Sulfur mineralogy and geochemistry of serpentinites and gabbros of the Atlantis Massif (IODP Site U1309)

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Geochimica et Cosmochimica Acta 72 (2008) 5111–5127 www.elsevier.com/locate/gca

Sulfur mineralogy and geochemistry of serpentinites and gabbros of the Atlantis Massif (IODP Site U1309) Ade´lie Delacour a,*,1, Gretchen L. Fru¨h-Green a, Stefano M. Bernasconi b a

Institute for Mineralogy and Petrology, ETH Zurich, CH-8092 Zurich, Switzerland b Geological Institute, ETH Zurich, CH-8092 Zurich, Switzerland

Received 14 June 2008; accepted in revised form 9 July 2008; available online 6 August 2008

Abstract In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150–200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high d34Ssulfide values and low d34Ssulfate values, which reflect a multistage history primarily controlled by seawater–gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity. Ó 2008 Elsevier Ltd. All rights reserved.

1. INTRODUCTION In mid-ocean ridge environments, interaction of seawater with the oceanic crust (basalts, gabbros and uplifted per*

Corresponding author. Fax: +33 1 44 27 99 69. E-mail address: [email protected] (A. Delacour). 1 Present address: Laboratoire de Ge´osciences Marines, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris Cedex 5, France. 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.07.018

idotites) is a fundamental process in the transfer of heat (e.g., Stein and Stein, 1994) and elements such as silicon, magnesium, boron or sulfur between the mantle and the oceans (e.g., Thompson and Melson, 1970; Edmond et al., 1979; Alt, 1995; Snow and Dick, 1995; Boschi et al., 2008). During fluid–rock interaction, oceanic basement rocks are affected by oxidation and reduction processes leading to variations in the speciation, concentrations and isotopic compositions of sulfur. These changes have consequences for the global sulfur cycle in

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the oceans (Alt, 1994; Alt and Shanks, 2003) and for elemental fluxes during dehydration of the oceanic crust in subduction zones, and in turn influence the chemical variability of the overlying mantle wedge (Chaussidon et al., 1987; Harmon et al., 1987) and the compositions of rocks formed in island arcs (Ueda and Sakai, 1984; Woodhead et al., 1987). Although a number of studies have documented variations in sulfur isotope compositions in hydrothermal fluids, basaltic rocks and hydrothermal deposits in black smoker systems (e.g., Kerridge et al., 1983; Shanks and Seyfried, 1987; Alt et al., 1989; Knott et al., 1995; Bogdanov et al., 1997; Gemmel and Sharpe, 1998; Knott et al., 1998) and in serpentinized ultramafic rocks in various environments (Alt and Shanks, 1998, 2003, 2006; Alt et al., 2007; Delacour et al., 2008), less is known of the distribution and isotopic compositions of sulfur in oceanic gabbros (Alt and Anderson, 1991; Natland et al., 1991; Alt, 1994; Puchelt et al., 1996; Alt et al., 2007). Recent drilling at Site U1309 (Holes 1309D and 1309B) during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) at the central dome of the Atlantis Massif provided an important opportunity to study variations in sulfur chemistry in in-situ lower crustal sequences of an oceanic core complex (OCC). During these two consecutive expeditions, 1.4 km of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Strontium and neodymium isotope studies indicate that the central dome has undergone relatively limited seawater–rock interaction and this portion of the massif remains relatively unaffected by hydrothermal circulation related to the Lost City Hydrothermal Field (LCHF), which is located 5 km to the south (Delacour et al., 2008). Therefore, sulfur geochemical studies of this section of gabbroic rocks and minor serpentinites provide constraints on the sulfur speciation, contents and isotope compositions of uplifted portions of oceanic crust and their influence on the sulfur cycle over time. In this paper, we characterize the sulfur mineralogy and geochemistry of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks drilled at Site U1309. These results are used to evaluate the alteration processes and the redox conditions that prevail at the central dome of the Atlantis Massif. We compare these data with results of a companion study of the southern wall (Delacour et al., 2005, 2008) to highlight the differences between the two parts of the massif and the influence of the LCHF on the sulfur geochemistry of the basement. 2. THE ATLANTIS MASSIF Located along the Mid-Atlantic Ridge (MAR) at 30°N, the Atlantis Massif (AM) forms the inside corner of the intersection of the ridge axis and the Atlantis Transform Fault (Fig. 1). This 1.5–2 Myr-old, dome-like massif is considered to be an oceanic core complex and was the target of many research cruises and laboratory investigations over the last ten years (e.g., Cann et al., 1997; Blackman et al., 1998, 2002; Canales et al., 2004; Schroeder and John, 2004; Kelley et al., 2005; Blackman et al., 2006; Boschi et al., 2006; Karson et al., 2006; Ildefonse et al., 2007; Boschi et al., 2008).

