific Ocean nodules is higher by 22% in siliceous sediment and 14% in red clay sediment. The influence of nutrient-rich cold AABW, overall low sedimentation rate and high biological productivity have been substantial in enhancing the metal grade in nodules from both the oceans (Friedrich et al., 1983; Glasby, 2000). The IONF situation when compared to that of nodule formation in the ENP brings out remarkable relations. The seafloor condition in sector A in the IONF can be compared with that of area F of the ENP, where high-grade (2.89 and 2.51%, respectively) nodules with low abundance (1.83 and 1.30 kg/m2) occur on siliceous debri-rich calcareous sediment. The contribution of metals to the nodules by diagenetic method is considered to be enormous in this area. The formational conditions of nodules in sectors B and C in the IONF can be compared to that of area C in the ENP. Bottomed by siliceous clay and influenced by tectonic and volcanic activities, nodules from these sectors respectively contain high grade (2.40, 2.64 and 2.48%) as also high abundance (4.12, 5.34 and 4.9 kg/m2). The conditions of formation of nodules in sector D in the IONF are comparable with that of area G of the ENP. Both these areas/sectors are highly disturbed by tectonic activities (Marques FZ runs through area G, whereas several closely spaced active reverse faults are present in sector D). The nodules show moderate-to-high abundance (5.82 and 3.1 kg/m2) but low grade (2.31% and 1.97%) in the absence of diagenesis. The mineralogy of nodules is extremely significant as uptake of transition metals into nodules is controlled by the atomic ratio of the incorporated metals relative to ˚ manganite (i.e. todorokite). In the nodules from the ENP the manganese into 10 A atomic ratio of 4 elements (Cu þ Ni þ Co þ Zn)/Mn is generally less than 0.10 (Friedrich et al., 1983). It is suggested that the supply of these divalent metal ions from the sediment column during diagenesis leads to the formation and stabilisation ˚ manganite). Increased biological productivity would facilitate of todorokite (10 A release of more divalent metal ions to the sediment column and supply of the same would influence formation of todorokite in nodules, resulting in the process, a highgrade nodule deposit (Glasby and Thijssen, 1982). Todorokite is found as the predominant mineral phase of nodules in the IONF, and normally occur associated with siliceous sediment with high-to-moderate biological productivity. It allows easy substitution for Mn2þ by Ni and Cu. At relatively shallow water depth, highly oxidised d-MnO2, birnessite and FeOOH are the predominant minerals in nodules. These minerals, closely associated with foraminifera, zeolites and illite, facilitate incorporation of Co and Pb in greater amount into the Mn4þ in birnessite, and Fe3þ in FeOOH (see Section 1.5). Therefore, biological productivity, mineralogy and composition appear interlinked.
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One of the early studies revealed that the factors controlling the enrichment of REE in nodules of the Pacific and the Atlantic oceans are apparently the same, and the REE pattern supports the idea that they have precipitated from normal sea water (Glasby et al., 1987; Piper, 1974). With an enormous amount of data now available from the world oceans, a comprehensive and critical assessment of all chemical, geological, biological and physical factors that influence the formation and enrichment of ferromanganese nodules should be carried out. Such an assessment would help understand the underlying processes and help evaluate the metal potential more realistically. Not only knowing well the resource potentiality of the areas explored in details would be sufficient, this exercise should bring about a predictive model with a mathematical base for poorly surveyed areas in the world oceans. The relation between formation of nodules and seafloor spreading, tectonic activities (like faults, folds, FZs to provide seeds and to supply extra oxygen for precipitation) and volcanic eruptions (seamounts, local secondary eruptions, release of shallow-seated heat to facilitate diffusion, hydrothermal activities) have been mentioned above. Preparation of databases on seafloor nodule photography to assess bioturbation and heat flow could help appropriate documentation. It is seen that nodule abundance rises with greater productivity up to a certain level and diminishes beyond that turnaround point. The depth of oxygen minimum zone needs to be calibrated accurately, so also the contents of organic carbon, calcium carbonate and silicate in the sediment. The crux is that the nodule research community today needs a model that can predict: What chemical, physical, biological and geological characteristics in the nodule environment could be used as indicators of the likely presence of high-grade deposits? This would help avoid both under- and over-optimistic assessment of return from marine manganese nodule resources.
C H A P T E R
S I X
Resource Management
Contents 225 226 228 230 231 233 233 234 235 249 250 250 252 254 255 256 258
1. Resource Identification 1.1. Seafloor characterisation 1.2. Resource sampling 1.3. Assessment of resource potential 2. Mining Technology 3. Environmental Impact Assessment 3.1. Possible impacts 3.2. The EIA studies in world oceans 3.3. The EIA studies in the IONF 4. Metal Extraction and Processing 4.1. Hydrometallurgical treatment 4.2. Pyrometallurgical treatment 4.3. Efforts in the IONF 5. Law of the Sea 6. Global Perspectives 6.1. Equatorial North Pacific 6.2. Indian Ocean Nodule Field
Resources need to be mined and used in a continuous and sustainable manner without causing any (or least possible) harm to the environment, biota and human life. With this understanding as the bottom line, the challenge for the scientists and technologists is to formulate suitable plans to undertake field and laboratory studies to identify the resources, select an appropriate methodology for mining, take cognisance of the possible impact on the environment, arrange an economically viable metal extraction process and take note of legal implications involved with resource mining. These aspects are now integrated under resource management.
1. Resource Identification Identification of ferromanganese resources is carried out by the detailed exploration and close-grid sampling of the possible resource areas and is followed by determining the distribution and extent of the resources (i.e. resource mapping) and the evaluation of the metal content of the deposit(s) for possible economic exploitation. With the development of newer technologies and instrumentation that can Handbook of Exploration and Environmental Geochemistry, Volume 10 ISSN 1874-2734, DOI: 10.1016/S1874-2734(07)10006-1
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2008 Elsevier B.V. All rights reserved.
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successfully sustain high water pressure, the exploration strategy had undergone major modifications. The principal methods employed in the Indian Ocean Nodule Field (IONF) for manganese nodule exploration over the last two decades are outlined below.
1.1. Seafloor characterisation Deep-sea exploration normally needs large ships with several built-in facilities and a host of advanced equipment. Among the ships used for extensive exploration work in the IONF, the contribution of ORV Sagar Kanya has been immense. This vessel has an overall length of 100.34 m, breadth 16.39 m and endurance of 45 days. Capable of attaining a speed of 14 knots, the ship has 13 onboard laboratories, sample storage facilities, a reasonably equipped hospital and can accommodate a total of 91 personnel including 32 scientific staff. The ship is equipped with magnetometer, gravimeter, single-beam and multi-beam echo-sounding bathymetry system, side-scan sonar, sub-bottom profiler (SBP), still and video cameras (for underwater photography), airgun seismic reflection system and also the requisite facilities of cranes, winches and A-frame to operate the various sampling devices. Besides Sagar Kanya, several other research and survey vessels from India (2), Britain (2), Norway (1) and Russia (2) have been used for exploring the IONF (Table 6.1). During the early years of exploration in the IONF, vessels used satellite navigation, which could calculate the position of a ship with an accuracy of 30–100 m on receiving stable 150–400 MHz signals from six transiting satellites placed in geostationery orbits at an altitude of 1075 km. Later, the Global Positioning System (GPS) brought a sea change in navigational exploration. Using 21 satellites arranged in six orbits and inclined at 55 , the GPS gives an accuracy of 5–10 m. In some turbulent sea-state conditions, the vessels also used the Dynamic Positioning System, which automatically controlled three horizontal motions of the ship—surge, sway and yaw—and prevented deviation of the ship from a specified location during precision sampling. The underwater acoustic navigation system was also used on a few occasions to precisely calculate the ship’s movement. The exploration strategy in the IONF involved computing precise position with real-time updating by integrating navigational and underway (bathymetry, gravity, magnetic and seismic) data through integrated navigation system (INS). Echo sounder, working on 12 kHz frequencies, was largely used during the early stages of exploration to measure depth of the seafloor in the IONF. To avoid side echoes and associated noise, the narrow-beam echo sounder, mounted on a specially designed gyro-stabilised platform, was used. By limiting the beam angle between 1 and 2 (compared to 5 or more for the conventional echo sounder), the narrowbeam echo sounder provided a more accurate measurement of depth. However, besides determining the precise depth, it was necessary to know the overall topography of the seafloor to assist in assessing the mineability of the deposits. To obtain detailed bathymetric information, the multi-beam swath bathymetry (MB) system was used. Using depth data from 59 reflected sonar beams, the MB system provided contoured bathymetry of the seafloor. The system works on a frequency of 15.5 kHz, can measure depths from 10 to 10,000 m and on a single run it can
Table 6.1
Major exploration ventures of India for manganese nodules in the IONF
Ship
Cruises undertaken
Freefall grab
Van Veen grab
Photo grab
Pettersson grab
Okean grab
Dredge
Gravity core
Box core
Spade core
RV Gaveshani ORV Sagar Kanya MV Skandi Surveyor MV Farnella MV GA Reay MV Nand Rachit AA Sidorenko RV Boris Petrov
03 09 17 08 03 04 08 03
141 1525 3373 1568 546 527 – –
– 05 17 – – 01 – –
– 480 654 315 167 101 – –
09 – 55 53 01 – – –
– – – – – – 502 72
– 33 393 91 58 01 02 –
– – – – – – 05 –
– 09 12 – 03 04 – 10
– 06 07 01 – 26 – –
Note: This data are up till December 2005. Gaveshani, Sagar Kanya, and Nand Rachit are Indian vessels; the rests are chartered vessels. The Department of Ocean Development, New Delhi in association with the National Institute of Oceanography, Goa used these 8 vessels for 55 expeditions under the Indian Nodule Programme between 1982 and 2005. IONF ¼ Indian Ocean Nodule Field.
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swath cover the seabed up to twice the water depth. To decipher the nature of the seabed sediments, it is necessary to generate pulses at low frequency. This was made possible by the SBP, which works on a low frequency around 3.5 kHz with a power output of 5–10 kW, so that the pulses of the SBP are able to penetrate the upper 30– 50 m of sediments to obtain information. Although not directly of much help to manganese nodule resource identification, several thousand kilometres of gravity and magnetic intensity data of the seafloor were collected during the exploratory cruises. Gravity is the attraction of the mass of the earth on any object, and variations in earth’s gravitational field, known as gravity anomalies (and expressed in milligals) may suggest differences in densities of subsurface rocks and mineral deposits. On the other hand, the magnetic intensity (expressed in gamma) is measured to characterise generally an iron-titanium rich mineralised zone. In addition to use in mineral exploration, magnetic data also provide valuable information on the paleo-plate movements, as well as on the rate and the nature of seafloor spreading, indicated by variable patterns of lineated magnetic anomalies (see Chapter 2). Pinger, an acoustic signal-transmitting device, attached to about 50 m (or more, as desired) above the box core, dredge or photo-sledge, was extensively used during the exploration in the IONF. The signal from pinger gives the exact distance between the sampling instrument and the seafloor. A Time Lapse Camera was sometimes deployed, which comprises a controller, an electronic flash, and a camera with a 250-frame capacity to take photographs at any interval, and for any period of observation. The camera, in fact, could accommodate a 100-feet roll of 35-mm film accounting for about 800 frames. Either the frequency of photographs could be adjusted or the snap timing could be preset onboard before lowering.
1.2. Resource sampling To assess the economic feasibility of a deposit, systematic sampling is essential. Sometimes, information about the sediment, rock and water associated with the deposit is also required as these intrinsically related parameters help define the environment of resource occurrence. In the beginning, the exploration involved grid sampling at an interval of 1 (i.e. 111 km), which was later revised and was progressively reduced to 55, 25 km and later to 12.5 km. Free-fall grabs (FFG), working on the principle of buoyancy, were commonly used in the IONF for sampling manganese nodules spread on the seafloor. An FFG has a self-working buoyancy mechanism capable of lifting þ20 kg from a sampling area of 0.13 m2 of the seafloor. The total weight of an FFG is 33 kg. Launched from the survey vessel, the FFG with open jaws descends at a speed of about 80 m/min, due to two attached ballast weights of solid iron blocks of 20 kg each. Upon touching the seafloor, the two weights fall on the bed, pushing the grab up because of buoyancy. Owing to this pull, the jaws get slowly closed trapping the nodules or rocks inside that were strewn on the seafloor and the FFG pops up at the sea surface after travelling through the water column. To photograph the part of the seabed that is being sampled, a camera was normally attached to the FFG (photo-FFG). By adjusting the length of rope, which carries a trigger weight hanging from the camera, one can programme (set) the focal distance of
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the camera. As soon as the trigger weight touches the seabed at least from 1 to 1.5 m before the grab lands on the bottom, a magnetic switch is activated and the camera takes a snapshot under flashlight. After taking the photograph, the photo-FFG touches the bottom, collects a sample and simultaneously throws off the ballast weights from the hinge-clips. Because of the weight loss, the grab returns to the sea surface. For the exploration of manganese nodules in the IONF, these two devices (the FFG and the photo-FFG) have been largely used to collect samples from desired spots. When several such spots are systematically covered, the data provide fairly accurate information on regional nodule coverage. Various types of dredges were used for bulk sampling of manganese nodules and crusts. For bulk collection of nodules, usually a net-dredge was used. This dredge has a large nylon net attached to a tethered steel frame having an opening of 1.5 m 0.5 m. This type of dredge proved useful in recovering nodules from areas largely devoid of rock outcrops. A box-dredge (of variable dimension, but commonly of 2.5 m 1.0 m 0.5 m), with a wire mesh fixed to a steel frame on five sides but with the sixth side left open for sample collection, was also used to collect ferromanganese crusts and large rock pieces from seamount regions. A chain-bag dredge was mainly used for sampling rocks from abyssal plains and seamounts. An undisturbed, long sediment core can provide valuable information on the depositional environment of the nodule deposit, and on the overall sedimentation history on a regional scale. Using the principle of gravity, various corers were utilised to recover undisturbed sediment cores. The box corer comprises an elongate steel liner (3- to 12-m long) with a square (30 cm 30 cm) or circular (12–15-cm diameter) front. This liner has led blocks as dead weight on its top proportionate to the need to retrieve a long sediment column. The whole system (total weight 1000–1500 kg) is operated from the ship by an 18-mm thick wire rope. A smaller and more compact version of box corer used was the spade corer. Housed in a steelframe and with a core-liner length of 45 cm and an opening of 20 cm 30 cm the spade corer weighed only 300–350 kg. Because of its reduced weight this devise was easy to handle and hence was extensively used. To understand the characteristics of the flux, colloidal material and particulate matter in the water column, which have a significant bearing on the metal adsorption (enrichment) processes in nodules, water samples were examined from various pre-determined water depths at several locations of the IONF. The samples were collected by a wide variety of samplers ranging from a wire-bound single sampling bottle to a sophisticated rosette water sampler that can collect water from any desired depth. The rosette sampler has a rack on which 24 bottles (interior coated with Teflon and polyethylene chloride to avoid contamination) were mounted. The system was lowered by hydrographic winch wire, and controlled by a deck command unit, which triggered to ensure that water samples are collected at desired depths. In general, several electronic sensors were lowered along with the rosette sampler to measure temperature gradient (CTD, conductivity–temperature–depth) down the water column. In addition, several other parameters such as pH, dissolved oxygen, light transmissivity and salinity were also measured with the help of CTD. Photographs and sketches of the sampling equipment used in the IONF are available in Ghosh and Mukhopadhyay (1999).
