Evidence for recent hydrothermal activity in the Central Indian Basin

Pergamon DeepSea Research I, Vol. 44, No. 7, pp. 1167-l 184, 1997

0 1991 Elwier Science Ltd

PII: so%7-o637(97)ooool~ All rights reserved. Printed in Great Britain

096743637197 $17.00+0.M)

Evidence for recent hydrothermal activity in the Central Indian Basin

S. D. IYER,* M. SHYAM PRASAD,* S. M. GUPTA* and S. NIRMAL CHARANt

(Received 5 October 1995; in revisedform 18 July 1996; accepted 5 October 1996)

Abstract-This study documents the first actual proof of recent intraplate volcanic-hydrothermal activity in the Central Indian Basin (CIB). Twenty-six surface sediments and a spade core (37 cm long) from the CIB were examined for the presence of volcanogenic-hydrothermal materials (vhm). High concentrations of vhm were discovered in a grab and the core top-both located at the base of an intraplate seamount. The vhm consist of ochrous metalliferous sediments, volcanic spherules and glass shards. The radiolaria associated with the vhm suggest a N 10 ka age for the hydrothermal episode. The metalliferous sediments are semi-indurated, yellow to orange colored Fe-Si oxyhydroxides with Fe0 and SiOl contents between 5473% and 1630%, respectively, and have been derived as a result of hydrothermal precipitation. Incipient formation of nontronite is noted to co-occur with these sediments. The CIB metalliferous sediments have close similarities to those reported from the intraplate regions of the Pacific Ocean. The volcanic spherules occur in various shapes and sixes and are dominantly composed of magnetite and lesser amounts of ilmenite, hematite and maghemite. Electron microscopy shows the arrangement of magnetite crystals in various textural forms. Inclusions within the spherules are of olivine, pyroxene and feldspar. The spherules have formed by a process of liquid immiscibility of a silicic-basic magma, dependent on oxygen fugacity. Rhyolitic glass shards are ubiquitous at l-2 cm depth in the core and constitute 55% of the coarse fraction. Microprobe analyses of the CIB shards show clear differences in Ti- and Si-Al ratios that, together with the vast differences in age of eruption, preclude their derivation from Toba (Indonesia). 0 1997 Elsevier Science Ltd

INTRODUCTION

The first discoveries of metalliferous sediments and mounds, unusually rich in Fe, Mn, Si, nontronite and barite, were reported from the Galapagos rift (Corliss et al., 1978; Honnorez et al., 1981); the East Pacific Rise (EPR), Bauer Deep and Central Basin (Dymond et al., 1973; Heath and Dymond, 1977); and from the Lau Basin (Bertine, 1974). Discoveries of these, massive sulfide deposits and high temperature vents near 21”N on the EPR (e.g. Francheteau et al., 1979; Spiess et al., 1980) and near 13”N on the EPR (Fouquet et al., 1988) were followed by similar discoveries along the Mid-Ocean Ridge (MOR) and rift zones (Rona and Scott, 1993).

In addition to the MOR, seamounts also may possess prerequisite conditions for hydrothermal activity. The conical shape of seamounts, presence of calderas and associated fracturing provide conditions conducive to hydrothermal discharge and accumulation of the resultant hydrothermal precipitates (Alt et al., 1987). Bonatti and Joensuu (1966) were among the first to report on the occurrence of spongy iron-oxides from a seamount in the

* National Institute of Oceanography, Dona Paula, Goa 403 004, India. t National Geophysical Research Institute, Hyderabad 500 007, India.

1167

1168 S. Iyer et al.

South Pacific and to suggest a hydrothermal origin. Later investigators have identified metalliferous sediments in mid-plate areas of the Pacific Ocean, (Dymond and Veeh, 1975), hydrothermal Mn oxides and nontronite from seamount environments (e.g. Batiza et al., 1977; McMurtry et al., 1983), Fe-rich slabs (Piper et al., 1975), ironstones (Hein et al., 1994) and hydrothermal Fe and Si oxyhydroxides (Hekinian et al., 1993).

Investigations in the Central Indian Basin (CIB) have led to the identification of volcanic ash layers in sediment cores (Martin-Barajas, 1988; Gupta, 1988), pumice (Iyer and Sudhakar, 1993a), basalts (Mukhopadhyay et al., 1995), spilites (Karisiddaiah and Iyer, 1992) and zeolitites (Iyer and Sudhakar, 1993b).

In this report we discuss the discovery of metalliferous sediments of recent origin recovered at the base of an intraplate seamount in the CIB. The sediments are composed of Fe-Si oxyhydroxides, volcanic spherules and glass shards. For simplification, we term these deposits volcanogenic-hydrothermal materials (vhm).

