Radiochronological investigation of hydrothermal deposits from the MESO zone, Central Indian Ridge

ELSEVIER

Marine Geology 149 (1998) 243–254

Radiochronological investigation of hydrothermal deposits from the MESO zone, Central Indian Ridge Claude Lalou a,Ł , Ute Mu¨nch b , Peter Halbach b , Jean-Louis Reyss a b

a Centre des Faibles Radioactivite ´ s, CNRS-CEA, 91198 Gif sur Yvette Cedex, France Free University Berlin, Department for Economic and Environmental Geology, Malteserstreet 74–100, 12249 Berlin, Germany

Received 2 October 1997; accepted 10 April 1998

Abstract Hydrothermal activity in the Indian Ocean has hitherto only been poorly documented. First indications of hydrothermal mineralization were observed in 1987 during the GEMINO-cruise over the 4th ridge segment of the Central Indian Ridge, 270 km north of the Rodriguez Triple Junction. Subsequent cruises in 1993 (HYDROTRUNC) and in 1995 (HYDROCK) were carried out in order to obtain detailed information about the geological setting and the extent and mineral distribution of the field, but primarily to sample the first massive sulfides of the Indian Ocean floor. This hydrothermal field, which was called the MESO zone, was studied by detailed mapping and by ocean bottom photography. Hydrothermal precipitates were recovered with a TV-grab sampler at four sampling locations, two in the northern part of the mineralized field, called the Talus Tips Site, and two in the central part, called the Sonne Field. The sulfide samples were age-dated using the 230 Th=234 U method. They show different periods of activity at each site: 52 š 5, 22:7 š 3 and 16 š 2 ka for the main events at the Talus Tips Site and 18 š 2 and 12:5 š 1:5 ka for the Sonne Field. We conclude from these data that the two sites were not active at the same time. If it is assumed that there is a complete cessation of activity between 52 and 23 ka, for more recent periods (from 23 to 10 ka) it may be postulated that the deep-seated hydrothermal circulation is continuous but that the discharge of hot fluid occurs alternately at the Sonne Field or at the Talus Tips Site. Comparing our results with those obtained previously at the different hydrothermal sites, we favor the conclusion of episodic activity. Moreover, in the Sonne Field, jasper formation resulting from a low-temperature hydrothermal event dated at around 11 ka is presently the youngest hydrothermal formation in the entire MESO zone.  1998 Elsevier Science B.V. All rights reserved. Keywords: Central Indian Ridge; hydrothermal activity; sulfides; jasper; radiochronology

1. Introduction Tectonic deformation, especially in external zones, is generally considered to be a prerequisite for convective heat transport and circulation of hydrothermal fluids and associated formation of Ł Corresponding

author. Tel. C33 (1) 6982-3539; Fax: C33 (1) 6982-3568; E-mail: [email protected]

mineral deposits. Exploration and investigations into hydrothermal sea floor mineralization has hitherto mainly been focused on spreading ridges in the Pacific and in the Atlantic, whereas the slow to intermediate spreading ridges in the Indian Ocean are still only poorly investigated. With the exception of a few low-temperature hydrothermal deposits (Cann et al., 1977; Rona et al., 1981), indications of high-temperature hydrothermal mineralization in the

0025-3227/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 0 4 2 - 5

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Indian Ocean were not found until 1987 when, during the GEMINO III cruise, it was first recognized north of the Rodriguez Triple Junction with a deeptowed camera (Plu¨ger et al., 1990). Color slides and video records indicate different hydrothermal products, such as sediment coloration and sulfide debris. Several research cruises as part of the GEMINO (Herzig and Plu¨ger, 1988; Plu¨ger et al., 1990), HYDROTRUNC and HYDROCK projects (Halbach et al., 1995, 1996) were then carried out in order to locate and study recent and=or former hydrothermal activity along the Central Indian Ridge. The focus of the last two cruises with R=V Sonne (1993) and R=V Meteor (1995) was to identify and recover massive sulfides from the Indian Ocean sea floor north of the Rodriguez Triple Junction. Detailed mapping of the hydrothermal field by means of ocean bottom photography was conducted to delineate the extent and zonation of the hydrothermal deposits. In addition, hydrocast stations were done to locate possible geochemical anomalies in the water column (Halbach et al., 1995). The objective of this study is to reconstruct the evolution (or rather the decay of chimney fragments

after hydrothermal activity ceased) of a massive sulfide deposit at the Central Indian Ridge. Ages are obtained, as in the previous fields studied (Lalou et al., 1990, 1993, 1995, 1996), from iron- and copper-dominated sulfides. Moreover, it has been established that sulfate-rich jasper samples may also be dated using the 230 Th=234 U method. Different episodes of hydrothermal activity are clear within the MESO zone.

2. Geological settings The hydrothermal field called the MESO zone (after the German research vessels R=V MEteor and R=V SOnne) is located in the central part of the 85 km long 4th ridge segment at about 23º23.500 S, 69º14.500 E on the Central Indian Ridge. Magnetic records indicate a spreading half-rate of about 2.5 cm=year in this ridge section (Plu¨ger et al., 1990). The N- to NNW-trending ridge segment is bounded by non-transform faults at both ends. The axial valley is about 20–25 km wide, whereas the inner graben is only 2–4 km wide; water depth varies between 3200

Fig. 1. Inset shows the general location of the studied area. Main map illustrates the 4th segment of the Central Indian Ridge (CIR) with the MESO zone. Areas deeper than 3000 m are patterned; bold line marks the ridge axis (after Briais, 1995).