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1.3. Assessment of resource potential The evaluation of the resources, identified through field exploration, as above, includes estimation of grade and reserve. The abundance of manganese nodules is expressed as the weight of nodules per unit area of seafloor, e.g. kilogram per metre square. Since the estimation of resource abundance largely depends on the sampling method and the tool employed (dredge, grab, etc.), the estimation of resources from a given area in the IONF was in a few cases randomly cross-checked by more than one method. The sampling areas (0.13 m2) of the FFG and the photo-FFG instruments and that of the Pettersson grab (0.18 m2) were considered to calculate the abundance of nodules, and the data were later normalised to a unit square metre area. The abundance of nodules was also estimated from the seafloor photographs by taking three measurements—average diameter of nodules, surface area covered in the photo-frame and the percentage of coverage. To measure the diameter of nodules, a camera trigger-weight of known diameter was used. The surface area of a photo-frame, on the other hand, depends on the height and angle of camera, angle of the light source and topography. The established procedures to estimate the nodule coverage from seafloor photographs were used. Coverage estimate, however, largely depended on the photo quality, method employed, number of photos taken and the nature of nodule distribution. Estimation of diameter, shape, size and coverage of nodules from photographs may have considerable error because of variable sediment cover. The abundance estimation of nodules in the IONF from photographs in general shows 20–45% error. For the estimation of grade, the content of elements in representative samples was determined by chemical analyses, largely through atomic absorption spectrophotometer, X-ray fluorescence and Inductively Coupled Plasma–Atomic Emission Spectroscopy (ICP–AES). The concentration of elements was usually expressed on the basis of dry weight of samples. Although various methods are used to calculate the chemical grade of nodules, in case of the IONF it is normally expressed as the cumulative weight percentage of nickel, copper and cobalt (Ni þ Cu þ Co). To assess the potential of underwater resources, the data must be reliable, reproducible and representative. For nodule deposits, which are two dimensional in occurrence, sporadic in coverage and heterogeneous in composition, one needs to be extra cautious in estimating the reserve. Using data from 182 nodule stations in the Pacific Ocean, Menard and Frazer (1978) found an inverse relation between abundance and grade at 99% confidence level. Similar studies from the IONF also confirm this relationship (Sudhakar, 1988). However, it has been observed that if grade and abundance are assumed as independent factors, a large area could be overestimated by 42%, whereas a smaller area is overestimated by only 2.5–5% (Assessment of Manganese Nodule Resources, 1988). The resource estimation of the IONF deposits was statistically computed following the Prime Area estimator and the Grid estimator. The data from only highly potential areas in the whole ocean were considered in the Prime Area estimation method. In contrast, the Grid estimation was based only on the data available from a particular site. Once the aerial extent of a mineable deposit in the IONF was established by these two methods, the total wet tonnage was next determined.
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For this calculation, the area was multiplied by the unit abundance. Finally, to convert to dry tons of the deposit, wet tons were multiplied by 0.7, assuming that nodules contain about 30% unbound water (cf. Assessment of Manganese Nodule Resources, 1988). The resource estimation in the IONF is given in Table 6.2. However, while making a final assessment of mineability of any nodule-covered area, the factor of topography was included along with abundance and grade. An appropriate and optimum combination of these three factors in the IONF has made sectors B and C as the most potential areas for nodule mining. This is despite sector A recording the highest grade (2.89%), and sector D exhibiting the highest abundance (5.82 kg/m2, Table 5.17, Mukhopadhyay et al., 2002). The resource estimation from the two Application Areas in the IONF (each of 0.15 million km2), as submitted to United Nations Convention on Law of the Sea (UNCLOS) by India, is furnished in Table 6.2.
2. Mining Technology Three sets of parameters are vital to evaluate a marine manganese mine site. These are nodule characteristics, site topography and certain physical properties of the seafloor sediments. Topography is important in designing a mining system since terrain features, such as fault scarps and large rocky outcrops, would be obstacles during mining. As a result, certain portions of a fully explored mine site may even be considered not mineable. The nodule recovery systems must also be able to travel along the surface of the seafloor, neither sinking out of control nor getting stuck, and able to accommodate small topographic obstacles and avoid large ones. Known as ‘trafficability’ this characteristic of a site also serves as the design criterion of the mining system (Padan, 1990). Developing an appropriate mining technology to mine nodules from a water depth of more than 4500 m, where the nodules mostly occur, has been a challenging task. A first generation mining technique was tested in 1970 at 750 m water depth in the Blake Plateau. Later, in 1978, manganese nodules were successfully recovered from abyssal depth (>5000 m) in the North Pacific. However, a commercial breakthrough is yet to occur. It is important that the metal to be used to manufacture Table 6.2 Resource estimate in the two Application Areas of the IONF
Area (km2) Abundance (kg/m2) Total nodules (million tons) Combined % (Ni þ Cu þ Co) (million tons)
Area A
Area B
Total
150,000 4.39 676.5 10.88
150,000 4.45 658.5 10.96
300,000 4.51 (avg.) 1335 21.84
Source: Sudhakar (1995). Note: In accordance with the PrepCom agreement, one of these two areas is returned to the International Seabed Authority for future exploitation for common benefit of mankind. IONF ¼ Indian Ocean Nodule Field.
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the mining equipment should have low weight, high tolerance limit to stress (compression and tension) and maximum immunity against corrosion. The equipment should be so designed that it is able to successfully encounter all types of seafloor topography, as well as accommodate the various motions of the ship. Although the technical feasibility of deep-sea mining is now established, in view of the tremendous risk involved from both technology and financial viewpoints, mining consortia with several partner companies are seen as reliable agencies to sustain cost of research and enhance technology to ultimately develop appropriate mining equipment. Primarily, three steps are involved in the mining of nodules. In the first place, nodules are collected from the seafloor, and lifted for on-board storage. Although this step demands tremendous technological sophistication, it may account for only 20–25% of the total project cost. The second step involves a less risky job of transporting the collected nodules to the land or to the processing plant and this step may account for 10–15% of the project cost. The third and the most expensive step is the processing of nodules, requiring about 70–60% of the total cost (Kunzendorf, 1986; Padan, 1990). Four systems are presently being tested globally to collect nodules from the seafloor. These are a. Hydraulic lift system: It involves collection of nodules by a self-propelled or towed-type collector and pumping the nodules up to the vessel on the sea surface in a slurry form. b. Air lift system: It involves injection of compressed air into a pipe that runs from the operating vessel to the seafloor. The buoyancy effect of the rising air brings up the nodules from the seabed. c. Shuttle or modular mining system: In this case, the collector propels independently on the sea bottom to collect manganese nodules in an attached bag, and ejects in the process an equal weight of ballast material from the bag. Mining terminates shortly before the total ejection of ballast material occurs. d. Continuous line bucket system: It is essentially a dredging method using a series of empty buckets (or dredges), which descend to the floor along a continuously rotating cable from the stern of the mining vessel, and return after collecting seabed materials at the bow of the same vessel. This system ranks better than other nodule mining methods due to its superior mechanical simplicity and lower power consumption (Masuda and Cruickshank, 1993). Efforts to develop a suitable technology to recover the mineral wealth of the IONF have also been initiated in India. Based on the data regarding seafloor condition and distribution of nodules provided by the Goa-based National Institute of Oceanography (NIO), the Central Mechanical Engineering Research Institute (CMERI, Durgapur) and the National Institute of Ocean Technology (NIOT, Chennai), in consultation with the Engineers India Limited (EIL, New Delhi), are presently engaged in developing suitable mining techniques. Meanwhile, an underwater mining system using strong, robust, yet flexible riser was developed by the NIOT in collaboration with University of Siegen (Germany) and successfully operated to a water depth of little more than 400 m. The integrated system when fully developed
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would have among others a collector-cum-crusher and an in situ soil tester to measure the strength of the soil. On the other hand, the CMERI is in the midst of developing a Remotely Operated Vehicle (ROV) for visual inspection of selected areas, remote cleaning using water jets, mapping and photo documentation of marine growth, detection and measurement of cracks, corrosion damage, denting, etc. The ROV was successfully tested off Chennai, east coast of India. Considering the immense applications of the ROV, manned and unmanned submersibles to underwater programmes (scientific research, underwater inspection, maintenance and repairing of the subsystems) in addition to seabed mining, India is making efforts to design a submersible capable of operating in deeper waters.
3. Environmental Impact Assessment Human activities influence the natural order of the environment and hence it is now considered essential to ponder over the possible impact of mining on the fragile oceanic environment. The impact of mining in terms of sedimentary processes, biogeochemistry, productivity and maintenance of marine life, particularly of the benthic communities, appear to be considerable. As required by Article 17 of the ‘Rio Declaration-1992’ a complete environmental impact assessment (EIA) is a prerequisite for sustainable development and global environmental management.
3.1. Possible impacts The U.S. Company Deep-sea Ventures Inc. was the first to successfully test a nodule mining technology in 1970 (Kaufman and Siapno, 1972). Since then, the environmental impacts that might occur when industry penetrates the vast and remote ocean space are under serious scrutiny. As the major impact of mining would occur mainly on the bottom sediment and overlying water column, a host of environmental parameters would need to be observed and measured during underwater mining. While collecting manganese nodules, the collector would disturb the top layers of the bottom sediment. In addition, the propulsion system of the collector will stir up unconsolidated or semi-consolidated sediments. These disturbances are likely to cause disruption, displacement and burial of bottom flora and fauna. Also, some of the proposed mining systems involve cleaning and crushing of nodules on the seabed and bringing the resources up to the ship in slurry form. Such mechanical activities on the seafloor, in addition to the partial discharge of the wastewater from the mother ship, would harm the ecology. Wastewater, containing particulate matter and trace metals, may hamper light penetration and thus, reduce photosynthetic activities in the water column (Thiel and Schriever, 1990). In this connection, an important question is what should be the ideal depth for the discharge of tailings from the mining ship and possibly from the ore carriers? Although several proposals were offered, experimental evidence is lacking and as yet there is no positive suggestion. A discharge depth of 1000 m was proposed for nodule mining so as to restrict the plume to depths greater than the biologically
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active zone defined by more intensive vertical migration (Foell et al., 1990). This proposal was based on the precautionary principle that the deeper the discharge, the less significant the discharge impacts would be. Accordingly, the bottom current regime must be considered prior to fixing the final discharge depth of the tailings (Thiel and Schriever, 1990). Put together, the various disturbances in the oceanic environment owing to seabed mining can be grouped under long- and short-term duration, with recovery rate varying from slow to rapid, respectively. For example, in the event of the mining of nodules, the seabed may register long-term disturbance in terms of physical impact and sediment suspension with a slow rate of recovery. The water column, on the other hand, may register both the effects—hindrance to nutrition due to the accumulated suspended sediment plume (long term) and the effect on fish and mammals (short term). The seabed and water column will be disturbed on a short-term basis also during the possible transportation and unloading of recovered nodule and sediment at a nearby port. Additionally, during the metal extraction process the atmosphere and the sub-surface groundwater would be affected on a long-term basis, while the effect on the water column might not last long (Amos and Roels, 1977; Berge et al., 1991).