Fig. 1. Bathymetric map showing the morphology of the Atlantis Massif at the intersection of the Mid-Atlantic Ridge (MAR) and the Atlantis Transform Fault (ATF). Based on morphologic and lithologic criteria, this 1.5–2 Myr-old massif is divided into three domains: (1) a peridotite-dominated southern ridge; (2) a gabbroic central dome; and (3) a volcanic eastern block. Also shown are the IODP drilling Site U1309, the deep and shallow-penetration holes, and the seismic reflection and refraction lines (Collins et al., 2001; Canales et al., 2004). IODP Hole 1309D (1415.5 mbsf) is the main hole drilled during IODP Expeditions 304 and 305.

Three regions are distinguished: a peridotite-dominated southern ridge, a gabbroic central dome, and a volcanic eastern block (Fig. 1). Corrugations and striations on top of the central dome have been interpreted as surface expressions of a long-lived low-angle detachment fault that led to the uplift and exposure of the massif on the ocean floor (Cann et al., 1997; Blackman et al., 1998, 2002, 2006; Boschi et al., 2006; Karson et al., 2006). Seismic refraction data (Collins et al., 2001) and multichannel seismic data (Canales et al., 2004) over the central dome (Fig. 1) documented high seismic velocities (>8 km.s1) and a strong reflector within several hundred meters below the seafloor. These data have been interpreted as indicating the presence of relatively unaltered ultramafic rocks and/or high-density mafic rocks in the core of the massif. Mafic and ultramafic rocks were recovered by dredging and submersible sampling at the central dome and southern wall of the AM (Cann et al., 1997; Blackman et al., 2002; Schroeder and John, 2004; Kelley et al., 2005; Boschi et al., 2006; Karson et al., 2006). Drilling at the central dome was thus aimed at recovering these sequences in-situ and to investigate processes of crustal accretion, deformation and alteration during formation and emplacement of an OCC. 2.1. The central Dome IODP Site U1309 is located between 1653 and 1885 m below sealevel (mbsl) on the crest of the central dome

Sulfur mineralogy and geochemistry of IODP Hole 1309D

(Fig. 1) and comprises two main holes, Hole 1309D and Hole 1309B (101.8 m depth), and five shallow-penetration holes (<10 m depth, Hole 1309A and Holes 1309E-H; Fig. 1). Hole 1309D was drilled to a depth of 1415 mbsf (meter below sea floor) (average core recovery 75%; Expedition Scientific Party, 2005a,b; Blackman et al., 2006) and recovered predominantly gabbroic rocks (91.4% of the total recovery) with minor intercalated ultramafic lithologies (5.7% of the total recovery). Tholeiitic basalts and diabases, forming 2.9% of the total recovery, are more common in the upper 130 m of Hole 1309D and occur locally throughout the hole where they intrude other lithologies. The gabbroic rocks can be subdivided in gabbro (55.7% including gabbronorite), olivine gabbro/troctolitic gabbro (25.5%), troctolite (2.7%), and oxide gabbro (7.0%). Olivine-rich troctolites (5.4%), characterized by up to 70 modal % olivine and poikilitic textures, are the dominant ultramafic lithology and are concentrated in a 140 m-thick layer between 1095 and 1236 mbsf in Hole 1309D. Variably serpentinized peridotites with relict mantle mineralogies and textures are rare (<0.3%) and are localized in the upper part of the cores. The basement rocks recovered at Site U1309 are interpreted to represent a lithospheric section in which numerous gabbroic melts intruded into mantle peridotites and resulted in complex melt-rock reactions and a thick gabbroic section (Ildefonse et al., 2007). The gabbroic rocks have primitive to evolved compositions and are characterized by a large range of MgO and SiO2 concentrations and low TiO2, Na2O and trace element contents (Expedition Scientific Party, 2005a,b; Blackman et al., 2006). Alteration mineral assemblages of the gabbroic rocks record cooling and local fluid–rock interaction from magmatic conditions to zeolite facies, with a predominance of greenschist facies conditions (<500 °C). The degree of alteration in the gabbroic rocks decreases from moderate in the upper 400 m (average 50%), to low between 400 and 800 mbsf (average 30%) and to minor between 800 and 1415 mbsf (average 10%; Expedition Scientific Party, 2005a,b; Blackman et al., 2006). Deformation under granulite–