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Fig. 2. Bathymetric map (10 m contour interval) of the MESO zone. Geological and tectonic information as inferred from deep-towed camera studies. Hydrothermal mineralization is structurally controlled by fissures, scarps and cracks running both parallel and orthogonal to the general strike direction (N153º) of the ridge. A division into three different sites is therefore possible Talus Tips Site; Sonne Field and Smooth Ground. Hydrothermal deposits are located in a pillow lava and basaltic talus=debris dominated terrain. The map also shows the positions of the TV-grab stations.

and 4000 m here (Briais, 1995). The central graben is subdivided by a neovolcanic ridge over a distance of about 20 km (Fig. 1). Hydrothermal precipitates were observed close to the top of the intrarift ridge

at a water depth of about 2850 m (Halbach et al., 1998). Ocean floor images of the Sonne Field have been published by Plu¨ger et al. (1990). The MESO zone (Fig. 2) contains three different

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sites that display various indications of hydrothermal activity. These are the Talus Tips Site, the Sonne Field and the Smooth Ground. The hydrothermal deposits investigated with deep-towed camera profiles appear as colored sediments, as basal mounds of accumulated sulfide talus, as chimney relicts, or occasionally as chimney edifices. At the northern edge of a small plateau chimney stumps=edifice often occur along cracks and fissures. This terrain is dominated by basaltic as well as by sulfidic talus and was therefore called the Talus Tips Site. Iron- and copper-dominated massive sulfides were recovered from this site (24 GTV). South of the plateau, in the center of the MESO zone, chimney stumps topping a hydrothermal mound were observed. Mound material is dominated by silicified Fe-oxides, sphalerite and barite (Halbach et al., 1998). This sulfide-bearing jasper breccia together with chimney fragments (again Feand Cu-dominated sulfides) were recovered from this site which is called the Sonne Field (18 GTV). In the most south-southeasterly part of the MESO zone, named Smooth Ground, mineral debris and sediment colorations were observed (Halbach et al., 1996, 1998). The decay of the hydrothermal activity is not only indicated by chimney stumps, ring structures and sulfidic talus, but also by the lack of geochemical anomalies in the water column; no indication of vent biota or even the presence of clam shells were found. In addition to these field observations, massive sulfide samples examined by means of polished sections show low porosity, and recrystallization and replacement processes. This contrasts with fresh, porous sulfides from active smokers.

sulfide samples and four sulfate-silica-rich samples (jasper) recovered from the central part of the Sonne Field (18 GTV) and two oxidized sulfide samples from the western edge of the Sonne Field (19 GTV). From the northern part of the MESO zone (Talus Tips Site) two sampling stations yielded seventeen massive sulfide samples and four ferromanganese crusts (24 GTV), with an additional three silica-rich samples (25 GTV). Polished sections were prepared from hydrothermal precipitates for petrographic examination. In addition XRD measurements were carried out to determine the mineralogical composition of the various samples.

3. Sampling of the MESO zone and description of samples

3.2. Jasper ore

The hydrothermal deposits were sampled with a TV-controlled grab (Halbach et al., 1995, 1996). Videotape recordings of the sea floor topography allowed visual evaluation of the sampling sites. The TV-grab allows the sampling of large quantities of different specimen. Various types of hydrothermal precipitates were recovered and a selection of representative specimen were chosen for age dating: eight

3.1. Massive sulfides Most of the sulfide samples comprise fragments of chimneys, and therefore it was possible to distinguish iron-rich from copper-rich portions. Fe-dominated samples are characterized by porous, fineto coarse-grained pyrite intergrown with marcasite. Massive pyrite is often cataclastic, crushed and replaced by chalcopyrite along cracks and grain boundaries. Cu-rich samples are dominated by massive chalcopyrite containing small recrystallized pyrite cubes. Chalcopyrite is often replaced by bornite, digenite and occasionally by covellite along cracks and rims of crystals. Sphalerite is rarely observed in sulfide samples. Some specimen are sealed by a thin layer of amorphous silica; these samples were largely protected from weathering and kept a quite fresh appearance. In more oxidized samples opal was not observed, and pyrite, as well as marcasite, is often replaced by Fe-hydroxides (Halbach and Muench, 1997).

The jasper ore, which is probably built by impregnation of Fe-hydroxide mud with silica-rich solutions, possibly forms the substrate and base of the sulfide chimneys in this area. The jasper samples show no chimney-like structure or concentric mineral zonation. The assemblage seemed to have been penetrated again later by sulfide-rich solutions because it contains large amounts of sphalerite and disseminated sulfides (pyrite and subordinately chal-

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copyrite); fissures and pores were again filled with barite and amorphous silica (Halbach et al., 1998). 3.3. Oxide samples The earthy material mainly consists of Fe-oxyhydroxides and=or Mn crusts, but subordinately contains oxidized sulfide particles. This observation and the close association with the massive sulfides suggests that the ochreous deposits were derived from weathered sulfide fragments. However, another explanation might be a formation as a primary phase under low-temperature conditions (<100ºC), following the description by Alt et al. (1987) for samples from the East Pacific Rise.