3.2. The EIA studies in world oceans An EIA study encompasses three aspects: environment, economy and social needs. However, most emphasis has been given to the environment, in particular to the extent of ecological disturbance caused by marine mining. Several experimental programmes (namely, MESADA, DOMES, DISCOL, BIE and JET) were undertaken in the world oceans along this direction. These programmes enabled the preparation of a list of some of the major types of ecological disturbances that would be caused by the seabed mining. The first EIA programme was conducted during 1977–1981, when Preussag AG, Germany, conducted the MESADA experiment (Metalliferous Sediment in Atlantis II Deep) in the central deep-graben area in the Red Sea. The mandate for this programme, funded by the governments of Saudi Arabia and Sudan, was to exploit metalliferous mud and sediment from a depth of 10 m below the seabed, covering an area of 60 km2. A total of 12,000 m3 of tailings containing 225 tons of particulate matter was discharged at 400 m water depth during this test. The plume thus created was traced down to a depth of 1100 m, with lateral extension up to 700–900 m around the discharge point, which extended to 5000 m within 10 days. Although MESADA failed to measure environmental impacts, the programme did reveal that a mining unit having a capacity of 100,000 tons per day would throw out 400,000 m3 of toxic tailings every day. Such an enormous amount of daily discharge during mining, on dissolution, is likely to make the seawater toxic for the food chain. The next environmental impact investigation was conducted by the National Oceanic and Atmospheric Administration (NOAA, USA) on a broad scale under a project acronym DOMES (Deep Ocean Mining Environmental Study). This investigation was conducted in the Pacific Ocean, in close succession to MESADA, and focused on a large variety of aspects, such as the surface discharge plume, its
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particulate and dissolved phases, trace metal concentrations, particle accumulation at the pycnocline and light attenuation. In addition, effects of mining (by the collector system, created plume and re-sedimentation) on bacterial growth and oxygen demand, on the standing stock of phytoplankton and primary production, on nutrients, on trace metal uptake, on fish and on the benthos have been investigated (Thiel and Schriever, 1990). The study prescribed monitoring on a regular basis to determine in situ settling velocities of mining particulate phases and to assess the long-term effect of these particulate on biological productivity. Disturbance and Recolonisation (DISCOL) was a long-term large-scale experiment conducted in the tropical South Pacific. Funded by the then West German government, it was carried out during 1989–1992, at 88 W-7 S, about 600 km south of the Galapagos Islands, and covered an area of 10 km2 at 4150 m water depth. This investigation involved creation of much larger disturbance at the sea bottom compared to those of the earlier MESADA and DOMES investigations. To study the rate of recolonisation process of the disturbed fauna, the area was crisscrossed 78 times with plough-harrow, at a speed of 1.5–2 knots, and about 20% bottom sediment was turned over in the process. Still and video photographs of the sea bottom taken before, during and after the disturbance distinctly showed the extent and intensity of the disturbance on sedimentation and faunal assemblages. DISCOL inferred that (1) bottom-dwelling and nodule-crevice fauna have little chance to recolonise, (2) stalked species, penetrating through the sediment blanket, can recover after initial shocks of the sediment plume, (3) tailings should preferably be discharged below the euphotic zone to ensure productivity and (4) scattering of tailings discharge depends on the nature of the water column and the current regime, which differs from place to place (Thiel and Schriever, 1990). Another investigation carried out in this endeavour was Benthic Impact Experiment (BIE), a joint venture of the USA and the then Commonwealth of the Independent States (CIS), in the North Pacific in 1991–1992. The area of investigation was experimentally disturbed in May 1992 and sampling was carried out after 4 months. Both continuous line bucket and hydraulic lift systems were used, and the varying degrees of injurious effects on the near-surface biological productivity, as well as on the bottom-dwelling communities, were examined. Since 1990, the Japan Deep Sea Impact Experiment (JET), another EIA investigation, was performed in the North Pacific (off Mexico) by the Metal Mining Agency of Japan in collaboration with the NOAA to study chemical, biological and physical responses of deepsea mining of manganese nodules. Under this programme, artificial disturbance was created in a selected area in August–September 1994, in conjunction with close monitoring to determine pre- and post-disturbance conditions. A towed, sledgemounted hydraulic dredge, known as Deep Sea Sediment Resuspension System (DSSRS), created the disturbance in the sediments.
3.3. The EIA studies in the IONF India has been the pioneer to lay claim on the mineral-rich areas in the IONF, located within the Central Indian Ocean Basin (CIOB). The Preparatory Commission (PrepCom) of the United Nations accepted the claim submission of India in
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1987 and following PrepCom’s recommendation, the UNCLOS allotted India an area of 150,000 km2 in the CIOB, with exclusive rights to carry out further investigation to develop and use the ferromanganese deposits. The next 10 years witnessed a major spurt in exploration to identify the resources. This included surrendering half of the allotted area to the International Seabed Authority (ISA). Simultaneously, intense research was carried out on various aspects of ferromanganese deposits, for example on nodule growth, the influencing factors and the dynamics of nodule formation. Added to these, a pertinent study was initiated in 1995 to evaluate the magnitude of environmental disturbance on the ecosystem caused by the mining of ferromanganese deposits. This was done through an experiment to simulate disturbance on the seabed of the IONF and to monitor its effect. Under the aegis of the Department of Ocean Development (DOD), Government of India, such multidisciplinary EIA studies, code named INDEX (Indian Deep-sea Environment Experiment), were carried out in the IONF by the NIO, Goa, with technical cooperation from Central Marine Geological and Geophysical Expedition, Gelendzhik, Russia. Based on the information collected between 1982 and 1996 on bathymetry, nodule characteristics, water column and bottom sediment, five areas, each of 342.25 km2 (18.5 18.5 km), were chosen for the INDEX preliminary study, of which three areas, R1, A1, T1, occupy sector A and the remaining two areas R2 and T2 are in sector C (Fig. 6.1, Table 6.3). Of these, a pair of final test sites and reference sites were selected based on the criteria: (1) a flat topography that would favour smooth manoeuvrability of the bottom crawling disturber, (2) a favourable seafloor environmental condition that would allow dispersion of the disturbed sediment plume to travel without hindrance, (3) a low nodule abundance area to avoid clogging of the disturber that pumps sediment to the water column and (4) to maintain an optimum distance from one another, so that disturbance at the test-site does not affect the reference-site (Sharma and Nath, 2000; Trueblood, 1993). During the INDEX programme, DSSRS (used earlier in JET experiment) was used to disturb the seafloor, and deep-sea moorings were installed to obtain information on the effect of disturbance. Each of the three deep-sea moorings, deployed during the INDEX, was equipped with five current metres and two sediment traps attached at different depths (Fig. 6.2). Lithogenic and biogenic fluxes from sediment traps were collected covering three periods: October 1995 – January 1996, April 1996 – September 1996 and November 1996 – May 1997. The DSSRS is a hydraulic disturber (Fig. 6.3) and used in the IONF during June–August 1997 from the ship R.V.Yuzhmorgeologia. The DSSRS was earlier used for the three deep-sea disturbance experiments in the Pacific Ocean conducted by the USA and Japan. The DSSRS has a tow-frame (dimension 4.8 m 2.4 m 5.0 m, weight 3.2 tons) connected to coaxial cable, which tows the disturber unit as well as transmits signals and power to it. The bottom of the frame was modified to collect nodules as it moves on the seafloor. The DSSRS has also a 5 m 0.3 m bellshaped stack, through which the sediment is sucked up and discharged from a height of 5 m above the seabed to create maximum disturbance. The disturber had two pumps: one to fluidise the sediment and the other to suck in the sediment slurry.
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75⬚E
76⬚
10⬚ T1
R1
A1
5000 5000
11⬚
5000
5000 5000
R2
12⬚S
T2
Figure 6.1 The five Indian Deep-sea Environmental Experiment (INDEX) sites in the Central Indian Ocean Basin (CIOB), which were subjected to multidisciplinary study to establish baseline conditions and evaluate the possible impact of deep-seabed mining.The study included analyses of 19 box-core and 40 Okean grab samples to decipher biological, physical and chemical characteristics of the water column, in addition to geology and biology of the seafloor (Sharma and Nath, 2000).
In addition, it had a rosette sampler to collect water with suspended sediment from selected depth intervals to quantify re-suspension rate. Also, a video camera was attached to film the complete disturbance activity (Sharma et al., 2000).
Table 6.3
Morphology of seafloor of the five short-listed sites for INDEX experiment Depth (m)
Area
Latitude ( S)
Longitude ( E)
Data points
Range
Average
Slope angle (degrees)
Number of 10 m peaks
Nodule abundance (kg/m2)
T1 R1 A1 T2 R2
10 000 -10 100 10 000 -10 100 10 000 -10 100 12 150 -12 250 11 550 -12 050
75 100 -75 200 75 350 -75 450 75 550 -76 050 75 450 -75 550 76 000 -76 100
1666 1857 1739 1581 3035
5150–5340 5200–5400 5180–5400 5120–5320 5200–5380
5217 5330 5327 5217 5297
0.78–1.07 0.93–1.40 0.68–1.00 1.02–1.76 1.36–1.67
2.9–4.0 3.1–4.7 2.9–4.3 4.0–5.8 7.8
1.94 1.14 2.10 3.41 5.05
Source: Sharma and Nath (2000). Note: INDEX ¼ Indian Deep-sea Environment Experiment.
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A
B Buoy
Current meter with transmissometer
Sediment trap
Current meter with transmissometer Autonomous videocamera
30 m
Anchor weight
7 43 mm m Sea bed
30 7 4 3 m mm m Sea bed
Figure 6.2 Configuration of two types of sediment trap mooring used in the Indian Ocean Nodule Field (IONF) (Parthiban, 2000).
The INDEX programme was divided into four time-bound phases, and a host of multidisciplinary parameters were analysed from five sites to assess the impact of mining on the surrounding ecology (Table 6.4). During the baseline survey (phase 1) between 1995 and 1997, bathymetric information and hydrographic data from 40 spots, spread over a little more than 1700 km2 area, were collected. In addition, sediment was obtained from 64 locations and spot photography of the seabed was undertaken, and three mooring systems deployed for a period of about 200 days. Sediment cores were also collected to study pore-water-diagenesis and other sedimentological properties. During phase 2 of INDEX, held between the years 1997 and 1998, two cruises were undertaken. During the first cruise, pre-disturbance studies were conducted, which included seafloor surveys with video and sonar sensors, sediment and water column sampling, deployment of time lapse cameras, deep-sea sediment trap mooring, acoustic navigation transponders, current metre and transmisometer to characterise the existing benthic conditions before the commencement of artificial disturbance. The DSSRS was then dragged to disturb the seabed. The concept was to study effects of suspension and resettlement of sediment particles on benthic environment at one test-site (A1) and also on one reference site out of the five selected areas. The disturber DSSRS was towed 26 times within an area of 3000 m 200 m. The INDEX site was divided into several 1–3 km long individual
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Figure 6.3 Deep Sea Sediment Resuspension System (DSSRS) is a benthic disturber used in the Indian Deep-sea Environment Experiment (INDEX) sites in the Indian Ocean Nodule Field (IONF) to disturb sediments.The disturber suspended about 3555 tons of sediment along a 88-km stretch (Sharma et al., 2000).
strips. The disturbance continued for about 47 and 1/2 hours over a period of 9 days, covering a total length of 88.3 km. The disturbance created suspension of about 3555 tons (i.e. about 6023 m3) of sediment (Sharma et al., 2001; Sharma and Nath, 2000). Impact of such disturbance on the seabed (Fig. 6.4), on distribution of radiolarians (Fig. 6.5), and that on macrofauna (Fig. 6.6), meiofauna (Fig. 6.7) and microbial and biochemical parameters (Fig. 6.8) is documented. Similarly, the impact of disturbance on the chemistry of surface sediment and particle flux collected in sediment traps of deep-sea moorings and rosette water sampler is shown in Tables 6.5 and 6.6, respectively. The temperature, salinity, potential density and geostrophic circulation decreased below 3500 m water depth. The abyssal circulation was characterised by a south-westward weak flow around 10 S (Ramesh Babu et al., 2001), which can probably be linked to Antarctic Bottom Water (AABW) entering the CIOB through a saddle in the Ninetyeast Ridge around the same latitude (cf. Warren, 1982). The direction and intensity of currents in the IONF at various depths with respect to a particular period is tabulated in Table 6.7. Impact of seabed disturbance by DSSRS in the IONF in terms of benthic mega-faunal group (Table 6.8), biomass distribution (Table 6.9) and geotechnical, geochemical and microbial parameters (Table 6.10) is also furnished.
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Table 6.4
Nature of studies made and parameters examined in the INDEX area
Study
01. Seafloor characteristics 02. Geochemistry of pore water and sediment 03. Sedimentology and clay mineralogy 04. Geotechnical analysis 05. Biostratigraphy 06. Particle flux analysis 07. Meteorological analysis 08. Watermass circulation 09. Hydrography 10. Seawater chemistry 11. Biological productivity 12. Mega-, Micro- and Meio-benthos 13. Microbes and biochemical environment
Parameter examined
Single-beam and multi-beam bathymetry, sediment type and thickness Eh, pH, alkalinity, phosphate, nitrate, nitrite, silica, TOC Sand, silt, clay fraction, mineral assemblage Water content, shear strength, specific gravity, porosity, plasticity index Radiolarian zonation, bioturbation Total flux, major and minor elements, biogenic silica Net radiation, sunshine, wind speed and direction, relative humidity Current speed & direction, total kinetic energy, spectral analysis Temperature, salinity, density, vertical stability, light transmission Dissolved oxygen, pH, alkalinity, nutrients, trace metals Phytoplankton, zooplankton, chlorophyll, optical properties Abundance, distribution, species variations Bacteria, fungi, ATP, labile organic matter, total organic matter
Source: Sharma (2001). Note: INDEX ¼ Indian Deep-sea Environment Experiment. TOC = Total Organic Carbon; ATP = Adenodine triphosphate
During the phase 3 post-disturbance phase, deep-tow photography, CTDrosette sampler (at 3 places), sediment coring (at 21 places), 10 mooring systems with sediment traps, current metres and transmisometer were used to study distribution and resettlement of the suspended particles, and the resultant impact on benthos. Before the disturbance of the seabed, 5 navigation transponders were deployed and calibrated (Sharma et al., 2000). It was seen that the daily flux rate (average 50 mg/m2/day) increased because of sediment disturbance by about 300% but then reduced to 33% within 5 days, indicating a rapid settlement of particles (Table 6.6). Variogram modelling and krigging estimation suggest that most of the 3600 tons of disturbed sediment discharged from 5 m above the seafloor in the water column did not spread far from the area of disturbance, and even did not go up the column (Parthiban, 2000). The disturbed sediment samples also record an increase in natural water content and a decrease in un-drained shear strength in the top 10–15 cm layers (Khadge, 2000).