PHYSIOGRAPHIC SETTING

The CIB is bounded on three sides by the Ninetyeast Ridge, Southwest Indian Ridge (SWIR) and Southeast Indian Ridge (SEIR). The basement age of the CIB is 50-60 Ma as noted from magnetic anomalies (Mukhopadhyay and Batiza, 1994). Three large N-S fracture zones, traverse at 73”E, 76”3O’E and 79”E in the basin (Kamesh Raju, 1993). Many seamounts dot the floor of the CIB (Mukhopadhyay and Khadge, 1990; Kamesh Raju et al., 1993), some of them having caldera (Kodagali, 1991; Kodagali, pers. commun.). The major physiographic features of the CIB are depicted in Fig. 1.

MATERIALS AND METHODS

Twenty-six surficial sediment samples and a spade-core were examined for vhm from geographically wide-spread locations in the CIB (Fig. 2). Of the 27 samples, two (SS2/89 and SS10/657; Fig. 3) had a large concentration of vhm and hence were considered for a detailed investigation. Sample SS2/89 (Pettersson grab; 14”.01’S and 75”.59’E), contained about 150 g of sediment. Sample SS10/657, a spade-core of 37 cm length (13”59.86’S and 75O58.15 l’E), was collected 1.4 km from SS2/89 and was sub-sampled at 2 cm intervals.

All the sediment samples were thoroughly washed in distilled water and the clays dispersed using 20 ml of 10% Na-hexametaphosphate and sieved through a 63 pm mesh sieve. A hand-held magnet was used to recover magnetic fractions from the coarse fractions. Glass shards and radiolaria were quantified by the method of Nigrini (1967). Mineralogical identifications of five magnetic particles were made on a Gandolifi twin-axis rotation camera (57.3 mm) attached to a Philips X-ray diffractometer (Xrd) system using Cu KU radiation and a Fe filter, with exposure times of 32-80 h.

Scanning electron microscopy (SEM) and electron probe microanalyses (EPMA) of the samples were carried out on a Camebax-571 microprobe. In all, 115 particles were mounted on Al stubs and coated with gold for SEM. An accelerating voltage of 15 kV and a beam current of 4.2-4.4 nA were used with a beam diameter of 3-5 pm. Although magnetite is the dominant component of the magnetic fractions, several other phases were also identified with a wavelength dispersive spectrometer (WDS) and their composition was later quantified by microprobe. For EPMA, 56 particles from SS2/89 and SS10/657 were mounted in epoxy, ground and polished to expose the internal features, and then sputter

Hydrothermal activity in the Central Indian Basin 1169

8”Oo’.

16”0(

Fig. 1. Physiographic features of the Central Indian Basin. FZ 1,2,3 = Fracture zones; Stars = seamounts. Contour interval = 100 m. See text for source of data.

coated with carbon. Silicate and oxide standards were used for calibration and the raw data were corrected for ZAF effects. Further, Ni and S contents were determined with pure Ni and pyrite standards, respectively. The polished sections of the spherules were analysed on a flat surface with an accuracy of 0.1%. The ochrous sediments, due to their porous nature, gave lesser totals and hence only those totals > 90% were considered.

Geology of the area and age of the sediments

Samples SS2/89 and SS10/657 were recovered from the base of a seamount located at 14.OO”S and 75.935”E (Fig. 3) and -45 km from the nearest fracture zone at 76”3O’E. The height of the seamount is 800 m and the water depth at the summit is 4440 m. The area of the seamount is 37.56 sq.km with a length of 5.77 km, a basal width of 6.51 km and a summit width of 0.18 km. The seamount lies on magnetic anomaly A 23b (Mukhopadhyay and Batiza, 1994) corresponding to a crustal age of 50.8 l-50.64 Ma on the time-scale of Cande and Kent (1992).

Siliceous microfossils like radiolarians and diatoms were identified to assign geological ages for the various subsamples of SS10/657. Two radiolarian zones, Buccinosphaera invaginata and Collosphaera tuberosa, were noted with a distinct boundary between the two

1170 S. Iyer et al.

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Fig. 2. Map showing the location of the 27 sediment samples studied from the Central Indian Basin. Numebers below each station are the numbers of magnetic particles present in 1 g of coarse fraction.

Star denotes the location of surface sediment SS2/89 and the spade core SS10/657.

zones at 30 cm depth in the core. This boundary is defined by the first appearance datum (FAD) level of B. invuginata in this core (Gupta, 1988). This FAD is paleomagnetically dated to be synchronous at 180 + 20 ka in nearby cores RC 14-22 and VM 34-53 by Johnson et al. (1989) and Caulet et al. (1993).