4. Analytical methods and results 4.1. Principle and limitation of the method Based on disequilibrium established in the uranium radioactive series during hydrothermal activity, dating methods were developed and applied for the first time to study the chronology of this activity by Lalou and Brichet (1981). Details of the different age dating methods were given by Lalou and Brichet (1987). Briefly, the method relies on the observation that, when chimneys form by mixing of the hydrothermal fluid with seawater, the high-temperature sulfides include some uranium, essentially from seawater which is at least 30 times richer in U than the hydrothermal fluid (Chen et al., 1986), and no thorium (neither 232 Th nor 230 Th). Thus the growth of 230 Th relative to 234 U may be representative of an age. However, there are limitations to this method, which is useful from some thousands of years to about 300,000 years, when classical α spectrometry is used. This range may possibly be extended to 500,000 years using the Thermo Ionization Mass Spectrometry (TIMS method). TIMS, which was developed by Edwards et al. (1987) has particularly been applied to corals dating (i.e. Bard et al., 1990). Its first application to hydrothermal high-temperature sulfides has been carried out by You and Bickle (1998). The limitations of the method are essentially due to the fact that the samples are aging in the seawater,

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which is relatively uranium-rich and at the contact of a substrate which is relatively rich in thorium. An uranium uptake would result in an apparently too young age, if that uptake occurs a long time after formation relative to the resolution of the 230 Th=234 U method (which is not better than a thousand years). A relationship between ages and uranium content, the samples richer in uranium having younger apparent ages, may be indicative of such a contamination and a cross-checking with other methods must be used, or the samples must be rejected. Because the studied samples are sulfides, a loss of uranium which would result in an apparent aging of the sample is geochemically difficult to consider due to the reducing conditions. If the recrystallization process took place during the first few thousand years, the method would not be able to quantify this recrystallization, and only a large scattering of the ages may appear. An uptake of 230 Th resulting in a too old age is more difficult to establish, but, if 230 Th enters the sample it is generally accompanied by 232 Th, and therefore a sample containing 232 Th must be rejected. 4.2. Validation of the method Validation of the method was established progressively as our work proceeded in various areas. Each time measurements are made on different parts or a sample, within statistical errors the same results are obtained. In the TAG area, samples have been found with different U contents in different parts of the same sample; even where the difference in U content was high (e.g. 95 ppm and 5 ppm), the same age was obtained. We also had the opportunity, by using different dating methods on the same sample, to test the internal consistency of our results in the TAG area: some samples of sulfide were found with their cavities lined with aragonite that gave 14 C ages that were slightly younger than their associated sulfide ages (Lalou et al., 1990). More recently, deep-seated sulfide samples from the cores drilled in the TAG active mound show the same age using the 230 Th=234 U and the 226 Ra=234 U ratio. Because the geochemistry of thorium and radium are very different we may prove that the samples act as a closed system (Lalou et al., 1998). Moreover, samples at intermediate depth in the same TAG-drilled core, dated by other authors by using another measuring method (TIMS), give

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results that are coherent with ours (M. Bickle, pers. commun., 1997). Finally, it is necessary that the ages accord with reliable field observations, when such observations are available. 4.3. Sample analysis The samples were carefully cleaned for any external oxidized or contaminated part and crushed. On this crushed fraction, the mineralogical study for the main constituents was done by X-ray diffraction, because of the mineralogical heterogeneity of the whole sample. To adapt the chemical treatment, it is necessary to obtain the mineralogical constituents of the subsample itself, which is then dissolved in a nitric=chlorhydric solution with addition of a spike (solution of 232 U–228 Th at equilibrium) with a known activity; the residue, if any, is dissolved in a mixture of nitric–perchloric–hydrofluoric acids. After successive precipitations, purifications, and separations of U and Th on resins, the final purification was obtained by extraction in TTA (thenoyltrifluoroacetone) solution and the separated U and Th fractions were deposited on aluminum films and counted by α spectrometry. The calculation of the age of the sample must take into consideration that [230 Th=234 U] increases with age, and [234 U=238 U] decreases with age. This leads to the equation (Kaufman and Broecker, 1965): ð230 Ł ð Ł Th=234 U D 238 U=234 U .1 exp. ½0 t// ð238 234 ŁÐ C 1 U= U ð ½0 =.½0

½4 /.1

exp.½0

½4 /t/

where ½0 and ½4 represent the decay constants of thorium 230 and uranium 234, respectively, and 238 U, 234 U and 230 Th are the activity measured in a sample of age t. To solve this equation, a computer program is given in Ku (1982) which, from the counting rate in each peak and the background associated gives the age of the sample, the count rate errors at the 1¦ level, the upper and lower limits of the age, and the initial ratio (234 U=238 U)0 . Until now, these methods have successfully been applied to pure sulfide samples (Lalou et al., 1990, 1993, 1995, 1996) and to thick manganese oxide layers (Lalou et al., 1986). In this study there are, associated with sulfides, different types of precipitates,