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amphibolite facies conditions is typically restricted to a few mm-thick shear zones in the core, which suggests that deformation related to detachment faulting was not recovered or occurred at lower temperature than in the gabbros from the southern wall (Schroeder and John, 2004). Shipboard paleomagnetic data indicate that the footwall did not rotate significantly, which is consistent with a model of uplift and OCC formation involving a shallow rooting detachment fault system, such as that proposed by MacLeod et al. (2002) and Escartin et al. (2003) for the domed massif at 15°450 N along the MAR. Ildefonse et al. (2007) revised this model and suggested that because of marked differences in rheological properties, the serpentinized peridotites intruded by the gabbroic rocks would concentrate deformation, and therefore enhance the uplift of an undeformed gabbroic body. 2.2. The southern ridge Dredging and submersible sampling revealed that the southern ridge of the AM, located 5 km to the south of Site U1309 (Fig. 1), differs significantly in composition and proportion of lithologies. The southern wall is composed predominantly of serpentinized peridotites (70%) with interspersed minor gabbroic bodies (30%), which form the basement of the LCHF (Blackman et al., 2002; Schroeder and John, 2004; Kelley et al., 2005; Boschi et al., 2006; Karson et al., 2006). A 100 m-thick detachment shear zone (DSZ) has been mapped at the crest of the massif and is composed of highly deformed serpentinized peridotites and lesser metagabbros that are affected by talc and/or amphibole metasomatism (Boschi et al., 2006; Karson et al., 2006). The top of the southern wall of the massif lies at water depths of 700–800 m and has experienced greater uplift than the central dome (Fig. 1). This area is cut by steep normal faults, which together with mass wasting have exposed a near-vertical 3800 m-high scarp facing the Atlantis Transform Fault (Fig. 1) and are likely important hydrological controls on hydrothermal activity below Lost City.

Table 1 Sulfide assemblages of serpentinized harzburgites and olivine-rich troctolites of the central dome of the Atlantis Massif Hole/Leg

Sample number

Rock type

Depth (mbsf)

Sulfide phases

304-1309D 304-1309D 304-1309D 304-1309D 304-1309D 305-1309D

27R-3 6–9 cm 31R-2 19–30 cm 42R-1 0–8 cm 58R-1 50–57 cm 65R-2 22–30 cm 227R-3 6–12 cm

Harzburgite Harzburgite Talc-rich harzburgite Harzburgite Cr-rich harzburgite Olivine-rich troctolite

155.1 173.2 224.3 300.9 335.7 1095.0

305-1309D 305-1309D

232R-1 20–31 cm 233R-1 24–30 cm

Olivine-rich troctolite Olivine gabbro

1115.3 1120.1

305-1309D

234R-2 102–110 cm

Olivine-rich troctolite

1127.2

305-1309D

237R-2 7–17 cm

Olivine-rich troctolite

1140.6

305-1309D 304-1309B

241R-2 97–108 cm 11R-1 23–30 cm

Olivine-rich troctolite Harzburgite

1160.7 57.2

Pentlandite ± smythite Pentlandite Pyrite + pentlandite + millerite Pentlandite Pentlandite + undefined phase Pentlandite + wurtzite + pyrrhotite + mackinawite Mackinawite + pyrrhotite Mackinawite + pyrrhotite + pentlandite + heazlewoodite Mackinawite + pyrrhotite + pentlandite + chalcopyrite Mackinawite + pyrrhotite + pentlandite + Cu-rich phase Mackinawite + pyrrhotite Millerite + violarite ± heazlewoodite