Three conspicuous radiolarian assemblages were identified from a factor analysis of percentage data of 47 radiolarian species in the CIB and these were related to the overlying sea surface temperature (SST) (Gupta, 1996). The warm SST factor, dominated by the Spongodiscids group, consisted of five species: Spongotrochus glacialis, Spongodiscus resurgense, Spongodiscus biconcavus, Spongurus spp. and Spongopyle osculosa. The cold SST factor, dominated by Pyloniids, comprises three species: Tetrapyle octacantha, Octopyle stenozona and Hexapyle dodecantha. The transitional SST factor was characterized by the Euchitoniids group, which is made up of six species: Euchitonia elegans, E. jiircata, Euchitonia sp., Dictyocoryne truncata, Dictyocornye profinda and Hymeniastrum euclidis (Gupta, 1996). These factor assemblages were quantified and their mutual ratios were plotted for core SS10/657 for the identification of datums within the latest Quaternary period.

The graphs of transitional to warm and transitional to cold fauna ratios show three distinct peaks with a maxima in the transitional group (Fig. 4a). Using the Pleistocene


Fig. 3. Seafloor map of the study area. Samples SS2/89 and SS10/657 are from the base of a seamount (water depth at summit = 4440 m), which has a pronounced bulge in an approximately E- W direction. Note the occurrence of minor seamounts to the NE and S and flexure of the seafloor

north of the present seamount. Contour interval = 100 m.

climatic scale of Martinson et al. (1987), these peaks are assigned ages of 130,70 and 10 ka at 28, 16 and 6 cm core depth, respectively. The top 6 cm of the core, which corresponds to N 10 ka, has the highest abundance of vhm. Cacsinodiscus nodulijkr diatoms are generally absent in most of the upper sections of the core but are found in remarkable numbers at 20- 28 cm and 36 cm (Fig. 4b). Maxima in C. nodulzjkr abundance are recorded just above and just below the B. invuginatu zonal boundary at 30 cm. Burckle and Mclaughlin (1977) found

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Fig. 5. (a) Photomicrograph of volcanic magnetite spherules exhibiting different shapes and sizes. Note adhering of ochrous sediments on some of the spherules. (b-1) Electron micrographs of ochrous sediments and volcanic spherules. Scale bars of a-d, g, h, j, 1 = 100 pm; e,i,k = 10 nm; f = 25 pm). (b) The largest ochrous sediment particle. An elongated crater is seen having overturned rims (see arrow) formed due to melt flow. The crater floor contains a bunch of spherules. (c) Higher magnification of the depression shown in (b). (d) Well-developed large euhedral magnetite crystals on a spherule. (e) Quench texture on the surface of a spherule. (f) Spherule with blow-holes created from escape of volatiles from within. (g) Tear-drop shaped spherule ejecting a %-rich blob. (h) Polished section of a spherule showing a central Si-rich capsule having a thick wall enclosing well-developed magnetite crystals. (i) Magnified view of magnetite crystals shown in (h). (i) Polished section of a spherule exhibiting an eccentric Si-rich capsule within a spherule. This capsule has thin walls and encloses numerous magnetite crystals. (k) Dodecahedral/octahedral magnetite crystals within the capsule

shown in 6). (1) Electron micrograph of platy glass shards found abundantly in the sediments.


Spherules

“Fresh” spherules with metallic luster dominate the magnetic fractions and range in size from a few microns to 475 pm. The spherules are dominantly spherical, but dumb-bell, tear drop and oval shaped spherules are also observed (Fig. 5a). During ultrasonic cleaning of the samples, several of the spherules broke-up, revealing hollow interiors or orange colored slag-like material. Mineralogically, the spherules are composed of magnetite crystals with lesser amounts of ilmenite, hematite and maghemite. SEM of the spherules shows an array of surface textures, reflecting the arrangement of magnetite crystals. This arrangement is due to quenching, i.e. melting followed by immediate supercooling - a process similar to that experienced by cosmic dust (Blanchard et al., 1980).

The commonly observed textures on the spherules are brick-work (Fig. 5d), interwoven textures (Fig. 5e), cork-screw type distortions and dendritic patterns. Some spherules are smooth with no apparent crystallinity vis-d-vis spherules with more than one type of surface texture. Several spherules contain vacuoles, some of which have overturned rims (Fig. 50. A possible process for the vacuole formation could be blow-out made by the escape of volatiles from within. In some rare cases, Si-rich blobs are found to protrude from an otherwise Fe- rich spherule (e.g. Figure 5g). In the polished sections of the spherules, unextruded Si-rich blobs show up as Si-rich inclusions.

An examination of 56 sectioned particles indicates that the external crystalline structure of the spherules is restricted to the top few microns. The interior of the spherules contain vacuoles that enclose either Si-rich blobs or glassy capsules which in turn enclose idiomorphic magnetite crystals (Fig. Sh-k) in an otherwise smooth matrix. In addition, there are discrete areas within the spherules that are enriched in Mg and have a composition similar to that of olivine and orthopyroxene (Table 2), but these do not exhibit any crystalline nature.