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Mukhopadhyay, Ghosh and Iyer
A
B
C
Figure 6.4 Seabed before and after disturbance (Rodrigues et al., 2001): (A) ¼ undisturbed seafloor, (B) ¼ track of disturbance, (C) ¼ sediment piles, also note resedimented areas.
These findings are important to develop an appropriate mining technology to avoid sinking of the collector during nodule recovery. Macrofauna, which are a highly sensitive group of metazoans, slowly recolonise after disturbance (Ingole et al., 2005). On the other hand, there has been an overall reduction in the megafaunal population and in benthic biomass after the disturbance (Rodrigues et al., 2001).
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10⬚1⬘S 0
533
13
53
40
13 13p 8 8p 8
5 5 5p
INDEX Depth (m)
5330
3 3p 3
10⬚2⬘S 14
11
7
Disturbance track
2p
2
2
Pre-Dist-Core Post-Dist-Core 2
10⬚3⬘S
16
14 14p
2p B. invagina in pre (2) and post (2P) Cores S. universus in 2 2p pre (2) and post (2P) Cores
11
20
40
53
53
7 7p
76⬚E
16 16p
6 6p 6
Scale 1: 15,000 Mercator Projection
76⬚1⬘E
76⬚2⬘E
Figure 6.5 Impact of disturbance on the distribution of radiolarians of species: B. invaginata and S. universus (Gupta, 2000).
Mean density (no./m2)
250 200 150 100 50 0
0−2
Sed 2−5 imen t
Post-disturbance
5−10 dep th (c 10−20 m)
Pre-disturbance
Figure 6.6 Distribution of macrofauna (benthos) along sediment depth during pre- and postdisturbance (Ingole et al., 2001).
Long-term monitoring of the effects of disturbance on sediment, metal and nutrient flux in the water column and on benthos re-colonisation was carried out during phase 4, which started in the year 2001. Overall, it is seen that in the IONF, shortterm disturbances have rapid rates of recovery, while long-term disturbances take long periods to recover.
244 60
60
45
45 Stn 16
30
15
0
0
60
60
45
45 Stn 15
30
15
0
0
60
60
45
45 Stn 13
30
15 0
60
60
45
45
30 15
30 15 0
60
60
45
45
4.0–6.0
4.0–6.0
2.0–4.0
1.5–2.0
0 1.5–1.5
0 0.5–1.0
15 0–0.5
15
2.0–4.0
30
1.5–2.0
30
1.5–1.5
Stn 8
0
0
Stn 3
30
0
Stn 12
Stn 4
15
0.5–1.0
Stn 5
15
30
0–0.5
Stn 6
15
30
0
Stn 7
Mukhopadhyay, Ghosh and Iyer
Sediment depth (cm)
Figure 6.7 Distribution of meiofauna along the sediment depth before disturbance (striped bars) and after disturbance (open bars) from 10 stations (Ingole et al., 2000).
It needs to be emphasised here that both the test (A1) and reference (T1) sites chosen in the INDEX fall in sector A, which is not encouraging from nodule potential (Mukhopadhyay et al., 2002, see also Chapters 1 and 5). The sedimentation rate and pattern, topography, grade and abundance of nodules in these two sites are different from the nodule-rich areas in the IONF (sectors B and C). Therefore, the INDEX areas only marginally represent the seafloor condition of the IONF.
TOM
LOM Pre
mg.g−1
mg.g−1
1
8 4 0
0 BC2 BC3 BC5
BC11 BC8 BC13
BC16 BC7 BC14
BC2 BC3 BC5
BC11 BC8 BC13
BC16 BC7 BC14
Disturbed track
North of track
South of track
Disturbed track
North of track
South of track
TC
Pre CFU
Post
10,000
100
Pre Post
100,000
CFU ⫻ 102.g−1
6 Counts ⫻ 10 .g−1
Post
12
Post
Pre
1000 10 0.1
0 BC2 BC3 BC5
BC11 BC8 BC13
BC16 BC7 BC14
BC2 BC3 BC5
BC11 BC8 BC13
BC16 BC7 BC14
Disturbed track
North of track
South of track
Disturbed track
North of track
South of track
Figure 6.8 Impact of disturbance on four major microbial and biochemical parameters (Nair et al., 2000). LOM ¼ labile organic matter, TOM ¼ total organic matter,TC ¼ total counts and CFU ¼ colony forming units. Stations BC2, BC3 and BC5 fall on disturbed tracks; BC11, BC8 and BC13 occur to the north; and BC16, BC7, BC14 are located to the south of the track of disturbance.
Table 6.5
Chemistry of surface sediment, and of particle flux collected in sediment traps and rosette sampler in the IONF
Sample type
Al
Fe
Ca
SiBOS
Ba
Ti
Mn
Cu
Ni
Surface sediment Pre-disturbed sediment in traps Rosette sample
NR 0.40 3.65
2.66 0.33 2.47
NR 20.96 0.72
NR 20.4 22.3
NR 841 2088
NR 455 1520
4600 377 4480
274 63 196
229 36 140
Source: Parthiban (2000). Note: Ba to Ni in ppm, rest in %, BOS ¼ biogenic opaline silica, NR ¼ not recorded, IONF ¼ Indian Ocean Nodule Field.
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Table 6.6
Impact of sediment disturbance on particle flux in the IONF Pre-disturbance
Syn-disturbance
Mooring station
Depth (m)
Flux
Duration
Flux
DMS-1 DMS-2 DMS-4 DMS-5 DMS-6 DMS-8 DMS-9
5320 5330 5330 5330 5320 5330 5320
51.5 49.4 52.1 22.3 33.3 41.1 55.1
41.5 41.5 40.5 35 25 39 38
244.1 8.5 172.3 8.5 102.3 8.5 Failed to collect Failed to collect 64.3 8.5 136.7 8.5
Post-disturbance
Duration
Flux
Duration
122.4 5.25 Failed to collect 90.5 5.33 Failed to collect Failed to collect 72.7 5.7 122.4 5.1
Source: Sharma et al. (2000). Note: Particle flux in mg/m2/day and duration in days as collected by sediment trap moorings. IONF ¼ Indian Ocean Nodule Field.
Table 6.7 Direction and intensity of current at various water depths in the IONF
Period of measurement
Sept. 1995 – Jan. 1996 (a)
Apr. 1996 – Sept. 1996 (b)
Nov. 1996 – Apr. 1997 (c) Sept. 1995 – Jan. 1996 (d)
Apr. 1996 – Sept. 1996 (e)
Velocity
Direction
Depth (m)
Average
Maximum
Average
Maximum
500 1200 3500 4900 500 1200 3500 4900 500 5100 600 1300 3600 4400 670 3670 4470
5.23 2.93 1.28 1.72 3.12 0.46 0.21 0.15 1.96 1.08 2.21 1.23 0.07 0.11 2.29 0.02 0.56
48.21 20.46 11.67 11.48 20.16 40.28 7.74 9.87 23.0 14.7 14.96 10.31 6.19 3.15 21.31 6.30 5.23
329 269 271 240 106 149 093 132 053 120 076 117 132 277 184 084 107
356 298 268 272 040 160 032 240 247 126 099 121 249 306 176 128 110
Source: Murty et al. (2001). Note: Velocity in cm/sec, Direction in degrees. Location of current meter moorings: (a) 09 56.7260 S/74 54.7940 E: sector A; (b) 09 55.8770 S/74 54.6120 E: sector A; (c) 09 57.2600 S/74 56.5600 E: sector A; (d) 14 46.1430 S/71 54.8430 E: sector D; (e) 14 47.1200 S/71 55.2530 E: sector D. IONF ¼ Indian Ocean Nodule Field.
248 Table 6.8
Mukhopadhyay, Ghosh and Iyer
Impact of sediment disturbance on benthic mega-faunal groups
Xenophyophora Sponge Sea anemone Sea pen Black coral Shrimp Starfish Brittle star Sea flower Sea urchin Sea cucumber Fish Unidentified Total
Pre-disturbance (%)
Post-disturbance (%)
Variation (%)
41 08 05 01 00 04 03 1.4 0.5 0.5 30 4.5 01 100
36 15 04 02 0.4 05 3.4 1.0 1.6 2.6 23 04 02 100
40 þ24 47 þ42 21 26 47 þ150 þ243 48 42 þ28 32.3
Source: Rodrigues et al. (2001).
Table 6.9
Impact of sediment disturbance on biomass distribution
(A) Biomass Protein LOM TOM TC 106 (B) Ratio LOM:TOM CHO:LOM Protein:LOM Lipids:LOM Bacterial Carbon/LOM
Pre-disturbance
Post-disturbance
0.330 (0.06) 0.736 (0.01) 5.508 (2.20) 2403 (1142)
0.076 (0.018) 0.387 (0.05) 7.36 (0.13) 9.3 (9.4)
15.28 16.9 45.9 37.1
7.48 24.3 37.6 38.1
4.66
0.022
Sources: Nair et al. (2000) for biomass distribution and Raghukumar et al. (2001b) for ratios. Note: LOM ¼ labile organic matter, TOM ¼ total organic matter, TC ¼ total counts in millions, CHO ¼ carbohydrates, variables unit in mg/g (dry wt.). Except TOM, all variables decrease after disturbance.
Hence, the EIA-related information from areas A1 and T1 (sector A) should be accepted with much caution while designing the mining strategy for the nodule-rich areas in the IONF. It should, however, be kept in mind that natural processes to bring about changes in deep sea benthic ecosystem, as observed, predominate over long-time scales (Raghukumar et al., 2006).
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Table 6.10 Impact of sediment disturbance on geotechnical, geochemical and microbial parameters Parameter
Sediment size Clay content (%) Geotechnical properties Shear strength (Kpa) Water content (%) Geochemical parameters Organic carbon (%) Nitrogen (%) Phosphorous (%) Microbial parameters ATP (mg per gm) TC (per gm) CFU (per gm) Fungi (per gm) Meiofauna (per10 m2) Macrofauna (per m2)
Pre-disturbance
Post-disturbance
Monitoring
35
40
62
1.80 541
2.38 552
5.27 508
0.35 0.084 0.008
0.46 0.093 0.0082
0.28 0.085 0.013
50–150 109 102 103 45 244
0.1–1.0 107 105 102 23 80
0.01–0.5 107 105 102 31 266
Source: Sharma et al. (2005). Note: ATP ¼ Adenodine triphosphate, TC = Total counts, CFU = Colony forming units.
With human attention increasingly focused on the oceans, EIA studies should also incorporate processes as well as methods for assessing the impact of policies, programmes and development on the socio-economic, legal and bio-physico-chemical environment. For example, the EIA should be able to identify parameters undergoing variations, select reference areas, determine environmental indicators, establish carrying capacity and conclude whether there would be any irreversible changes. However, the information available from all the investigations conducted so far in the Pacific and the Indian oceans appears largely inadequate to predict the exact extent of pollution from full-scale deep-sea mining. In fact, different methods provide different conclusions about the understanding the environmental impact. Hence, it is necessary to globally standardise the data collection methods, analytical procedures and encourage a mutually agreed design to calibrate and compare results. This would make the proposition of the deep-sea mining cost-effective, synergistic and environment sustaining (Sharma and Nath, 2000).
4. Metal Extraction and Processing The selection of an appropriate technique to extract economically important metals (such as nickel, copper and cobalt) from mined manganese nodules constitutes an important stage to determine the commercial viability of the resources. While doing so, few major considerations are generally taken into account:
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Mukhopadhyay, Ghosh and Iyer
(1) techno-economic feasibility of the process selected, (2) extractable metal types, based on their market demand, (3) plant location, with respect to availability of power, water, transport, etc., and (4) environmental consequences of extraction route chosen, that is waste disposal. For extraction of metals from manganese nodules, two distinct process routes have been recognised: hydrometallurgy (low-temperature aqueous processing) and pyrometallurgy (high-temperature smelting process). The former uses sulphuric acid, hydrochloric acid or ammonia as lixiviant. Subsequently, techniques like cementation, precipitation, gaseous reduction, electro-winning and solvent extraction are used to purify the leach liquor and produce pure metals or their salts. In contrast, through pyrometallurgy, the nodule is smelted under reducing condition to produce a crude alloy of Cu, Ni, Co and Fe. The process also produces an enriched manganese-bearing slag, which can be used for the production of ferromanganese metals. Both these processes efficiently extract Ni (90%) and Cu (80–90%), whereas recoveries of Co and Mn are high through pyrometallurgy process compared to hydrometallurgical process. Metallurgical processes followed by various organisations in India and outside to extract metals from the ferromanganese nodules are shown as flow charts (Fig. 6.9).
4.1. Hydrometallurgical treatment This involves selective leaching of elements with sulphuric acid, hydrochloric acid or ammonia as a lixiviant in the presence of appropriate reducing agents. In the case of sulphuric acid, the consumption of the acid is about 300 kg for each ton of treated nodules. It was observed that leaching with H2SO4 at higher temperature improves recovery of Ni and Cu, though that of Co was lowered. Low acid consumption (100 kg/ton of nodule) due to hydrolysis of Fe and fast dissolution are some of the advantages of elevated temperature, and therefore elevated pressure. On the other hand, shortage of material needed for the construction of the reactor, and operational problems are the detriments of this method (Tangri and Suri, 1999). Manganese nodules are often leached in ammonia and ammonium carbonate/ sulphite. As most of the metals present in nodule are in the form of their oxides, and these perhaps in their higher oxidation states, prior reduction by carbon monoxide, sulphur dioxide, hydrazine, or charcoal, or methanol or ethanol is mandatory to bring the oxides into solution and record higher recovery of metals, particularly of Co (Tangri and Suri, 1999).