especially jasper samples, which we have tested for their chronological value, as in the case of hydrothermal silica chimneys (Herzig et al., 1988; Stu¨ben et al., 1994). We were especially interested in knowing if the jaspers can provide an indication of the variability in time of the hydrothermal system. 4.4. Results for sulfide samples Tables 1 and 2 present the results obtained for station 18 GTV and 24 GTV, respectively. With the exception of sample 23 from station 18 GTV, all samples have a very low uranium content (less than 1.3 ppm for samples from station 18 GTV and generally less than 0.2 ppm for samples from station 24 GTV). The samples are made of pyrite or a mixture of pyrite and marcasite (at station 18 GTV) and pyrite and chalcopyrite (at station 24 GTV). There is no relationship between mineralogy and U content; there is no unequivocal relationship between age and U content. 232 Th is always very low, below the detection limit (within 2¦ /. Therefore we can be confident about the calculated ages for the different samples from the two fields. 4.4.1. Chronology of the sulfides in the Sonne Field In Fig. 3 we reported the ages found for the eight sulfide samples from station 18 GTV. These range from 10 to 20 ka. These ages may be tentatively grouped around two periods of activity, the first at 18 š 2 ka followed by a reactivation of the system 12:5 š 1:5 ka ago. An abnormally high U content (16.8 ppm) is found in a chalcopyrite sample 23 which nevertheless presents the same age as sample 102. This last sample is composed of marcasite and pyrite and contains only 0.37 ppm of uranium, showing probably that the uranium enrichment of sample 23 is concomitant to the formation of the sample. 4.4.2. Chronology of the sulfides at the Talus Tips Site With the exception of sample 2-36 all samples fall into two age classes, one around 52 š 5 ka, and the second around 20 ka (Fig. 4). Because the dispersion is large in the younger class, we attempted (Fig. 5) for this last event to decide between only one event with a relatively long duration (20 š 5 ka) or two

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Table 1 Radiochemical results for station 18 GTV sulfide samples (Sonne Field) Site No. GTV 6 GTV 19 GTV 23 GTV 76 GTV 77 GTV 78 GTV 80 GTV 102

238 U

234 U

(ppm)

(dpm=g)

0:15 š 0:01 0:51 š 0:02 16:82 š 1:04 0:95 š 0:05 1:29 š 0:05 0:52 š 0:02 0:29 š 0:01 0:37 š 0:02

0:14 š 0:01 0:40 š 0:02 13:88 š 0:85 0:76 š 0:04 1:05 š 0:04 0:46 š 0:02 0:24 š 0:01 0:31 š 0:01

234 U=238 U

1:220 š 0:061 1:072 š 0:056 1:121 š 0:046 1:089 š 0:036 1:114 š 0:041 1:215 š 0:030 1:151 š 0:041 1:154 š 0:044

232 Th

230 Th

(dpm=g)

(dpm=g)

<0.001 <0.005 <0.031 <0.004 <0.009 <0.001 <0.002 <0.003

0:022 š 0:001 0:049 š 0:005 1:44 š 0:05 0:13 š 0:01 0:14 š 0:01 0:05 š 0:002 0:039 š 0:003 0:032 š 0:003

230 Th=234 U

Age (ka)

Mineralogy a

0:16 š 0:01 0:12 š 0:01 0:10 š 0:01 0:17 š 0:01 0:14 š 0:01 0:11 š 0:01 0:16 š 0:01 0:10 š 0:01

18:8 š 1:6 13:9 š 1:7 11:9 š 0:9 20:4 š 1:8 16:2 š 1:4 12:5 š 0:7 18:8 š 1:6 11:8 š 1:5

P, M (S) P, M (C, A?) C P (?) P (S?) P (C C ?) P (?) M (P)

a P D pyrite, M D marcasite, C D chalcopyrite, A D atacamite, S D sphalerite. The quoted uncertainty is 1 standard deviation.

Table 2 Radiochemical results for sulfide samples from station 24 GTV (Talus Tips Site) Site No. GTV 2-2 GTV 2-5 GTV 2-6 GTV 2-36 GTV 2-38 GTV 3-10 GTV 3-17 GTV 3-20 GTV 4-2 GTV 4-34 GTV 4-46 GTV 5-1 GTV 5-2 GTV U GTV 5-4 GTV 5-7 GTV 5-12

238 U

234 U

(ppm)

(dpm=g)

0:18 š 0:01 0:041 š 0:002 0:13 š 0:01 0:070 š 0:004 0:045 š 0:003 0:054 š 0:003 0:12 š 0:01 0:13 š 0:01 0:14 š 0:01 0:12 š 0:01 0:085 š 0:004 0:09 š 0:01 0:17 š 0:01 0:07 š 0:01 0:05 š 0:00 0:10 š 0:01 0:10 š 0:01

0:16 š 0:01 0:033 š 0:001 0:11 š 0:01 0:056 š 0:003 0:038 š 0:002 0:045 š 0:002 0:11 š 0:01 0:10 š 0:01 0:11 š 0:01 0:101 š 0:004 0:077 š 0:003 0:08 š 0:01 0:14 š 0:01 0:055 š 0:004 0:045 š 0:002 0:079 š 0:004 0:09 š 0:01

234 U=238 U

232 Th

230 Th

(dpm=g)

(dpm=g)

1:185 š 0:049 1:082 š 0:061 1:179 š 0:043 1:095 š 0:057 1:149 š 0:088 1:129 š 0:066 1:204 š 0:106 1:042 š 0:059 1:114 š 0:051 1:109 š 0:049 1:238 š 0:068 1:122 š 0:130 1:106 š 0:054 1:152 š 0:117 1:158 š 0:086 1:058 š 0:068 1:265 š 0:085

<0.0008 <0.0013 <0.0007 <0.0011 <0.0007 <0.0006 <0.0041 <0.0012 <0.0011 <0.0013 <0.0009 <0.0036 <0.0015 <0.0008 <0.0028 <0.0010 <0.0016

0:022 š 0:001 0:012 š 0:001 0:042 š 0:002 0:042 š 0:002 0:007 š 0:001 0:018 š 0:001 0:025 š 0:003 0:015 š 0:001 0:016 š 0:001 0:042 š 0:002 0:016 š 0:001 0:015 š 0:003 0:017 š 0:002 0:010 š 0:001 0:013 š 0:002 0:014 š 0:001 0:018 š 0:002