Glass shards

Smear slides of sub-samples of SS 1 O/657 revealed abundant platy glass shards of 15ct 250 pm length occurring trapped within the metalliferous sediments or as individual fragments (Fig. 51). The glass shards range from a maximum of 7000 per slide (- 55%) at l- 2 cm to 2555 per slide at 5-6 cm. Their abundance gradually decreases downward to the bottom of the core (Fig. 4c).

From the microprobe analysis we have classified the morphologically different samples as follows:

(1) Metalliferous sediments with a mixed composition of SiOz (N 1630%) and Fe0 (54- 73%) and with variable contents of other oxides (#l-10; Table 1).

(2) The sphemles are dominantly magnetite as revealed by XRD, with high Fe0 (range 87-99%, avg. -95%) and minor amounts of Ti and Mn (#l; Table 2). At times, Si-rich zones encase the magnetite crystals forming clear chemical differences at their contacts (Fig. 5g): the glassy capsules have enhanced Si, Mg, Al, Ca and Na (as determined qualitatively by WDS), whereas the spherule matrix is dominantly Fe-rich.

(3) Sectioned spherules show various types, of inclusions of which three are Mg-rich. One is of hyalosideritic olivine (#4; Table 2; cf. Deer et al., 1967a), with no crystallinity, while the other two have orthopyroxene-like composition (bronzite-enstatite, #5-6; Table 2; cf. Deer et al., 1967b). However, the Al here is slightly higher. Besides these, the other types of

1176 S. Iyer et al.

Table 1. Representative microprobe analysis of metalliferous sediments (#l-10) from the Central Indian Basin

S. No. Sample SiOa TiOa A1203 Fe0 MnO MgO CaO NaaO KaO CrrOs BaO Total Remarks

1 2 3 4 5 6 I 8 9 10 11

89 16.31 0.33 3.02 71.01 1.35 1.66 0.92 1.66 0.65 0.11 0.28 97.30 T-l,Si/Fe=O.l4 89 16.18 0.46 3.23 69.32 1.19 1.63 1.01 0.74 0.63 0.30 0.13 94.82 T-l, Si/Fe=O.l4 89 29.99 0.14 6.62 53.87 0.37 0.28 0.14 0.04 0.05 0.30 - 91.80 T-3, Si/Fe=0.33 89 25.87 0.17 4.06 58.79 0.13 0.46 0.98 0.25 0.19 0.03 - 90.93 T-l/3, Si/Fe=0.26 89 25.75 0.04 5.07 60.49 0.40 0.46 1.21 0.25 0.20 0.09 -- 93.96 T-l/3, Si/Fe=0.26 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 T-1/3,Si/Fe=0.27 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 T-1/3,Si/Fe=0.26 657 20.77 0.10 0.34 71.06 0.08 1.13 1.59 0.05 0.26 0.19 0.11 95.68 T-l,Si/Fe=0.18 651 22.86 0.05 2.25 59.26 - 6.15 6.29 0.28 0.23 1.38 0.20 98.95 T-l,Si/Fe=0.23 657 21.93 - EPR 31.67 -

Society 17.24 - 6.84 56.22 0.03 - - - - 0.02 - 80.35

EPR 41.82 -

Red Seamount 8.25 0.04 0.50 56.06 0.01 0.68 0.86 0.67 0.26 0.002 0.005 67.34

Red Seamount 17.27 0.05 0.85 56.45 0.18 0.90 1.04 0.86 0.39 0.002 0.03 78.02

16 Galapagos 47.06 - 17 Fukujin 53.91 0.41 18 Red Seamount 47.50 0.04

0.23 73.50 0.29 1.12 1.81 0.16 0.20 - 0.07 99.31

- 41.20 0.18 2.45 2.37 1.23 0.52 - - 79.62

1.06 26.63 0.65 - - - - -- - 70.16

0.18 32.72 0.31 2.44 0.74 1.54 1.78 - - 86.77 0.71 26.65 1.04 1.88 0.84 2.02 1.50 - 0.04 89.00 0.56 32.16 0.02 1.92 0.09 3.16 1.90 0.001 0.005 87.36

T-l, Si/Fe=0.18 Fe-hydroxide from seamounts, T-3, Si/Fe = 0.46 Fe-hydroxide.T-1, S = 0.3%; Si/Fe = 0.18 Fe-Si hydroxide. T-3, S= 0.4%; Si/Fe=0.95 Fe-oxide mud of active vent, PaciticOcean(3),Si/Fe=0.09 Fe-oxide mud of inactive deposit, Pacific Ocean (3), Si/ Fe=0.19 Nontronite (3), Si/Fe= 0.86 Nontronite, Si/Fe= 1.22 Nontronite (2), Si/Fe= 0.89

EPR, East Pacific Rise; l-10, present study; 11, AR etai. (1987); 12,13 andT-I, T-3, T-l/3, Hekinianet al. (1993); 14,15 and 18, Alt (1988); 16, Corliss et al. (1978); 17, McMurtry et al. (1983).