4.2. Pyrometallurgical treatment This route has two main processes: smelting and hydro-chlorination. In smelting, dry nodules are mixed with silica, coke and pyrite, and reduced at 1400 C to recover Cu, Ni, Co as mixed alloy. The recovery of Cu is about 80%, while those of Ni and Co are between 93 and 98%. A major part of the Mn (and Fe, if desired) may be recovered from the slag. Sulphation roasting, which involves treatment of nodules with SO2 and air at 400 C or with H2SO4 at 650 C, is another common technique. In the
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A
Raw nodules Crushing and grinding Cu metal
CO Ammonical solution
Leaching
Filtration
Tailings
40% LIX 64N Kerosene
Cu-Ni Extraction 3 stages
Raffinate to Co-Mo recovery Second ammonia scrub 2 stages
First ammonia scrub 2 stages
Ammonium bicarbonate solution
Dilute sulphuric acid
Ammonium sulphate to waste Ni stripping 3 stages
Pregnant electrolyte 75g/l Ni, pH3
To ammonia recovery
Cu stripping 2 stages
Return electrolyte 50g/l Ni, 40g/l H2SO4 Pregnant electrolyte 45g/l Cu, 145g/l H2SO4
Figure 6.9
(Continued )
Return electrolyte 35g/l Cu, 160g/l H2SO4
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Mukhopadhyay, Ghosh and Iyer
hydro-chlorination process, nodules are reacted at 500 C in a stream of hydrogen chloride gas to convert most of the metals, including Mn, into water-soluble chlorides. The chloride is next leached with water and separated from the undissolved solids. The leached liquor, when suitably treated by solvent extraction and electro-winning, yields pure individual metals. Segregation roasting is another technique in which nodule is mixed with carbon and chloride salts and roasted between 500 C and 700 C in a stream of N2 gas.
4.3. Efforts in the IONF The extraction of metals from the IONF nodules has been tried since the late 1980s at three organisations: Regional Research Laboratory, Bhubaneshwar, India (using ammoniacal sulphur dioxide leach route), National Metallurgical Laboratory, Jamshedpur, India (following roast reduction ammoniacal-leach route) and Hindustan Zinc Limited, Udaipur, India (using acid leach-pressure leach route). Over the years more than 50 tons of the IONF nodules have been processed in these laboratories. However, the high-energy cost of pyrometallurgical processing and the complexity of the nodule structure had called for advanced research in improving the cost-effectiveness of procedures of extraction of metals from the manganese nodules. It was later found that a combination of hydrometallurgical and pyrometallurgical processes seems to work best. While pyrometallurgy process could possibly concentrate the metal and alter the physical, chemical and mineralogical characteristics of nodules, the high temperature acid leaching and electro-winning processes of B
Raw nodules
Raw nodules
Drying and selective reduction
Crushing and grinding
Cl2
Leaching
S/L separation
Manganiferrous slag
Residue
Oxidising⬘ sulfidising and converting
Stripping for Fe
SX for Fe removal H2S Cu PPTN
CuS
FeCl2
H2S
MgO
Metal or oxide production
HCl Metal/oxide
Figure 6.9
(Continued )
Slag
Oxidative pressure leaching Solution purification
Pyro hydrolysis
Mn PPTN Cl2
Leach residue
NiS-CoS
Ni-Co PPTN
Smelting
Iron
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hydrometallurgy could enhance metal extraction to about 99.8% pure metals, comprising recoveries of Cu (93.9%), Ni (94.2%) and Co (62.5%; Parida et al., 1997; Sen, 1999). In fact, recovery of Co could be increased to even 79% on using liquid and gaseous reductants (Srikanth et al., 1997). While the newer experiments on increasing metal recovery continue, the next generation of nodule metallurgy should address the limitations of NH3-NH4OHSO2 process in terms of high cost of NH3 and SO2 and high cost of transportation. However, by using natural gas as a source of reducing agent, seawater as process
C
Water recycle
Water
Nodule
Solid/liquid separation
Grinding
Nodule
2 stage leaching
Soln prep
Ammonia
Sulphur dioxide Wash water containing ammonia
Solid/liquid separation
Washing
Air
Fe/Mn precipitation
Metallic cu Ammonia recycle Slurry preparation
Cu SX/EW
Filtration Stripping
Ammonia recycle
Raffinate (Ni, Co, Zn) Sulphide Mixed pptn sulphide
Tails stripping
Residue Pressure acid leach
S/L sepn
(Ni, Cu, Zn)
SX-EW
Final residue Ni metal
Raw nodules
Co Zn metal metal
H2SO4
Crushing and grinding
Pressure leaching
Pre-leaching Leach solution
S/L separation
Impurity removal Purified solution CuSO4SOLN
SX for copper
Raffinate NiSO4SOLN
SX for nickel
Raffinate for cobalt recovery
Figure 6.9
Residue Electrowinning Cu metal Electrowinning Ni metal
(Continued )
Make up H2SO4
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Mukhopadhyay, Ghosh and Iyer
water and elemental sulphur for simultaneous production of energy and SO2, an alternative mechanism for the future pilot plants may be envisaged (Das, 2001).
5. Law of the Sea For sustained development and rational use of marine minerals, managing the ocean wealth is an important subject. Towards this, the International Law Commission (ILC) of the United Nations intended to codify the Law of the Sea to develop D Nodules
Size reduction Material preparation
Mixing
Reductant
Pelletisation Reduction roasting
Reduction
Ammonia leaching
Leaching
Copper and nickel recovery
Filtration
Residue (Mn recovery)
Solvent extraction
Raffinate
Electrowinning Ni-metal
Cobalt recovery
Cu-metal
NH3 stripping
NH3 recycle
Cobalt oxide/metal
Figure 6.9 Flow chart of metallurgical processes developed and followed to extract metals from ferromanganese nodules by various consortia/agencies/institutions (Premchand and Jana, 1999): (A) Kennecott Copper Corporation, (B) Metallurgie Hoboken-Overpelt (MHO): left, and International Nickel Company (INCO): right (C) Regional Research Laboratory (RRL, Bhubaneshwar, India): top, and Hindustan Zinc Limited (HZL, Udaipur, India): bottom and (D) National Metallurgical Laboratory (NML, Jamshedpur, India).
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mechanism to govern mining in international waters, share resources between two coastal neighbouring countries and compensation to the landlocked countries. The ILC’s report, prepared in 1956, reached a broad agreement among the majority of the UN member countries only during the 11th session of 3rd United Nation Convention on the Law of the Sea (UNCLOS III) in 1982. The agreement focuses almost entirely on the provision relating to the mining of ferromanganese nodules. With all the countries signing, the Law of the Sea came into force on 16 November 1994. According to a provision of this Law, any state or company interested to make a claim on exploration and exploitation of manganese nodules in any part of the international sea needs to register with the Preparatory Commission (PrepCom) and make initial investments in pioneer activities, thereby securing Pioneer Investor (PI) status. The terms and conditions to obtain PI status includes (1) spending an amount not less than US$30 million towards pioneering activities in nodule exploration, (2) spending not <10% of the amount under (1) to locate, survey and evaluate specific mining sites (e.g. CIOB/IONF), (3) paying a registration fee of US$0.25 million to the PrepCom, and also depositing an equal amount during application, (4) presenting to the PrepCom prospecting data of the demarcated site, which should include two pioneer areas of 1,50,000 km2 each having equal resource potential, defined by coordinates (not necessarily continuous) to allow for two mining sites and (5) relinquishing 50% of that allocated Pioneer Area (i.e. half of 150,000 km2) from the date of award, according to the following schedule: 20% by the end of third year, 10% by the end of fifth year and the remaining 20% by the end of eighth year. Until now, only a few countries namely, France (The Institut Francias de Recherche pour l0 Exploration de la Mer, IFREMER), India (DOD, rechristened now as Ministry of Earth Sciences), Japan (Deep Ocean Resources Co. Ltd.), Russia, China and South Korea have been registered as PIs. Later the Eastern block countries comprising Bulgaria, Cuba, the Czech Republic, Slovakia and Poland jointly formed Inter Ocean Metal Joint Organisation (IJO) and their application for PI status, in the name of Inter Ocean Metal Co., was acceded to in 1991. In addition, four multinational consortia (Kennecott Group, Ocean Mining Associates, Ocean Management Inc. and Ocean Minerals Co.) were also duly recognised as PIs. Exploration and exploitation of marine mineral resources need enormous investment, which should be supported by close monitoring and sustained profit. This includes undertaking EIA and other measures to collect, compile and disseminate relevant information. A centralised integrated database for providing coordinated information and advice to the PIs is being envisaged (Chaturvedi, 1996; Glasby, 2000; Murthy, 1991; Shyam, 1982).
6. Global Perspectives The initial phase of investigating the possibility of mining deep-sea manganese nodules lasted from 1972 to 1982, shortly after the discovery of its widespread abundance in the world’s oceans (Mero, 1965). However, criteria for economic
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Mukhopadhyay, Ghosh and Iyer
deposits, a combined Ni, Cu and Co content >2.5% and an abundance on the deepsea floor >10 kg/m2, indicated that only a small percentage (<5%) of the nodule deposits could be considered potentially economic. Such deposits are mainly found in the area between the Clarion and Clipperton fracture zones in the equatorial North Pacific (ENP) in the Pacific Ocean, and in the IONF in the Indian Ocean. Economic-grade manganese nodules are generally found at water depths exceeding 4500 m. Lifting nodules from that depth of the seafloor is a formidable proposition, and a commercially feasible scale would be to recover 3 million metric tons of nodules per year for 20 years from an individual mine site covering an area of little more than 6000 km2. But not that all goes against the argument favouring nodule mining. It is agreed that the world metal prices have remained depressed until the beginning of this century, but a clear increasing trend in metal prices is recorded since 2003. It is again proved that mining of manganese nodules from deep-sea floor is technically feasible. With ever-improving robotic and cable technology scooping nodules from seabed may not be difficult in years to come—efficiently and profitably. Moreover, the concern for environmental degradation of land mining for metals may turn consortia towards the deep sea. Hence the debate over the viability of mining deep-sea nodules within the foreseeable future rightfully continues. We discuss below perspectives with regard to two most resourceful areas in the world’s oceans.
6.1. Equatorial North Pacific In the late 1970s, it was calculated that the nodules from the ENP high-grade area, covering about 6 million km2, would contain about 11 billion tons of Mn, 115 million tons of Co, 650 million tons of Ni and 520 million tons of Cu (Mero, 1977). However, the most recent resource estimate indicates that this area is not quite as rich as calculated earlier but still contains not <7.5 billion metric tons of Mn, 78 million tons of Co, 340 million tons of Ni and 265 million tons of Cu (Morgan, 2000). On the other hand, nodules cover 68% of the eastern Central Pacific Basin (CPB) seafloor and record an average abundance of 7.83 kg/m2 (maximum 31.6 kg/m2), on average Mn/Fe ratio of 1.89 (maximum 6) and on average grade (Ni þ Cu þ Co) at 1.73% (maximum 3.55%). Nodules with high grade (>1.3%) and high abundance (>10 kg/m2) are found mostly between 4900 and 5600 m water depth (Exon, 1983). The CPB is also considered significant for its Co-rich ferromanganese crusts, which generally contain >0.8% Co, and are characterised by >4-cm thick layers of ferromanganese precipitation occurring on subdued smooth topography at a depth <2400 m. Among the favourable environment suggested for the formation of economically potential Co-rich ferromanganese crusts globally are shallow-sea areas (<1500 m), large volcanic edifice not capped by atolls or reefs, older substrates (>20 Ma), strong current activity, well-developed oxygen minimum zone, stable slopes and in areas away from riverine and aeolian inputs (Hein et al., 1988). With the above possibility of metal availability from the Pacific Ocean, seven consortia were set up, mainly from the USA, Germany, France, Britain and Japan, to investigate the feasibility of commercial exploitation of nodules. This work culminated in the successful testing of a system to mine deep-sea nodules at the pilot-plant
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stage in 1978, though the entire mining system was lost (Glasby, 2000). The exploration programmes of the USA, Germany, Russia and France had essentially comprised not less than 200 cruises to the Pacific Ocean. From the early 1960s to 1984, more than US$650 million (1982 dollar rate) had been spent on developing technologies and exploring for deep-sea manganese nodules (Broadus, 1987). However, the initial enthusiasm in manganese nodule mining in the Pacific Ocean had waned by around the mid-1980s. The new phase of commercial interest in deep-sea nodules in the Pacific Ocean, which was developed over the last decade or so, has involved Japan, China, South Korea and Russia. All these countries aim for a share of the richest ferromanganese deposits lying in the ENP (Fig. 6.10). South Korea, in particular, has a poor reserve of the land-based mineral resources. Moreover, with continual decrease of the resources and increasing demand for such metals as Ni, Cu, Co and Mn, South Korea has expressed serious interest on the seabed resources, especially the polymetallic nodules. South Korea evaluated a deep-sea mining venture as a possible option for a longterm stable procurement of strategic metals for ongoing economic growth (Moon et al., 1997). This evaluation accelerated a national seabed exploration effort as one of the important marine policies since the beginning of the 1980s. The Korea Ocean Research and Development Institute (KORDI) conducted Korea’s first deep-seabed survey in 1983 in the ENP between Clarion and Clipperton fracture zone. In 1989, KORDI resumed the survey programme and carried out a 3-year joint exploration with the U.S. Geological Survey (USGS), focusing on the reconnaissance of Co-rich crusts as well as on the exploration for ferromanganese nodules. At the end of 1993, South Korea finally selected 300,000 km2 of a commercially prospective deep-sea mining area based on the result of the exploration covering more than 1,000,000 km2. The PrepCom of the United Nations approved the South Korean application for registration as a PI on 2 August 1994. The application area was divided into 11 sectors with average nodule abundance from 5 to 8 kg/m2 in each sector,
Pacific Ocean
Group I Group II 15⬚N
10⬚
155⬚W
145⬚
135⬚
125⬚
115⬚W
Figure 6.10 Sketch of the mining claims by Pioneer Investors (PIs) in the Pacific Ocean (from Glasby,2000). Eachcolourbox represents acountry/consortia in order. Group1: Russia, Japan, France, China, Interoceanmetal Joint Organisation and South Korea. Group 2: Ocean Mining Associates, Ocean Minerals Company, Ocean Management Inc. (OMI-I and OMI-II), Kennecott Consortium, and white areas ¼ International Seabed Authority (ISA).