230 Th=234 U

Age (ka)

Mineralogy a

0:14 š 0:01 0:37 š 0:04 0:38 š 0:02 0:74 š 0:06 0:19 š 0:02 0:39 š 0:03 0:23 š 0:03 0:15 š 0:01 0:15 š 0:01 0:41 š 0:03 0:20 š 0:02 0:20 š 0:04 0:12 š 0:01 0:18 š 0:02 0:29 š 0:05 0:17 š 0:02 0:20 š 0:02

16:8 š 1:2 48:9 š 6:4 51:0 š 3:80 140:0C23 19 22:7 š 2:70 53:5 š 4:4 28:5 š 4:50 17:5 š 1:80 17:2 š 1:60 57:2 š 4:50 24:4 š 2:00 24:1 š 5:00 14:7 š 1:56 21:4 š 2:70 36:4 š 6:80 20:5 š 2:00 23:3 š 2:90

P P P P P (C, M?) P P (S, A, C) P (C?) C (P, M) P (C) P C (P) C, P (S?) fond C (P) fond C (G) P (C) P, (C) (?)

a P D pyrite, M D marcasite, C D chalcopyrite, A D atacamite, S D sphalerite, G D geerite. The quoted uncertainty is 1 standard deviation.

shorter events. Fig. 5 shows that two events at 22:7 š 3 ka and 16 š 2 ka can possibly be distinguished (at 1¦ uncertainty as generally used, and for a restricted population of, respectively, 6 and 4 samples), but two samples (3.17) and (5.4) cannot be included in this scheme. An apparent succession of hydrothermal events at the Talus Tips Site may be summarized as follows: a first high-temperature event occurred at 52 š 5 ka, followed by a reactivation at 23 š 3 ka and another more recent event at 16 š 2 ka. An intermediate event, around 32 ka may be assumed,

but this is only supported by two samples. Sample 2.36 which is older than all others, may demonstrate a high-temperature event around 140 ka, but this has not been corroborated by other measurements. 4.5. Results for miscellaneous samples Different types of miscellaneous samples that are either directly or indirectly of hydrothermal origin have been sampled in the Sonne Field and at the Talus Tips Site. The conditions of formation of those

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Fig. 3. Ages for sulfide samples from station 18 GTV showing the possibility of two episodes. Samples are in numerical order, ages are given at the 1¦ confidence level.

Fig. 4. Ages for sulfide samples from station 24 GTV showing the possibility of at least two distinct episodes. Samples are in numerical order, ages are given at the 1¦ confidence level.

GTV (17 and 17b) are oxidized sulfides or possibly low-temperature hydrothermal material. samples (instantaneous or continuous), and consequently the chemistry of uranium and thorium, are however not well known. They can however provide information on the variability in time of the environment throughout the lifetime of the fields, and as some constraints in time have been established previously by the ages obtained on sulfides, they have been analyzed and ‘ages’ have been calculated when possible to test the value of these miscellaneous samples as indicators of time of mineral formation. 4.5.1. Sonne Field miscellaneous samples Two types of samples other than sulfides have been found in the Sonne Field at stations 18 GTV and 19 GTV (Fig. 2). The XRD analysis of samples 144a and 144b and JIV and JVI from 18 GTV indicate jasper. Samples 144a and 144b are yellow-red in color, JIV and JVI are darker. The samples from 19

4.5.1.1 Jasper samples (Table 3). Jasper is described as due to silica-rich solutions permeating a low-temperature Fe-hydroxide mud (Halbach et al., 1998), and might represent the older pre-chimney mineralization of colloidal origin (Table 3). Barite, on the other hand, is considered as representing the final stage of the hydrothermal process (Halbach et al., 1995, 1998). Jasper samples 144a and 144b contain only few barite crystals, whereas jasper JIV and JVI are very BaSO4 -rich. Neither contains 232 Th or an excess of 230 Th, which may imply that they have not been exposed for a long period as loose sediment able to include, before being cemented, some of the detrital sedimentation (with its 232 Th) or part of the regular oceanic rain of 230 Th. This last feature prompts us to believe that the calculated ‘ages’ may be taken into consideration. The ages so obtained (13:4 š 0:4 ka, 10:9 š 0:5 ka, 9:3 š 0:65 ka,

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istics succeed each other in a time span that is too short relative to the time resolution of the method (at least 1000 years) to be clearly separated. Only the last episode at station 18 GTV has been accompanied by a siliceous event.

Fig. 5. Details of the youngest episode indicated in Fig. 4 showing that this episode may be divided into two events, one around 23 ka and the other around 16 ka. Samples are in numerical order, ages are given at the 1¦ confidence level.

and 10:9 š 1:1 ka) fall just in the time span of the most recent sulfide-forming episode registered in the Sonne Field. We conclude therefore that these ages may be real ages; and that diverse hydrothermal episodes with different physico-chemical character-