Serial numbers 11 to 15 are the composition of F&i oxyhydroxides, and 16 to 18 of nontronite from the Pacific Ocean

inclusions are enriched in Si, Al, Fe, Ca, and Na (#7-9; Table 2), indicating the presence of feldspars but the Fe content is too high to be pure plagioclase. It is worth noting that feldspars have been reported from hydrothermal deposits on the Mid-Atlantic Ridge (MAR, cf. Hoffert et al., 1978). A possible reason for these phases in the present samples

Table 2. Microprobe analysis of volcanic magnetite spherules, inclusions within the spherules and volcanic glass shards of the Central Indian Basin. The numbers of analyses usedfor the averages are given in parentheses

S. No. Sample No. SiOl TiOs AlrOs Fe0 MnO MgO CaO NaaO KaO CrrOs BaO Total Remarks

Magnetite Spherules

1 89 and 657 1.91 0.60 0.68 94.98 0.46 1.04 0.45 0.39 0.08 0.16 0.13 100.88 Magnetitespherules(24) 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 Ti-richareasinspherules(5) 3 89 2.81 60.76 4.41 23.92 6.82 0.40 0.65 - 0.09 0.03 1.04 100.93 TLrichareasinspherules(2)

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 Hyalosideritic Ohvine 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 Bronzit~nstatite 6 657 59.32 - 2.00 9.15 - 28.76 0.08 0.03 0.02 0.60 0.03 99.99 Bronzite-enstatite 7 89 43.26 - 36.76 8.88 0.38 0.51 4.58 5.23 0.23 0.19 - 100.02 Feldspar

8 89 59.85 0.27 25.76 2.60 - 0.05 3.55 6.40 0.93 - - 99.41 Feldspar 9 89 48.14 0.29 25.18 21.25 0.28 0.50 0.74 0.48 1.53 0.08 0.29 98.76 Feldspar

Glass Shards 10 89 77.48 1.27 9.28 6.87 0.28 0.32 1.08 0.95 2.33 0.27 0.14 100.27 Rhyoliticshards(6)


could be admixing of the surrounding aluminosilicate sediments with the large amount of Fe, which tends to mask the original mineral composition.

(4) One spherule and an inclusion show substantial Ti enrichment (5260%; #2,3; Table 2), but their composition does not correspond to any known Ti mineral. This probably represents contamination of a titanium mineral (ilmenite?), which was observed in the XRD of a spherule, with the ubiquitous iron oxides.

(5) The platy glass shards have rhyolitic composition with Si02 ranging from 74 to 81.5% (avg. 77.5%; #lo; Table 2).

(6) The inclusions within the spherules and metalliferous sediments were analysed for Ni and S (Table 3). Ni ranges between 0.02 and 0.14% and S between 0.01 and 1.55%, in the spherules; in the metalliferous sediments Ni ranges between 0.02 and 1.40% and S between 0.21 and 4.65% (Table 3). The highest Ni content (up to 2.24%) and S content (up to 9.82%) were noted at one probe spot in the inclusions within the metalliferous sediments.

DISCUSSION

We note that the - 10 ka age vhm, of varying morphological and compositional nature, occurs at the base of an intraplate seamount of > 50 Ma. The probable genesis of the vhm and similarities to deposits at intraplate seamount locations and other sites in the Pacific Ocean are discussed.

Metalliferous sediments

Four possible modes of formation of metalliferous sediments have been proposed: derivation from hydrothermal emanations (Zelenov, 1965); reaction of sea water with hot

Table 3. Microprobe analysis for Ni and S (wt. %) of inclusions within the volcanic spherules and melailiferous sediments, from the Central Indian Basin.

The numbers of analyses usedfor the average are given in parentheses

S. No. Sample Ni(%) S(%) Remarks

Inclusions in volcanic magnetite spherules 1 89 0.13 0.08 Metallic matrix 2 89 0.02 0.04 Oval shaped spherule 3 89 0.11 0.06 Tear drop spherule (3) 4 89 0.14 Matrix 5 89 - 0.01 Matrix 6 89 1.55 Edge of matrix 7 657 0.11 0.54 Matrix (2)

Inclusions in metalliferous sediments 8 89 0.02 4.65 Metallic (3) 9 89 0.06 0.53 Metallic (2)

10 89 0.03 0.23 Metallic 11 89 0.08 0.21 Metallic (2) 12 657 1.40 0.37 Metallic (2) 13 657 1.09 0.43 Metallic (4)

Samples 89 and 657 (S. Nos 8 and 13) show the highest S (9.82%) and Ni (2.24%) at one probe spot.