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where mean size of the nodules ranges from 2 to 6 cm in diameter. Eventually, South Korea became the seventh PI country to secure a registered mining site in the ENP area of the central Pacific. South Korea plans to conduct a detailed exploration in the pioneer area using multi-frequency exploration system (MFES) at close-grid, deep-tow side-scan sonar, deep-tow camera system, and including deep-seabed mining operations.
6.2. Indian Ocean Nodule Field The major deposits of ferromanganese nodules are found in the CIOB. The IONF within this basin is considered as the second richest and second largest ferromanganese deposit in the world (Mukhopadhyay et al., 2002). India’s interest in deep-sea nodule deposits was mooted during the early 1970s. The broad objectives were to explore and delineate nodule-bearing areas in the Indian Ocean, which are located at about 2800 km south of Goa at a water depth varying between 4500 and 5500 m. The objectives, among others, were to demarcate two Application Areas of 150,000 km2 each, obtain recognition as PI and finally concentrate on the Self-Allocated area (i.e. 75,000 km2 after relinquishment) for resource exploitation. RV Gaveshani, a research vessel of the Goa-based NIO, collected the first nodule for India on 26 January 1981 from the seafloor of the Arabian Sea. Subsequently, a detailed work plan for the exploration of nodules in the CIOB was prepared by the NIO. With the establishment of the DOD in July 1981, the concept received a boost. This envisaged exploration in three phases: regional, detailed and finally the search for a mine-site. With the demarcation of about 300,000 km2, having an estimated nodule reserve of 1335 million tons (Table 6.2), India was recognised as a PI in April 1982, and it became the first country to be allocated exclusive rights for further exploration and developmental work in the CIOB. Of the two Application Areas of 150,000 km2, one was awarded to India by the United Nations as Pioneer Area. As per the Law of the Sea, the other 150,000 km2 area is kept by the United Nations as common heritage for the betterment of humankind. India was to relinquish 50% of 150,000 km2 in phases to the ISA. While relinquishing, the most critical aspects considered have been the required abundance and grade of nodules, as well as a comparatively smooth unobstructed topography with a slope angle <3 . Dividing the entire pioneer area into several blocks, the relative weightage of the three combined parameters (grade, abundance and topography, Table 6.11) was determined for each block to ease the process of relinquishment. The detailed exploration resulted in demarcating the IONF within the CIOB having a metal grade >2% and nodule abundance >5 kg/m2. India progressively relinquished 50% of the pioneer areas by 2003. The area retained after relinquishing (retained area) is being surveyed with focused scientific and technical objectives. Mineability of nodules in various sectors (A–D) was decided after considering resultant values of important parameters, such as bathymetry (slope angle and roughness), grade and abundance. For example, when the parameter of seabed topography was added to the grade and abundance estimation, the results had a profound effect on the resource map of the IONF. Hence, the low combined values
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Table 6.11 Distribution of mid-plate volcanoes along with abundance and grade of nodules in the four sectors of the IONF
Crustal age (Ma)
Sector A
Sector B
Sector C
Sector D
>58
58–56
56–61
<51
1263 276
900 509
Number of mid-plate volcanoes in million km2 Abundance 00 2403 Volcanoes >500-m 00 449 height Seafloor crenulations Wavelength (km) 8729 Amplitude (m) 62 RMS roughness (m) 65 Stress condition Stretching
5754 67 194 Compression
5659 78 90 Stretching
5269 118 154 Compression
Nodule abundance (kg/m2) Maximum 8.72 Average 1.83
12.69 4.12
18.79 5.34
13.73 5.82
Nodule grade NiþCuþCo (%) Mn (%)
2.40 24.20
2.64 25.33
2.31 24.10
2.89 24.50
Sources: Mukhopadhyay and Batiza (1994), Jauhari and Pattan (2000) and Mukhopadhyay et al. (2002). Note: For sectors, see Chapter 1. RMS ¼ root mean square, IONF ¼ Indian Ocean Nodule Field.
of these three parameters make sector D least potential for mining in spite of hosting the highest abundance of nodules (Mukhopadhyay et al., 2002; Table 6.11). As the next phase to identify the richest possible area in the CIOB begins, a locale of about 17,500 km2 is brought under critical scrutiny. This area has high nodule abundance, rich grade and reasonably flat topography and hence can form the nucleus for the first generation mine site in the IONF. Detailed sampling at 6.25-km spacing and resource evaluation based on 35 elements, as planned, may better appreciate mineability of the polymetallic nodules in the IONF. Efforts to develop suitable technology to recover mineral wealth of the IONF are continuing in several institutions through collaboration. Also intense research is being carried out to characterise nodule deposit and its surroundings geologically, and to estimate the effect of mining (and tailings) on the ecosystem. Added to this is the effort to improve upon cost-effective metallurgical route to enhance metal extraction and develop mining technology. Hence, one should look upon the manganese nodule programme not merely as a commercial venture, but an opportunity to understand the complex deep-sea phenomenon. This approach is well appreciated and should offer the concerned country an advantage when next global campaign of deep-sea mining begins in the future. Humans have travelled millions of kilometres against gravity to explore the outer space, and should now be able to fathom the gravity friendly 6-km deep ocean. Probably ocean can bring some degree of sustainability and equilibrium in this increasingly untenable world.
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Author Index
Page numbers in italics refer to the reference list A Achache, 53, 275 Allan, 65, 70, 261, 266 Allegre, 36, 265 Alt, 89, 91, 261 Ambre, 123–6, 280 Amos, 234, 261 Anderson, 50, 79, 261, 281 Andreev, 157, 168, 261 Andrews, 106–7, 261 Andrushenko, 187, 190, 279 Ansari, 28, 261 Aplin, 202, 261 Arrhenius, 108, 261 Ashalata, 14, 261 Augustithis, 70, 261 Aumento, 100, 214, 261 B Baksi, 12, 261 Banakar, 123, 128–9, 131–2, 135, 139, 142–3, 145–6, 149, 151, 169, 172, 174, 176–7, 179, 181, 188, 194, 202, 205, 207–10, 212–13, 222, 261–62, 275 Banerjee, 160, 186, 273 Banerjee, 19, 21, 108–9, 118, 172, 174–5, 177–78, 183–4, 198, 207, 262, 269, 273, 279 Baragar, 105–7, 262 Barr, 63, 262 Basu, 55, 57, 262 Batiza, 20, 39, 55, 58, 62, 65, 259, 262, 266, 273 Baturin, 190, 199, 262 Beck, 53, 262 Berge, 234, 263 Bergman, 50, 262 Berner, 149, 263 Berzukov, 116, 263 Bhattacharya, 20, 263 Bhattacharya, 17, 33–4, 263 Biscaye, 108, 118, 270 Bischoff, 127, 263 Blanchard, 94, 263 Blatt, 136, 263 Bolton, 220, 263 Bonatti, 92, 103–4, 108, 199, 263 Borole, 123, 128, 135, 176–7, 191, 202, 205, 207–9, 211–12, 263, 272, 276, 279 Bowin, 14, 263 Brewer, 171, 274
Brichet, 163, 272 Broadus, 257, 263 Broecker, 177, 211, 219, 263, 271 Bull, 7, 263 Burckle, 86, 263 Burns, 165–7, 169–170, 200, 263 Buser, 165, 263 Byerly, 76, 79, 264, 272, 280 C Cassidy, 134, 264 Caulet, 86, 264, 269 Chamot-Rooke, 7, 264 Chaturvedi, 257, 264 Chaubey, 17, 33, 54, 263, 264 Chesner, 101, 102, 277 Chester, 115, 118, 264 Christie, 76, 264 Chu, 10, 264 Clague, 92, 274 Cloetingh, 46, 50, 264, 279 Cochran, 177, 271 Cohen, 20, 264 Conaghan, 53, 276 Conrad, 12, 266 Corliss, 89, 91, 264 Coumes, 34, 270 Courtillot, 12, 264 Cox, 34, 268 Cronan, 132, 136, 156, 165–6, 169, 177, 202, 214–15, 222, 261, 263, 264, 279, 281 Cruickshank, 232, 272 Curray, 118, 264 D Das, 39, 42–3, 196, 201, 256, 264 De Carlo, 218–9, 264 Deffeyes, 110, 265 Dehn, 101, 265 Del Monte, 96, 265 DeMets, 10, 22–3, 53, 265, 267 De Sousa, 26–8, 265, 277 Detrick, 76, 279 De Wit, 32, 34, 277 Dia, 137, 265 Dickens, 142, 265 Drolia, 22–3, 265 Duncan, 11, 13, 55, 265, 271, 279 Dunham, 176, 225, 265 Dupre, 36, 265
283
284
Author Index
Dyment, 20, 40, 44–5, 52, 75, 265 Dymond, 151, 187–89, 190, 195, 266 E Einsele, 142, 266 Elderfield, 170, 172, 266 Engel, 68, 266 Epp, 59, 266 Ewing, 218, 266 Exon, 217, 258, 266, 267 F Fagel, 69, 127, 132, 266 Fewkes, 174, 176, 279 Fisher, 11, 14, 53, 266, 278 Foell, 234, 266 Fornari, 55, 59, 266 Foster, 219, 275 Fraley, 218, 264 Francis, 14, 266 Francis, 11, 266 Frazer, 217, 230, 266, 272 Frick, 85, 266 Friedrich, 161, 182, 189–190, 196, 202, 223, 267 Fujii, 82, 273 Furnes, 106, 266 G Galli, 109–10, 267 Gass, 59, 266 Gehrels, 33, 35, 266 Ghosh, 64, 108, 161–2, 163–5, 167–69, 174, 176, 187, 191, 192, 195, 197, 230, 266, 273 Gibson, 218, 276 Glasby, 144, 161–3, 165, 167, 173–4, 177–78, 189–90, 192, 194, 196, 199, 217, 222–4, 255, 257, 265–7, 271–2, 279 Goodell, 178, 267 Gordon, 7, 9–11, 264, 267, 277 Gottardi, 109–10, 267 Greaves, 172, 266 Grutter, 165, 263 Gude, 109, 112, 278 Gupta, 86, 110, 123, 135, 143, 145, 205, 243, 265, 267, 269, 272, 275–6, 279 H Halbach, 161, 189, 197, 199, 202, 222, 267 Hall, 67, 269 Hallam, 140, 267 Hamlein, 20, 267 Hargraves, 11, 13, 265 Harrison, 92, 267 Hart, 34, 264 Hartland, 53, 268 Hawkins, 83, 268
Hayes, 87, 268 Hedge, 20, 24, 279 Heiken, 89, 94, 268 Hein, 139, 207, 209–10, 213, 219, 222, 257, 262, 268 Heirtzler, 14, 271 Hekinian, 68, 89, 91, 268, 279 Henstock, 9, 268 Heye, 174, 176, 196, 223, 268 Hirst, 153, 268 Hodge, 100, 281 Hofmann, 20, 24, 277 Honnorez, 89, 103–4, 268 Horn, 178, 199, 220, 222, 268 Hughes, 80, 268 I Iijima, 108, 268 Ingole, 28–9, 242–4, 268 Iyer, 3, 18–19, 21–2, 24, 38, 52, 64, 68, 70, 72–3, 77, 81–2, 84, 86, 91–2, 94, 96–8, 100, 102, 105–10, 135, 163, 165, 183–4, 193, 205–09, 220, 262, 264, 265, 269, 272–74 J Jana, 254, 276 Jauhari, 128, 132, 167–69, 182, 189–90, 193, 199, 259, 262, 269 Johnson, 87, 143, 269 Johnson, 72, 280 Juyal, 53, 272 K Kamesh Raju, 22, 40, 41, 43–5, 80, 269–70, 277 Karisiddaiah, 68, 72, 81, 269–70, 280 Kastner, 108, 270 Kaufman, 234, 270 Kent, 85, 266 Kennet, 138, 270 Kessarkar, 42, 46–8, 265, 270 Khadge, 61, 80, 123, 151–4, 169, 189, 241, 270, 273, 280 Khedekar, 123, 134, 276 Kidd, 69, 110, 116–18, 139, 144, 270, 280 Klein, 24–5, 76, 270 Klein, 24, 278 Klootwijk, 53, 270 Knawer, 195, 272 Kodagali, 42, 59, 270 Kolla, 34, 116–18, 139, 146, 270, 278 Kreemer, 5, 270 Krishna, 8, 15–17, 35, 44, 265, 270, 277 Krishnaswami, 177, 271 Krishnaswami, 177, 271 Ku, 176–7, 271 Kunzendorf, 169, 171, 232, 266, 271 Kutzbach, 145, 276