4.5.1.2 Oxidized sulfides (Table 4). The two analyzed samples come from station 19 GTV, on the western periphery of the Sonne Field (Fig. 2). These samples are composed of iron silicate and amorphous silica. 19 GTV 17 is yellow and 17b brown yellow, due to the presence of MnO2 . Theoretically, such samples are not good candidates for chronological purposes because they are frequently very enriched in uranium and thorium, which is scavenged continuously from the seawater. Samples 17 and 17b do not contain 232 Th and their U concentrations are not very high. Even though these U concentrations are different the two samples present ‘ages’ that are in relatively good agreement (19 š 1:8 and 24:7 š 2:5 ka). If we believe these ages this indicates that probably, as deduced from the field observations, and in contrast to the jasper samples, they represent an early stage, low-temperature precipitate (Halbach et al., 1995) which would have formed just before the oldest event defined by the sulfides of the Sonne Field. A more complete study of the conditions of formation (direct and rapid precipitation from a saturated solution or slow modification of a pre-existing deposit) would be necessary before accepting these ages, however. 4.5.2. Talus Tips Site miscellaneous samples The miscellaneous samples from station 24 GTV and 25 GTV present a large age span (Table 5), including samples in which 230 Th is in excess relative to 234 U (essentially Mn oxide samples). These sam-

Table 3 Radiochemical results for ‘jasper’ samples from station 18 GTV (Sonne Field) Site No. GTV 144a GTV 144b GTV JIV GTV JVI

238 U

234 U

(ppm)

(dpm=g)

4:4 š 0:1 6:6 š 0:2 3:4 š 0:2 1:7 š 0:1

3:7 š 0:1 5:7 š 0:1 2:9 š 0:1 1:4 š 0:1

234 U=238 U

1:143 š 0:012 1:142 š 0:017 1:143 š 0:031 1:156 š 0:044

a T D tridymite; Cr D cristobalite. The quoted uncertainty is 1 standard deviation.

232 Th

230 Th

(dpm=g)

(dpm=g)

<0.0038 <0.0096 <0.0057 <0.0150

0:43 š 0:01 0:55 š 0:02 0:24 š 0:01 0:14 š 0:01

230 Th=234 U

Age (ka)

Mineralogy a

0:117 š 0:003 0:096 š 0:005 0:082 š 0:006 0:096 š 0:009

13:4 š 0:40 10:9 š 0:50 9:3 š 0:65 10:9 š 1:1

Jasper Jasper Jasper Jasper

(T, Cr?) (T, Cr) (C barite) (C barite)

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Table 4 Radiochemical results for oxidized sulfides from station 19 GTV (Sonne Field) Site No. GTV 17 GTV 17b

238 U

234 U

(ppm)

(dpm=g)

4:8 š 0:2 2:1 š 0:1

4:0 š 0:2 1:7 š 0:1

234 U=238 U

1:138 š 0:047 1:128 š 0:037

232 Th

230 Th

(dpm=g)

(dpm=g)

<0.032 <0.025

0:65 š 0:05 0:35 š 0:03

230 Th=234 U

Age (ka)

Mineralogy

0:16 š 0:01 0:21 š 0:02

19:0 š 1:8 24:7 š 2:5

oxid. sulf. oxid. sulf.

The quoted uncertainty is 1 standard deviation.

Table 5 Radiochemical results for miscellaneous samples from Talus Tips Site 24 GTV and 25 GTV Site No.

238 U

234 U

(ppm)

(dpm=g)

234 U=238 U

232 Th

230 Th

(dpm=g)

(dpm=g)

230 Th=234 U

Age (ka)

24 GTV GTV 2-29 7:0 š 0:2 GTV 2-42 9:3 š 0:2 GTV 7-1 2:5 š 0:1 GTV 7-2 9:8 š 0:5 GTV 7-2-1 10:2 š 0:3

5:99 š 0:20 7:98 š 0:18 2:05 š 0:08 8:29 š 0:45 8:78 š 0:27

1:160 š 0:011 1:168 š 0:008 1:124 š 0:037 1:144 š 0:014 1:168 š 0:012

<0.0032 <0.0023 <0.0023 <0.0018 <0.0041

0:36 š 0:02 0:57 š 0:01 1:87 š 0:09 1:82 š 0:07 0:31 š 0:02

0:060 š 0:003 0:072 š 0:002 0:91 š 0:06 0:22 š 0:02 0:036 š 0:002

25 GTV GTV 7 GTV 17a GTV 23a GTV 23b GTV 17b GTV 17c

0:87 š 0:09 3:92 š 0:07 0:11 š 0:01 0:18 š 0:01 0:95 š 0:05 0:73 š 0:02

1:088 š 0:009 1:147 š 0:007 1:105 š 0:111 1:213 š 0:114 1:163 š 0:062 1:112 š 0:038

<0.0052 <0.0026 <0.0034 <0.006 0.05 š 0.03 <0.008

0:18 š 0:01 0:34 š 0:01 0:12 š 0:01 0:17 š 0:01 2:55 š 0:11 0:94 š 0:04

0:21 š 0:02 25.1 š 3.25 0:087 š 0:004 9.9 š 0.4 1:11 š 0:11 >300 0:95 š 0:10 254C168 67 2:70 š 0:19 n.d. 1:29 š 0:07 n.d.

1:1 š 0:1 4:6 š 0:1 0:13 š 0:01 0:2 š 0:0 1:1 š 0:1 0:9 š 0:0

6.7 š 0.37 8.0 š 0.30 234.8C61 40 26.8 š 2.00 3.9 š 0.24

Mineralogy

oxides sulf. ox. Fe–Mn crust Fe–Mn crust Fe–Mn crust

amorphous silica smectite? amorphous silica amorphous silica Fe–Mn crust Fe–Mn crust

The quoted uncertainty is 1 standard deviation. n.d. D excess of Th-230.

ples are enriched in uranium relative to the sulfide samples, and no general trend can be established. We therefore do not use these results, which confirm the impossibility of using oxidized sulfides as indicators of time of mineral formation.