1178 S. Iyer et al.

lava (Corliss, 1971; Dymond et al., 1973; Honnorez et al., 1981); hydrothermal precipitates (Hekinian et al., 1978,Hekinian et al., 1993; Alt et al., 1987); and bacterial processes (Alt et al., 1987; Alt, 1988).

It is conceived that during hydrothermal solution discharge from the igneous crust into the sediments, metal deposits can form either within the sediment column or at the sediment-water interface, resulting in intra-sedimentary hydrothermal deposits (Bonatti, 1983). Further, F&i oxyhydroxides may form as a result of alteration of unstable sulfide deposits on the seafloor (Hekinian et al., 1993), or may be of primary origin, formed by direct precipitation from hydrothermal fluids (e.g. Alt et al., 1987; Binns et al., 1993). Our analyses of SS2/89 and SS10/657 sediments make it difficult to attest to a single mode of formation, but the geologic setting of the samples (Fig. 3) and strong chemical evidence (Table 1) suggest that these sediments were derived from hydrothermal precipitation.

Hekinian et al. (1993) identified Fe- and Si-rich oxyhydroxide deposits from intraplate volcanoes in the South Pacific and the EPR. From submersible observations, compositional variations and Si/Fe ratios, they divided the deposits into four types: Fe oxyhydroxides (total Fe 27-45 wt.%) depleted in trace metals; Fe oxyhydroxides (Fe 3&50X) associated with sulfides and trace metals; F&i oxyhydroxides (Fe 20-30%, Si 7-20%) enriched in nontronite; and Fe oxyhydroxides (Fe < 10%) Si > 35%) enriched in opaline silica.

On comparing our analyses of the metalliferous sediments (#l-10; Table l), with those from the Pacific (#l l-l 5; Table 1) we observe that the CIB samples are akin to F&i oxyhydroxides of the Pacific Ocean. On the basis of Si/Fe ratios, Hekinian et al. (1993) found that type 1 has Si/Fe < 0.3 1 as compared to 0.30.94 for type 3, which is intermediate between Galapagos mounds nontronite (Hekinian et al., 1978) and type 1 Fe oxyhydroxide deposits. Interestingly, the CIB samples show similar Si/Fe ratios (0.14-0.33) and are also comparable to the Fe oxide mud of inactive deposits of Red Seamount (cf. Alt, 1988).

The above facts suggest that favourable conditions exist in the CIB wherein hydrothermal solutions can percolate through the basaltic crust, leach the rocks (cf. Seyfried and Bischoff, 1977) and mix with the sea water. The cooling and oxidation of such solutions result in precipitates of FeOOH (Corliss, 1971). The textural characteristics of Fe-Si oxyhydroxides make it difficult to ascertain their original site of deposition, due to their easy transport and dispersal (Hekinian et al., 1993); but the samples from the CIB show a high concentration in a localized area, which rules out the possibility of the vhm observed here having been transported by bottom currents from a far off place to be deposited at the base of the seamount. Additionally, recent observations with deep-towed instruments and examination of several thousand underwater photographs have indicated the absence of bottom currents in the CIB (R. Sharma, pers. commun.). Therefore, the hydrothermal deposit identified here seems to have formed in situ.

Other than the Fe-rich sediments, a composition close to nontronite, a typical hydrothermal mineral, is also observed. The CIB samples (Table 1) show similar enrichment trends in Si and Fe, but their A&O3 and MgO are enriched in some samples compared to marine nontronites (#16-l& Table 1). The higher Si/Fe ratios in type 3 deposits, coated with manganese and associated with inactive hydrothermal fields, are believed to be related to an increase in nontronite (cf. Hekinian et al., 1993). Nontronite forms when hydrothermal solution cools slowly under reducing conditions while percolating through the sediments and precipitating Fe and Si (Corliss et al., 1978). Honnorez et al. (1981) suggested that iron could precipitate in pelagic sediments as amorphous FeOOH from upward-diffusing hydrothermal solution and the FeOOH further


react with the hydrothermal solution and the biogenic silica to form nontronite (Barrett and Friedrichsen, 1982). Such a process was earlier propounded by Heath and Dymond (1977) for the Bauer Deep Fe-rich smectite.