285
Author Index
L Lallier-Verges, 82, 85, 101–2, 110, 135, 177, 272 Lalou, 163, 177, 271 Lane-Bostwick, 192, 219, 272 Laughton, 35, 271 Le Pichon, 14, 271 Le Roex, 76, 271 Leven, 34 Levitus, 141, 271 Lisitsin, 190 Lisitzin, 67, 69, 115, 118, 132, 137, 271 Liu, 35, 271 Liu, 146, 271 Lofgren, 71–2, 94, 268, 271 Lyle, 176–7, 271 M Ma, 160, 271 Mahoney, 20, 24, 36, 271, 279 Malinverno, 47–8, 73, 271 Mangini, 142, 271 Manheim, 192, 214, 219, 271 Marchig, 177, 268 Martin, 195, 272 Martin-Barajas, 82, 85, 101, 110, 135, 177, 272 Martinson, 86, 272 Mascarenhas-Pereira, 103, 272 Masuda, 232, 272 Mathur, 53, 272 Matthews, 106–7, 272 McKelvey, 210, 215, 272 McKenzie, 12, 281 McKenzie, 32, 34, 267, 272 McLaughlin, 86, 263 McMurty, 91, 219, 264, 272 McQueen, 89, 94, 281 Meade, 118, 272 Melson, 19, 100, 106–7, 264, 272 Menard, 110, 230, 272 Merkouriev, 21, 272 Mero, 213, 255, 256, 272 Meylan, 161, 263 Milliman, 118, 272 Minshull, 9, 268 Mislankar, 70, 272 Mitchell, 35, 272 Mitchell, 100, 261 Mizuno, 217, 273 Molnar, 53, 273 Moon, 257, 273 Moore, 118, 264 Moore, 103, 273 Morgan, 14, 256, 273 Morgenstein, 109, 273 Moritani, 218, 222, 280 Mudholkar, 20, 82, 118, 123, 127–8, 130, 144, 149, 149, 209, 211, 273, 274–5 Mukherjee, 38, 82, 273
Mukhopadhyay, 3, 22, 24, 35, 38–9, 42, 48, 51–2, 55–6, 58–9, 61–2, 64–6, 69, 72, 75, 77, 80, 82, 102, 109, 135, 139, 159–160, 162–69, 176–7, 180, 182, 184–93, 195–7, 202–3, 207–8, 217, 220–2, 229–1, 244, 258–9, 265, 267, 269, 273–4 Muller, 12, 32, 35, 274, 277 Murnane, 92, 274 Murray, 171, 211, 274 Murray, 147, 274 Murthy, 36, 274 Murthy, 255, 274 Murty, 26–7, 247, 265, 272–4 N Naini, 34–5, 274 Nair, 245, 248, 274 Narain, 11, 274 Nath, 100, 118, 123, 127–8, 130, 132, 135–6, 141, 144, 146–7, 149–151, 163–4, 167–69, 174, 186, 189, 191, 205, 207, 217, 221–2, 237–38, 249, 274, 276–7, 278 Neprochnov, 8, 275 Nigrini, 140, 146, 269 Ninkovich, 101–2, 275 Nishimura, 218, 275 Norton, 32, 36, 275 O Oldenburgh, 47, 275 Opdyke, 219, 275 Osborn, 96, 276 Owen, 142, 265 Ozkara, 197, 202, 269 P Padan, 230, 232, 275 Parida, 253, 275 Parker, 47, 275 Parsons, 157, 275 Parthiban, 130, 178, 180–2, 239, 241, 246, 262, 273, 275–6, 278, 280 Patriat, 44, 53, 265, 275, 277, 280 Pattan, 101–2, 123–4, 127–8, 130, 135, 144–6, 148–9, 167–69, 172, 174, 178–82, 188–90, 193, 199, 205, 211, 213, 261, 262, 269, 273, 275–6 Paul, 196, 276 Peckover, 94, 276 Peng, 211, 263 Pickering, 138, 276 Piper, 147, 177, 194, 211, 217–8, 224, 266, 276 Powell, 53, 276 Prakasa Rao, 54, 276 Prasad, 87, 210, 276 Prasad, 123, 133–4, 276 Prell, 145, 276 Premchand, 254, 276 Price, 20, 276
286
Author Index
R Raab, 163, 263 Raghukumar, 28–9, 31, 250, 276 Raitt, 14, 266 Rajendran, 54, 276 Ramana, 16–17, 277 Ramana, 159, 189, 273 Ramesh Babu, 25–6, 240, 277 Ramprasad, 45, 270 Rao, 36, 123, 127–8, 130, 167, 277 Rao, 205, 275, 277 Rao, 207, 277 Rao, 166, 277 Rawson, 156–7, 217, 277 Ray, 3, 18, 24, 265, 269 Reeves, 32, 34, 277 Rehkamper, 20, 24, 268 Richards, 152, 268 Richards, 55, 265 Rodrigues, 30, 242, 249, 268, 276, 277 Roels, 234, 261 Rogers, 80, 278 Rose, 101–2, 277 Roy, 178, 277 Royer, 3–4, 9–11, 32, 274, 277 Ryan, 78, 277 Ryan, 156–7, 219, 277 S Sager, 220, 277 Sahoo, 161, 274 Sandwell, 9, 21, 277 Sardessai, 28, 265 Scarfe, 106–7, 278 Scheidegger, 76, 278 Schlich, 32, 278 Schmitz, 132, 138, 278 Schriever, 234, 266, 280 Schwarzer, 80, 278 Sclater, 14, 32, 34, 36, 53, 266, 272, 275, 277, 280 Searle, 53, 278 Segoufin, 44, 275 Sempere, 24, 278 Sen, 253, 278 Shackleton, 87, 268 Shah, 24, 278 Shane, 102, 278 Shankar, 128, 278 Sharma, 176–7, 278 Sharma, 70, 163, 237–38, 240–1, 247, 249, 269, 278 Sheppard, 109, 112, 278 Sholkovitz, 150, 278 Shor, 11, 266 Shyam, 255, 278 Siddiquie, 168, 176, 217, 278
Sinha, 16, 278 Sinton, 76, 264, 279 Skornikova, 190, 279 Smith, 106–7, 279 Smith, 64, 279 Smith, 9, 21, 279 Smoot, 59, 266 Solomon, 50, 262 Somayajulu, 176–7, 181, 278, 279 Sorem, 174, 176, 222, 279 Sorokin, 194, 198, 279 Sotchevanova, 21, 272 Sparks, 82, 281 Srikanth, 253, 279 Staudigel, 105, 279 Stein, 10, 279 Stein, 6, 51, 279 Stein, 51, 265 Stoffers, 185, 279 Stonecipher, 108, 270 Storey, 13, 34, 36, 265, 271, 279 Stowe, 116, 279 Subbarao, 19–20, 24, 278, 279 Sudhakar, 109–10, 134–5, 165, 197, 230, 269, 274, 276, 279 Sugisaki, 183, 279 Sukumaran, 101, 279 Sun, 19, 279 Sundkvist, 160, 279 Suri, 250, 280 Svalnov, 82, 280 Sykes, 6, 280 Sykes, 69, 280 T Talwani, 34–5, 274 Tangri, 250, 280 Tapponier, 53, 273 Tapscott, 36, 42, 280 Taylor, 170, 280 Thiel, 233–5, 266, 280 Thijssen, 167, 224, 266 Thompson, 76, 79, 103–4, 280 Thy, 76, 280 Tooms, 176, 264 Trueblood, 236, 280 U Udintsev, 117, 280 Usui, 189, 217–8, 222, 280 V Vallier, 69, 100, 110, 272, 280 Valsangkar, 123–6, 160, 162, 169, 189, 278, 280 Vanko, 59, 65, 262 Vogt, 68, 72, 76, 264, 280 Von Stackelberg, 194, 281
287
Author Index
W Warren, 25, 140, 242, 281 Weis, 16, 281 Weissel, 6, 35, 51, 261, 279, 281 White, 12, 281 Whitham, 82, 281 Wijayananda, 132, 136, 281 Williamson, 176, 194, 276 Wilson, 76, 281
Wilson, 217, 266 Windley, 35, 281 Wohletz, 89, 94, 96, 100, 268, 281 Wortel, 46, 50, 264 Wright, 100, 268, 281 Z Zelenov, 89, 281
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Subject Index
A Aseismic ridge Chagos-Laccadive ridge, 3, 5, 11–13 Ninetyeast ridge, 3, 5, 11–17, 117 B Basin Central Indian Ocean, 2, 67, 116, 157, 235 Central Pacific, 217–9, 256 Crozet, 168, 215–16 Drake Passage-Scotia Sea, 215 Equatorial North Pacific, 3, 37, 159, 219–20, 256 Inter-basin model, 211, 220 Madagascar, 19, 32, 34–5, 82, 165, 216 Mozambique, 216 Northeast pacific, 217 Northern Peru, 217 Penrhyn, 217–18 Seychelles-Somali, 216 Somali, 67, 145, 216 South Australian, 216 Southern Pacific, 217 Wharton, 6, 9, 16–17, 35, 168, 217 Bottom watermass Antarctic Bottom Water, 25, 139, 167, 240 North Atlantic Deep Water, 139, 214 E Environmental impact assessment Benthic impact experiment (BIE), 234–5 Deep-sea sediment resuspension system (DSSRS), 235–6, 239–40 Disturbance & Recolonisation (DISCOL), 234–5 Deep ocean mining environmental study (DOMES), 234–5 Indian deep-sea environment experiment (INDEX), 25, 236, 239, 241, 244 Japan deep-sea impact experiment ( JET), 234–6 Metalliferous sediment in Atlantis II Deep (MESADA), 234–5 Expeditions Albatross, 1 Challenger, 1–2 Dana, 1 Deep Sea Drilling Project (DSDP), 2, 11, 16, 67–8, 217 Geothermal metallogenesis in the Indian Ocean (Gemino), 2 International Geophysical Year (IGY), 1 International Indian Ocean Expedition (IIOE), 1
Indian Ocean Experiment (INDEX), 236–41, 244 Integrated Ocean Drilling Program (IODP), 2 Murray, John, 1 Ob, 1 Ocean Drilling Project (ODP), 2, 11, 13, 15–16, 68 Snellius, 1 Trans-Atlantic Geotraverse (TAG), 221 F Ferromanganese crust Atlantic Ocean, 193, 214–15 characteristics, 161, 205–8, 213, 217–19, 221, 229, 256 hydrothermal, 217 Indian Ocean, 206, 215 occurrence, 205–8, 215, 217 Pacific Ocean, 217–19, 256 paleoceanographic, 208–11 Ferromanganese nodule abundance, 157, 215, 220 acoustic properties, 159 acoustically transparent sediment layer, 189–92 age, 176–7 Archimedes principle, 196 Atlantic Ocean, 214–15 buried nodules, 178, 181–2 chemistry, 144, 150, 151, 158, 250 distribution, 157–59 dynamics of formation, 194–204, 236 epitaxial intergrowth, 167, 172 factors for Nodule formation, 184–94 grade, 184, 205, 220, 256 growth, 173–6, 192 Indian Ocean, 144, 148, 152, 155, 157–8, 178, 184, 220, 250, 258 internal structure, 144, 155–6, 165, 173–6, 178, 220 intra-nodule diagenesis, 167 magnetic properties, 160 micronodule, 82, 108, 182–184 mineralogy, 165–7, 202, 214–15, 218, 223 model of formation, 202–3 morphology, 158, 161–3, 167, 186, 188–9 nucleus, 160, 163–5, 196–7, 205 Pacific ocean, 217 physical properties, 151, 159–160 rare earth elements, 146, 171, 181 secondary eruption, 192–4, 202, 204, 207 sediment-water interface, 183–4, 189, 195–6, 219–20 tectonics, 220–21
289
290
Subject Index
Ferromanganese nodule (cont.) topography, 184–88, 215 volcanism, 41, 163, 192, 220–1 Fracture zone Atlantis II, 35 Clarion, 219–20, 256–7 Clipperton, 219–20, 256–7 Galleni, 35 Indira, 15, 40, 44 Indrani, 4, 35, 38, 40, 41, 43–4, 52, 59, 68, 73, 80 Vema, 22–3 Vishnu, 4, 35, 38, 40–4, 52, 59–60, 63, 73, 87 Vityaz, 94, 99, 221 G Gondwanaland break-up, 32–5 Himalayas, 6, 8, 132 Hindukush, 54 India-Eurasia Collision, 52–4 Madagascar-India separation, 32, 34 Plate reorganisation, 34–5, 52 Seychelles-India separation, 32, 34
Institut Francias de Reserche pour l’Exploration (IFREMAR), 255 Korea Ocean Research and Development Institute (KORDI), 257 Ministry of Earth Sciences (MoES), 255 National Institute of oceanography (NIO), 227, 232 National Institute of Ocean technology (NIOT), 232 National Metallurgical Laboratory (NML), 252, 254 Regional Research Laboratory, Bhubaneshwar (RRL, B), 252 University of Siegen, 232 United States Geological Survey (USGS), 257 M Metal extraction Hydrometallurgical, 250, 252 pyrometallurgical, 250–2 Mining technology Air lift system, 232 Consortia, 232, 254–5 Hydraulic lift system, 232 Line bucket system, 232 Shuttle or Modular system, 232
H Hotspot Afar hotspot, 12 Austral hotspot, 82 Crozet hotspot, 35 Kerguelen hotspot, 13, 16–17, 36 Marion hotspot, 34 Reunion hotspot, 11, 13, 34–5 Society hotspot, 82 I Indian Ocean nodule field bacteria, 28–9, 197–8 biological properties, 25, 28–31 chemical properties, 27–8 Geostrophic current circulation, 25 macrofauna, 28, 240 megafauna, 28, 30, 207 microfauna, 28, 249 oscillation, 26–7 physical properties, 25–7, 119, 152, 154, 159–160 three layers of water mass, 27 Institutes Central Marine Geological and Geophysical Expedition (CMGGE), 236 Central Mechanical Engineering Research Institute (CMERI), 232–3 China Ocean Mineral Resources Research and Development Association (COMRA), 217, 220 Department of Ocean Development (DOD), 227, 236, 255, 258 Engineers India Limited (EIL), 232 Geological Survey of Japan (GSJ), 217 Hindustan Zinc Limited (HZL), 252, 254