5. Conclusions The MESO zone is a mature sulfide deposit on the Central Indian Ridge. Field observations indicate chimney stumps and sulfidic talus, and a thin sediment coverage (approximately 2–4 cm) on sulfide edifices=relicts. No indications of active venting such as geochemical anomalies or vent biota were observed in any part of the area (Halbach et al., 1998). The principal aim of our study was an attempt to establish the chronology of the high-temperature events in the two hydrothermal fields of the MESO

zone using sulfide samples. Another objective was to obtain more information about the variability of the hydrothermal system, using different hydrothermal precipitates such as jasper or oxidized samples which reflect the low-temperature hydrothermal system (<100ºC). Before interpreting our results, however, we must emphasize that the relatively small number of samples studied precludes a statistical treatment, and the mean values we calculated to detect a possible periodicity in the hydrothermal activity of the Sonne Field must be considered only as trends. Moreover, as is generally the case in these studies, the difficulties of sampling, either by TV-grab from a research vessel or from deep submersible vehicles, raises questions of whether the sampling is representative. With a TV-grab sampler, a large quantity of mixed material is obtained over a surface of about 3.5 m2 (representing about 300 to 500 kg). With deep submersible vehicles in contrast,

C. Lalou et al. / Marine Geology 149 (1998) 243–254

253

Fig. 6. Summary of the results of the chronological study. The shaded areas indicate the periods of activity for: (A) the Talus Tips Site, (B) the Sonne Field, and (C) the Sonne Field jaspers.

point sampling is attained and interrelations between diverse samples may be observed, but the field of action and the visibility are restricted. In Fig. 6, we summarize the different periods of activity established using the ages of the sulfide samples of the two relict hydrothermal fields within the MESO zone (Figs. 3–5). This enables us to present the following chronological scale. Within the limits of the representativity of the sampling (only one station in the sulfide-rich area for each field), the hightemperature hydrothermal activity possibly began 140 ka ago at the Talus Tips Site (although note that only one sample of this age has been detected). After a period of quiescence, around 52.5 ka ago (four samples) hydrothermalism was reactivated again in the same area for a period of several thousand years. Hydrothermal activity ceased at the Talus Tips Site for at least 10 ka, if we take into account the two or three samples around 30 ka, or, for more than 20 ka if we consider for our interpretation only the nine samples dated around 22 ka. Another interruption for several thousand years occurred at the Talus Tips Site, whereas during this period, high-temperature activity is present southeastward in the Sonne Field (18 š 2 ka; four samples). The last high-temperature hydrothermal event at the Talus Tips Site has been

dated at 16 š 2 ka (four samples) and terminates the activity in the northern part of the MESO zone. The formation of sulfides in the Sonne Field was interrupted at about 16 ka and reactivated at 12:5 š 1:5 ka for a period of about 3 ka (four samples). The jasper formation in the Sonne Field has been dated at around 11 ka (four samples). This is the youngest hydrothermal event determined in the MESO zone and may mark the end of hydrothermal activity. The jasper samples seem to give good results, which implies that they are pure, quasi-instantaneous precipitations. Iron and manganese oxides do not give such good results. The above chronological sequence, with episodic and alternating hydrothermal activity in topographically separated fields, is not as clear as that found on the Mid-Atlantic Ridge at TAG (Lalou et al., 1995) and 14º50 (Lalou et al., 1996), although this is probably due to the restricted number of samples analyzed. Assuming that the hydrothermal activity affects the two separated fields alternately, it is possible that, for the last 30 ka, activity has been continuous, the conduits being alternately blocked or open, dispatching the fluid towards one field or the other. Our experience in other fields, however, leads us to favor the hypothesis of an episodic phenomenon.

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Acknowledgements U.M. and P.H. gratefully acknowledge the Ministerium fu¨r Forschung und Technologie (grant 03G0092A) and the Deutsche Forschungsgemeinschaft (grant Ha563=2) for financial support. C.L. and J.L.R. acknowledge with thanks CNRS-INSU and CEA for financial support. We wish to thank Dr. M. van Gerven for carefully reading our manuscript, Drs. Chamley, McMurtry and Stoffers for constructive reviews of the paper, and Dr. Bickle who gave us permission to use unpublished results. This is CFR contribution N. 2002. References Alt, J.C., Lonsdale, P., Haymon, R., Muehlenbachs, K., 1987. Hydrothermal sulfide and oxide deposits on seamounts near 21ºN, East Pacific Rise. Geol. Soc. Am. Bull. 98, 157–168. Bard, E., Hamelin, B., Fairbanks, R.G., 1990. U=Th ages obtained by mass spectrometry in corals from Barbados. Sea level during the past 130,000 years. Nature 346, 456–458. Briais, A., 1995. Structural analysis of the segmentation of the Central Indian Ridge between 20º300 S and 25º300 S (Rodriguez Triple Junction). Mar. Geophys. Res. 17, 431–467. Cann, J.R., Winter, C.K., Pritchard, R.G., 1977. A hydrothermal deposit from the sea floor of the Gulf of Aden. Mineral. Mag. 41, 193–199. Chen, J.H., Wasserburg, G.J., Von Damm, K.L., Edmond, J.M., 1986. The U–Th–Pb systematic in hot springs of the East Pacific Rise at 21ºN and Guaymas basin. Geochim. Cosmochim. Acta 50, 2467–2479. Edwards, R.L., Chen, J.H., Wasserburg, G.J., 1987. 238 U– 234 U– 230 Th– 232 Th systematics and the precise measurement of time over the past 500,000 years. Earth Planet. Sci. Lett. 81, 175– 192. Halbach, P., Muench, U., 1997. Mineralogical and geochemical investigation of massive sulfide deposits from the Central Indian Ridge. In: Papunen (Ed.), Mineral Deposits: Research and Exploration — where do they meet. Proc. 4th Biennal SGA Meeting, Turku=Finland, 11–13 August. Balkema, Rotterdam, pp. 357–360. Halbach, P., Blum, N., Plu¨ger, W., van Gerven, M., Erzinger, J., SO92 Shipboard Scientific Party, 1995. The Sonne Field, first massive sulfides in the Indian Ocean. InterRidge News 4 (2), 12–15. Halbach, P., Blum, N., Mu¨nch, U., Plu¨ger, W., Kuhn, T., 1996. The Sonne Sulphide Field is not alone in the Indian Ocean. Bridge Newslett. 10, 51–54. Halbach, P., Blum, N., Mu¨nch, U., Plu¨ger, W., Garbe-Scho¨nberg, D., Zimmer, M., 1998. Formation and decay of a modern massive sulfide deposit in the Indian Ocean. Miner. Deposita 33, 302–309. Herzig, P.M., Plu¨ger, W.L., 1988. Exploration, for hydrothermal