Thus, we infer that the CIB metalliferous sediments are hydrothermal precipitates similar to other such oceanic deposits. The hydrothermal precipitates were formed through submarine exhalations and subsequently deposited in the surrounding siliceous sediments. It is worth mentioning that the influence of dilute, low-temperature hydrothermal solutions was earlier suggested from a study of the ferromanganese crusts occurring at the base of the a seamount (12”34’-12”39’E and 76”03’-76”3O’E, -45 km from the present area; Iyer, 1991) spilites in the vicinity of the 79”E fracture zone (Karisiddaiah and Iyer, 1992) and the widespread occurrence of zeolitites in the basin (Iyer and Sudhakar, 1993b). Further, the MOR, nearest to the presently studied location viz., the Central Indian Ridge, is at a distance of w 1000 km. It is noteworthy that an enhancement of such fragile vhm has not been found in any of the other samples studied by us, nor has it been reported in the literature. These observations therefore indicate localized intraplate hydrothermal activity in the CIB.

Volcanic spherules

The CIB volcanic spherules are either smooth (Fig. 5e) or have magnetite crystals arranged in various textural patterns (Fig. 5d) that are commonly seen on cosmic spherules (Blanchard et al., 1980) and from those recovered from ferromanganese crusts from the MAR (Aumento and Mitchell, 1975). Paradoxically, we note that the CIB spherules have textural similarities to cosmic spherules, but the interiors consist of vacuoles containing either Si-rich blobs or glassy capsules enclosing magnetite crystals (Fig. 5h-k), in contrast to the Ni-rich cores typically found in Fe-rich cosmic spherules (Blanchard et al., 1980). Considering that magmaphile elements dominate the composition of the spherules, we term the CIB spherules as “volcanic”, adopting the criteria of Del Monte et al. (1975): the dominance of magnetite associated with silicate fragments, the absence of wustite and alpha-Fe minerals, and the presence of elements such as Si, Ti, Al, Mn, Ca, Mg and Cr. Furthermore, it has been shown that Ti (El Goresy, 1968) and Mn (Finkelman, 1970) are diagnostic elements in spherules of volcanic origin.

We attempt to explain the formation of the CIB volcanic magnetite spherules by a process of liquid immiscibility of a silicic-basic magma, coupled with oxygen fugacity. It has been suggested that oxygen controls the fractionation of a liquid (Muan and Osborn, 1956; Osborn, 1979) giving rise to two contrasting or fractionated paths of cooling liquids: Fe- rich and Fe-depleted and with both cooling liquids precipitating magnetite (Presnall, 1960). As the temperature of the cooling liquid falls, Fe oxidizes faster and combines with oxygen forming magnetite in the cooling melt depending on the relative oxygen fugacity. Due to their large density differences, the Fe and Si fractions form distinct chemical differences (Fig. 5h). In case the Si forms a glassy microlayer for the Fe-rich liquid cooling in the interior of the spherules, then idiomorphic magnetite crystallizes (Fig. 5i). With a more complete oxidation, unmixing of the phases occurs, with the result that the Si phase is either pushed aside (Fig. 5h) or else separates from the Fe phase as blobs (Fig. 5g).

According to Anderson (1974), sulfur in silicate melts seems to be almost universally saturated with a sulfide phase, generally an immiscible melt but possibly a crystallized mineral at low igneous temperatures. A similar relation between S and Fe0 content of the

1180 S. Iyer et al.

FAMOUS basaltic glass suggested the strong dependence of S on Fe0 content in the melt (Czamanske and Moore, 1977). The sulfur content of the vhm of the CIB seems to suggest its origin from a deep-seated source and co-existence with the abundant Fe. We observe that sulfur contents in the inclusions of the spherules range from 0.01 to 1.55%, whilst those in the metalliferous sediments range from 0.21 to 4.65% (Table 3). During their deposition onto the sea floor, the S from the magnetite crystals might have been lost by degassing as indicated by some of the blow-holes observed on the spherule surface (Fig. 5f). This degassed S was probably admixed with the metalliferous sediments, which show inclusions with the highest S content (up to 9.82%; Table 3) in one instance.

Volcanic glass shards

It has been postulated that glass shards and pumice in the CIB have originated as distal fallout from the Indonesian Arc volcanism and from Toba, respectively, based on the Ti/Al ratio of the tephra (Martin-Barajas and Lallier-Verges, 1993). But an examination of the distribution of Toba tuffs and equivalent ash layers in deep-sea cores indicates confinement of the Toba ash to the north-east Indian Ocean and the Bay of Bengal (Fig. 1 of Ninkovich et al., 1978; Dehn et al., 1991). We find significant dissimilarities in terms of certain elemental concentrations and ratios, between the CIB shards and those from the Toba eruption (Table 4). Considering the association of the CIB shards with the vhm at l-2 cm depth of the core, the shards could have been formed in situ. Moreover, the major Indonesian volcanic activities have been dated to have erupted at N 74 000,450 000,840 000 and 1.2 Ma BP (Rose and Chesner, 1987), which do not correspond to the N 10 ka age of the vhm. At present we are unable to resolve the origin of the glass shards as to whether these are contemporaneous with the formation of the metalliferous sediments or are pre-existing entities in the CIB sediments.