P Plateau & Islands Blake, 165, 214–15, 222, 231 Hawaii, 209, 219 Line Island chain, 217 Manihiki, 217 Marshall-Gilbert Island, 217 Physical properties Crust, 206, 208 Nodules, 159–1, 231 Sediment, 119, 151–3, 231 Plate boundaries Australian–Capricorn, 3, 8–11 Australian, 3, 4, 6, 9–11, 22, 24, 35, 44 Deformation, 6–9, 11, 35, 46, 50 Eurasian, 53, 187 Indian, 3, 5, 46, 49 Indian–Australian, 4, 22 Triple junction trace on the Indian Plate (TJT-In), 4, 38–40, 44, 60, 63, 73, 86–7 R Research vessels AA Sidorenko, 227 Boris Petrov, 227 Farnella, 227 GA Reay, 227 Gaveshani, 227, 258 Glomar Challenger, 2 JOIDES, 2, 67 Mabahiss, 1 Nand Rachit, 227 Sagar Kanya, 226–7 Skandi Surveyor, 227 Yuzhmorgeologia, 236
291
Subject Index
Resource potential application area, 158, 231, 257–58 assessment, 211, 224, 230–1 First Generation Mine Site (FGMS), 159, 259 grid estimate, 229 International Seabed Authority (ISA), 159, 231, 236, 257 Krig estimation, 241 para-marginal, 157, 215 Pinger, 228 pioneer area, 156, 158, 255, 258 Pioneer Investor, 255, 257 Preparatory Commission (PrepCom), 235, 255 prime-area estimate, 159, 219, 230–1, 256 sampling instrument, 228 Sub-bottom profiler, 226, 228 Sub-marginal, 157, 215, 217 United National convention on the law of the sea (UNCLOS), 156, 231, 236, 255 Variogram modelling, 241 S Seafloor anomalies crenulations, 41, 47–8, 259 geoidal (gravity, magnetic), 49–52 lineations/lineaments, 35, 46, 52 normal faults, 48–9, 53, 187, 204 reverse faults, 7, 39, 48–9, 52, 187–8, 203–4, 223 root Mean Square, 47–8, 73, 203, 259 tectonic (seismic, earthquake, stress), 49, 220 thermal (heat flow), 49–50 Seamount abundance, 56, 58–60, 66 distribution, 56, 59–61 emplacement, 63–4 flatness, 54, 58, 60–1, 69 growth, 54, 61, 63–4, 66 height, 6, 42, 54–5, 57–61, 64, 69, 72, 74–5, 164 morphology, 38, 58, 60–1, 63, 66 near-axis seamounts, 22, 38, 58–9, 61–4, 72 neutral buoyancy, 66 petrology, 55, 56, 61–3 red seamount, 89, 91 secondary off-axis eruption, 64 volume, 58, 60–1, 64, 69, 75 Sediment biological productivity, 127, 138, 145, 149, 167, 193–4, 218, 224 biogenous, 115, 137, 144 bioturbation, 144, 149, 154, 196, 208 calcareous/carbonate ooze, 52, 116–118, 124, 127–8, 130, 132, 137, 139, 146–7, 150, 160, 181, 190, 215–16, 218, 223 carbonate compensation depth, 130, 178 consolidation, 151–4 cosmogenous, 115 diagenesis, 124, 127, 149–51, 154, 189, 191–3, 201, 223, 239 distribution, 87, 115–18, 136, 138
geochemical tracer, 127–35 hydrogenous, 115, 124, 127, 136, 149, 189 liquid limit, 153–4 lithogenous, 115, 118, 124, 130, 132 lysocline, 138 mineralogical tracer, 124–7 ochrous metalliferous sediment, 87–92 physical properties, 119, 151–4, 231 plastic limit, 152–3 pore water, 149–51, 176, 189, 196–7, 201, 239, 241 red/pelagic clay, 86, 116, 118, 124, 128, 132, 147, 149, 154, 157, 181–2, 184, 199, 201, 215–16, 223 radiolarians, 86, 116, 123, 127, 135, 137, 141–3, 145, 152, 154, 183 rare earth elements, 123, 146–8, 172–4, 182 sedimentation rate, 108, 117, 145, 182, 218, 220 shear strength, 152–4, 241, 249 siliceous ooze, 116, 128, 130, 138, 141, 180, 184, 200, 215–16 tektites/microtektites, 87, 115, 120–21, 132–4 Subduction zones Magellan, 217 Nova-Canton, 217 Okinawa Trough, 82 Tonga Trench, 82 Spreading ridge Carlsberg, 17–21 Central Indian, 17, 22–4, 44–5 Chain, 11, 16–17, 56 East Pacific Rise, 20, 56 Galapagos, 76, 78–9 Indian Ocean Ridge System, 7, 17, 20, 35, 46, 52, 56, 63 Indian Ocean Triple Junction, 22, 24, 35, 44 Juan-De-Fuca, 59 Lau, 221 Mid-Atlantic, 20, 56 Overlapping spreading centre (OSC), 54 Propagating rifts, 72 Ridge-Transform Intersection (RTI), 44 Southeast Indian, 24, 35, 44–5 Southwest Indian, 44 Spiess, 76, 78 Spreading rate, 18–19, 21–2, 35, 42, 44–5, 54, 56, 58 Tonga, 221 Tonga-Kermadec, 221 Triple junction trace on the Indian Plate (TJT-In), 44 V Volcanics Alteration, 103–12 Basaltic glass, 103–8 Ferrobasalt, 72–80 Fuel-Coolant Interaction (FCI), 94 Glass shards, 86–8, 93, 101–3, 114 halmyrolysis, 103–4, 124 Indonesian volcanic arc, 69, 82, 101, 103, 132, 135
292 Volcanics (cont.) Intraplate volcanism, 204 In situ, 82, 86, 114, 135, 219 Isotopes, 69 Krakatoa volcanics, 82, 86 Microlapilli, 94, 99 Mid-plate volcanism, 103, 131, 193 Magnetite Spherules, 87, 92–100, 114 Magnetic telechemistry, 72 Mid-ocean ridge basalt (MORB), 67–70, 72–3, 76, 78, 80, 135 Nontronite, 89, 91–2 Ochrous metalliferous sediment/precipitates, 87–92
Subject Index
Palagonite, 74, 75, 103–9, 111–13 Pillow basalt, 108 Pumice, 73, 82–6, 103, 109–12, 131 Sheet basalt, 62 Spilite, 73, 80–1 Tholeiite, 70, 83 Titanium spherules, 89 Toba volcanics, 82, 86, 101–2 Volcanic Hydrothermal Materials (VHM), 86–92, 113, 207 Zeolitites (phillipsite/clinoptilolite), 73, 104, 108–13, 163, 165
20⬚E
40⬚
60⬚
80⬚
100⬚ 20⬚ N
0⬚
20⬚ S
40⬚
Mukhopadhyay, Ghosh and Iyer, Figure 1.1 General physiography, essential tectonic elements and bathymetry of the Indian Ocean and neighbouring seas (base map from NGDC).The Indian Ocean Nodule Field (IONF) within the Central Indian Ocean Basin (CIOB) is shown in square box. Note inverted Y-shaped profusely fractured mid-ocean ridge system, the long north-southtrending Chagos^Laccadive Ridge and Ninetyeast Ridge on either side of the IONF, and the Andaman^Sumatra subduction zone to the east.
60⬚
30⬚
0⬚
30⬚
60⬚ 0⬚
30⬚
60⬚
90⬚ 120⬚ 150⬚ 180⬚ 210⬚ 240⬚ 270⬚ 300⬚ 330⬚
0⬚
Mukhopadhyay, Ghosh and Iyer, Figure 1.3 Plate boundaries of various types. Green ¼ subduction zones, light blue ¼ mid-oceanic ridge, deep blue ¼ deformed boundary zone, red ¼ continental deformation zone (Kreemer et al., 2002).
69E
Gravity (mgal)
68E
50
70E
71E
72E
73E
74E
CIR 5S
0 −50 6S
CB −100
20N
IND
7S DG
0
8S 20S
CAP
80E
M = 5.0 AUS
100E
Mukhopadhyay, Ghosh and Iyer, Figure 1.6 Plate configuration in the Central Indian Ocean Basin (CIOB) from satellite-derived gravity field (Sandwell and Smith, 1997). IND ¼ Indian, CAP ¼ Capricorn, AUS ¼ Australian plates, CB ¼ Chagos Bank, DG ¼ Diego Garcia, CIR ¼ Central Indian Ridge. Earthquake epicentre data (courtesy: ANSS) in open and black circles in the main and inset figures. Gray in inset represents plate boundary zones, while horizontal lines represent extension of India^Capricorn plates (Henstock and Minshull, 2004).
A
9N
6
3
0
−160
−100
−120
58E
−50
−60
−20 −40
60
10 0
62
40 20
80 60
64
110
3S 66
68E
B 1.4 1.3
S, west flank (Cm/y)
1.2 1.4
S, east flank
2
1.3 1.2 1.4
1
3
S, mean
1.3 1.2
Mukhopadhyay, Ghosh and Iyer, Figure 1.13 (A) Mantle Bouguer gravity anomaly map of a part of the Carlsberg Ridge (CR) based on satellite altimetry data (Sandwell and Smith,1997).The NW-SE-trending thick black line is the ridge axis. (B) Spreading rate distribution for anomalyA5 along the CR for the west (top), east (middle) flanks and average spreading rate (bottom). The solid dots are rates as measured, thin lines are running average, while the thick lines show theoretical rate calculated using anomalyA5 and CR pole finite rotation (Merkouriev and Sotchevanova, 2003).
160W
100
40W
0
40E
100E
60 N
30
0
30
60 S
Ice rafted
Carbonate
Siliceous
Red clay
Terrigenous
Siliceous/red clay
Mukhopadhyay, Ghosh and Iyer, Figure 4.1 Sediment distribution in the world oceans (Kolla and Kidd,1982;The Open University,1995).
60E
70
80
90 10 N
0
10 S
20
30
Calcareous Ooze
Siliceous Clay
Calcareous Clay
Brown Clay
Terrigenous Clay
Terri-Siliceous Clay
Mixture of Terrigenous, Calcareous, Siliceous Clay Southern limit of Indonesian Volcanic Tephra
Mukhopadhyay, Ghosh and Iyer, Figure 4.2 Sediment distribution in the Indian Ocean (Udintsev,1975; Kolla and Kidd,1982).
A
B 80E
20N
100
80E
100
Fe (%)
Al (%)
20N
0
0
20S
20S > 6.5 6.5 - 5.5 5.5 - 4.5 4.5 - 3.5
> 5.5 5.5 - 4.5 4.5 - 3.5 3.5 - 2.5
3.5 - 2.5 2.5 - 1.5 1.5 - 0.5 < 0.5
C
2.5 - 1.5 1.5 - 0.5 < 0.5
D
20N
Smectite (%)
>70 50 - 70 30 - 50 10 - 30 <10
MYANMAR
INDIA
2
1
20N
3 4
4 Sri Lanka
3
0
0
0
0 40
3
00
40
5
3 5
20S
20S 80E
100
80E
100
Mukhopadhyay, Ghosh and Iyer, Figure 4.6 Geochemical zones in the Indian Ocean Nodule Field (IONF) sediments showing distribution of (A) Al, (B) Fe and (C) smectite.The overall sediment distribution process (Wijayananda and Cronan,1994) is sketched in (D):1 ¼ Mg, Al,Ti,V, Cr, Fe and smectite-rich subcontinental sediments, 2 ¼ Himalayan range sediments, 3 ¼ sediments from subduction zone, 4 ¼ terrigenous sediments, 5 ¼ Mn, Co, Cu and Ni-rich hydrogenous sediments (after Nath, 2001).
A
Warm surface current
B
Present day Productivity
0 Depth (km)
Deep cold current
728 3
752
OMZ
721
754
757 722 731
6 Late Miocene−Early Pliocene Productivity
Depth (km)
0
721 728 3
752
OMZ
754
757
722 731
6 0
4000 Distance (km)
8000
Mukhopadhyay, Ghosh and Iyer, Figure 4.11 (A) Distribution and exchange of warm surface and colder deep water along the conveyor belt in the world’s oceans (Einsele, 2000). (B) Fluctuation in oxygen minimum zone (OMZ): present day and at Late Miocene (Dickens and Owen,1994).
Mukhopadhyay, Ghosh and Iyer, Figure 6.3 Deep Sea Sediment Resuspension System (DSSRS) is a benthic disturber used in the Indian Deep-sea Environment Experiment (INDEX) sites in the Indian Ocean Nodule Field (IONF) to disturb sediments. The disturber suspended about 3555 tons of sediment along a 88-km stretch (Sharma et al., 2000).
Pacific Ocean
Group I Group II 15N
10
155W
145
135
125
115W
Mukhopadhyay, Ghosh and Iyer, Figure 6.10 Sketch of the mining claims by Pioneer Investors (PIs) in the Pacific Ocean (from Glasby, 2000). Each colour box represents a country/consortia in order. Group 1: Russia, Japan, France, China, Interoceanmetal Joint Organisation and South Korea. Group 2: Ocean Mining Associates, Ocean Minerals Company, Ocean Management Inc. (OMI-I and OMI-II), Kennecott Consortium, and white areas ¼ International Seabed Authority (ISA).