activity near the Rodriguez triple junction, Indian Ocean. Can. Mineral. 26, 721–736. Herzig, P.M., Becker, K.P., Stoffers, P., Ba¨cker, H., Blum, N., 1988. Hydrothermal silica chimney fields in the Galapagos Spreading Center at 86ºW. Earth Planet. Sci. Lett. 89, 261–272. Kaufman, A., Broecker, W., 1965. Comparison of Th-230 and C-14 ages for carbonate materials from lakes Lahontan and Bonneville. J. Geophys. Res. 70 (16), 4039–4054. Ku, T.L., 1982. Appendix A: Derivation of the expressions for the 234 U=238 U and 230 Th=234 U activity ratio age equation. In: Ivanovich, Harmon (Eds.), Uranium Series Disequilibrium, Applications to Environmental Problems. Oxford Science, Oxford, pp. 512–519. Lalou, C., Brichet, E., 1981. Possibilite´ de datation des de´poˆts de sulfures me´talliques hydrothermaux sous marins par les descendants a` vie courte de l’uranium et du thorium. C. R. Acad. Sci. Ser. II 293, 821–826. Lalou, C., Brichet, E., 1987. On the isotopic chronology of submarine hydrothermal deposits. Chem. Geol. (Isot. Geosci. Sect.) 65, 197–207. Lalou, C., Thompson, G., Rona, P.A., Brichet, E., Jehanno, C., 1986. Chronology of selected hydrothermal oxide deposits from the transatlantic ‘TAG’ area, Mid Atlantic ridge, 26ºN. Geochim. Cosmochim. Acta 50, 1137–1143. Lalou, C., Thompson, G., Arnold, M., Brichet, E., Druffel, E., Rona, P.A., 1990. Geochronology of TAG and Snakepit hydrothermal fields, Mid Atlantic Ridge: witness to a long and complex hydrothermal history. Earth Planet. Sci. Lett. 97, 113–128. Lalou, C., Reyss, J.L., Brichet, E., Arnold, M., Thompson, G., Fouquet, Y., Rona, P.A., 1993. New age for Mid-Atlantic hydrothermal sites: TAG and Snakepit chronology revisited. J. Geophys. Res. 98, 9705–9713. Lalou, C., Reyss, J.L., Brichet, E., Rona, P.A., Thompson, G., 1995. Hydrothermal activity on a 105 year scale at a slow spreading ridge, TAG hydrothermal field, Mid Atlantic Ridge, 26ºN. J. Geophys. Res. 100, 17855–17862. Lalou, C., Reyss, J.L., Brichet, E., Krasnov, S., Stepanova, T., Cherkasev, G., Makov, V., 1996. Initial chronology of a recently discovered hydrothermal field at 14º45N, Mid Atlantic Ridge. Earth Planet. Sci. Lett. 144, 483–490. Lalou, C., Reyss, J.L., Brichet, E., 1998. Age of sub-bottom sulfide samples at the TAG active mound. Proc. ODP Sci. Results 158, 111–117. Plu¨ger, W.L., Herzig, P.M., Becker, K.P., Deissmann, G., Scho¨ps, D., Lange, J., Jenisch, A., Ladage, S., Richnow, H.H., Schulze, T., Michaelis, W., 1990. Discovery of hydrothermal fields at the Central Indian Ridge. Mar. Min. 9, 73–86. Rona, P.A., Bostro¨m, K.L., Widenfalk, E.G., Mallette, M., Melsen, W.B., 1981. Preliminary reconnaissance of the Carlsberg Ridge, Northwestern Indian Ocean, for hydrothermal mineralizations. Eos 62 (45), 914. Stu¨ben, D., Eddine Taibi, N., McMurtry, G.M., Scholten, J., Stoffers, P., Zhang, D., 1994. Growth history of a hydrothermal silica chimney from the Mariana backarc spreading center (southwest Pacific, 18º130 N). Chem. Geol. 113, 273–296. You, C.F., Bickle, M.J., 1998. Evolution of an active sea floor massive sulphide deposit. Nature, in press.