Intraplate volcanic and hydrothermal activity

Numerous seamounts of different morphology have been observed in the CIB to occur in linear chains (Mukhopadhyay and Khadge, 1990; Kamesh Raju et al., 1993). Some of the

Table 4. Representative elemental concentrations and ratios in volcanic glass shards of the Central Indian Basin

and of Toba. Indonesia

1 2 3 4 5

Si 36.22 33.75 33.56 36.59 25.39 Ti 0.19 0.07 0.08 0.04 1.01 Al 4.91 6.83 6.93 6.72 9.24 Ti/Al 0.04 0.01 0.01 0.01 0.11 Si/Al 7.38 4.94 4.84 5.44 2.75 K 1.94 4.23 4.23 3.54 2.87

1, present study; 2, 3, Toba glass shard (Ninkovich et al., 1978); 4, Average of 88 Toba glass shards (Rose and Chesner, 1987); 5, CIB glass shard (Martin-Barajas and Lallier-Verges, 1993)


secmounts have caldera (Kodagali, 1991). The origin of the CIB seamounts has been suggested to be from the Reunion hotspot or from two separate hotspots (Mukhopadhyay and Khadge, 1990), or to be related to the fracture zones (Kodagali, 1991; Kamesh Raju, 1993). It is well known that fracture zones are zones of weakness in the oceanic crust and form favourable conduits for an ascending magma. The CIB has been noted to contain a high incidence of oceanic intraplate earthquake zones wherein the earthquakes can be triggered by magmatism due to their association with pre-existing zones of weakness (Bergman and Solomon, 1980) during a phase of reactivation (Mukhopadhyay and Khadge, 1992).

Recent observations indicate that a majority of the CIB seamounts have an E-W elongation. This is attributed to subsequent addition of magmatic mass from eruptions that took place after the seamounts reached their present position (Mukhopadhyay, pers. commun.). This suggests that more than one episode of volcanism has occurred at some seamount sites in the CIB. The seamount in the present study area also has an E-W elongation (Fig. 3), which leads us to postulate an episode of recent volcanic activity. The above factors, when considered in the light of the present findings, suggest that the presently studied seamount has been active in the recent past, Although the seamount is -45 km from the 76”3O’E fracture zone, the influence of the fracture zone in relation to the hydrothermal deposits is not ruled out. Interestingly, hydrothermal signatures on ferromanganese crusts recovered near the 76”3O’E fracture zone had been suggested earlier (Iyer, 1991). Further, the occurrence of recent metalliferous sediments has been reported at three isolated locations in the North Pacific manganese nodule area, N 200 km from a fracture zone trace (Bischoff and Rosenbauer, 1977). These studies suggest that fracture zones may or may not influence the formation of metalliferous sediments.

CONCLUSION

We conclude from an observation of 27 sediment samples from the CIB that at two locations, at the base of an intraplate seamount (> 50 Ma), high concentrations of metalliferous sediments and volcanic spherules are associated with sediments of N 10 ka age.

Considering the enrichment of metalliferous sediments at the two sites, intraplate volcanism and associated hydrothermal activity in the CIB may have occurred on a local scale on the Indian Ocean plate, similar to that reported for the Pacific Ocean (e.g. Dymond and Veeh, 1975; Heath and Dymond, 1977; Hekinian et al., 1993). We speculate the probable existence of a sulfide deposit near the presently studied region. Our discovery of a recent (w 10 ka) hydrothermal deposit in the CIB, within the manganese nodule area, might have relevance for the type and eruptive mechanism of intraplate seamounts at abyssal depths.

Acknowledgements-We thank our Director, Dr E. Desa for strong encouragement, Mr R. R. Nair, Project Co- ordinator, for discussion and the Director of NGRI for kindly providing the microprobe facilities. We acknowledge the late Dr L. R. Bhargava, Mr K. D. P. Singh and Mr G. V. G. K. Murty of the Atomic Minerals Division, Hyderabad, for carrying out the painstaking XRD work. We thank Mr K. Ali Sheikh and Mr U. Sirsat for excellent support in printing the photomicrographs and Mr U. Javali and Mr K. G. Chitari for draughting the figures. We thank Dr M. Sudhakar for constructive comments on an earlier version of the manuscript. We are obliged to Drs T. L. Vallier and R. Batiza for critical assessments that vastly improved the paper. NIO contribution 2525.

1182 S. Iyer et al.

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Evidence for recent hydrothermal activity in the Central Indian Basin

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