Other phases ±Native Cu

+Native Cu +Native Cu +Native Cu

+Native Cu

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Karson et al. (2006) postulated that a similar normal fault zone and/or the limit of the large gabbroic intrusion may be responsible for the differences in topography and lithology observed between the central dome and the southern wall. 3. MATERIALS AND METHODS Our studies were conducted on 19 selected samples of gabbroic rocks, serpentinized mantle peridotites, and olivine-rich troctolites from Hole 1309D and one serpentinized mantle peridotite from Hole 1309B (Fig. 1 and Tables 1 and 2) with the aim at documenting primary sulfur compositions and evaluating changes related to fluid circulation and alteration. Because alteration is most prevalent in the ultramafic lithologies and olivine-rich gabbroic rocks, these were targeted for more detailed studies and sulfur phase extraction. Representative samples of olivine gabbro, oxide gabbro, gabbro and troctolite recovered at different intervals in Hole 1309D were measured for bulk-rock sulfur contents and bulk-rock sulfur isotope compositions (Table 2) to monitor the variations in compositions with depth. Samples of the mantle peridotites in Hole 1309D and Hole 1309B and the 140 m-thick layer of olivine-rich troctolites intercalated with olivine gabbros at Hole 1309D were extracted for sulfate and sulfide fractions, on which sulfur isotope compositions were measured. One anhydrite vein in a gabbro at 739 mbsf in Hole 1309D (Expedition Scientific Party, 2005a,b; Blackman et al., 2006) was also analyzed for its sulfur isotope composition (Table 2). Sulfur isotope analyses and sulfur contents were determined for total sulfur in bulk-rock powders of all samples and on separate fractions (monosulfides, disulfides and sulfates) of 12 ultramafic and gabbroic samples (Table 2) using a Carlo Erba Elemental Analyzer (EA) (NCS 2500, CE Instruments) interfaced in continuous flow mode to a GV Instruments OPTIMA mass spectrometer. For extraction of the sulfate and sulfide fractions, approximately 10 g of powdered sample was reacted with HCl and a CrCl2 solution under N2 atmosphere according to the method of Tuttle et al. (1986). During the first step, reaction of the sample powder with HCl caused dissolution of the acid-soluble sulfates and decomposition of monosulfides, which liberated H2S. The H2S was then precipitated as Ag2S, whereas the liberated acid-soluble sulfates were precipitated as BaSO4. In a second step, the addition of CrCl2 solution to the sample powder residue reduced the disulfide minerals, again releasing H2S, which was precipitated and analyzed as Ag2S. The amounts of collected sulfate-sulfur, monosulfide-sulfur and disulfide-sulfur were determined gravimetrically on a highprecision balance. Sulfate-sulfur contents determined by weight were corrected based on the sulfur content determined by EA. The correction was necessary because during the washing step subsequent to the HCl treatment and dissolution, some of the very fine-grained rock powder likely passed through the filter resulting in an overestimation of the amount of sulfate-sulfur determined gravimetrically. Sulfur isotope ratio measurements were determined on approximately 0.5–1 mg of extracted Ag2S and BaSO4 and on approximately 100 mg of bulk-rock powder, weighed into tin capsules. Vanadium pentoxide (V2O5)

was added to the sample in the tin capsule to enhance combustion. The samples were combusted in the EA and analyzed in continuous flow by isotope ratio mass spectrometry (IRMS). Sulfur isotope values are reported as standard d-notation relative to the Vienna-Canyon Diablo Troilite (V-CDT) standard. The system was calibrated using the international standards IAEA-S1 (d34S = 0.3&), IAEA-S2 (d34S = 22.7&), NBS123 (d34S = 17.4&) for sulfides, and IAEA-SO5 (d34S = 0.5&), IAEA-SO6 (d34S = 34.1&) and NBS127 (d34S = 21.1&) for sulfates. Detection limit for a reproducible sulfur isotope measurement on the Micromass system was about 28 lg S. Analytical reproducibility of replicate measurements of standards is ± 0.3& (n = 178) and relative precision for the sulfur content is of 3%. Primary and secondary sulfide phases were identified by reflected light microscopy, on the basis of morphology, textures, and the presence or absence of replacement or crosscutting relationships, and their chemical composition was determined with a JEOL JXA-8200 electron microprobe. The operating conditions were 15 kV accelerating potential, 20 nA current, and 1–10 lm beam size. Natural and synthetic sulfide mineral standards were used for calibration. 4. RESULTS 4.1. Mineralogy 4.1.1. Serpentinized mantle peridotites Mantle peridotites represent less than 0.3% of the total recovery at the central dome and were only recovered in the upper 225 m of Hole 1309D and in the upper 60 m of Hole 1309B (Fig. 2a) (Expedition Scientific Party, 2005a,b; Blackman et al., 2006). Primary minerals include olivine, orthopyroxene and chromium spinel. Alteration occurred dominantly under greenschist facies conditions and varies from 50% to 90%, which is higher than the average degree of alteration in the gabbroic rocks in the upper 400 m (50%). Olivine is replaced by serpentine and magnetite, which form well-developed mesh to ribbon texture. Relics of olivine are present in some samples that are not pervasively altered (e.g., 304-1309D-42R-1 0–8 cm), and locally talc with minor chlorite and amphibole replace the serpentine mesh texture (Fig. 2b). Orthopyroxene is replaced by pseudomorphic bastite (Fig. 2a) rimmed by tremolite and minor chlorite. Some orthopyroxene phenocrysts show bending characteristic of early high-temperature crystal– plastic deformation (e.g., 304-1309B-11R-1 23–30 cm). Several generations of veins of serpentine, calcite, or with an assemblage of talc, tremolite, and minor chlorite were observed. The sulfide minerals in the serpentinized peridotites of Hole 1309D consist of pentlandite [(Fe,Ni)9S8], pyrite [FeS2], millerite [NiS] and smythite [(Fe,Ni)9S11] (Table 1 and Fig. 3a). Magmatic pentlandite is the dominant phase and occurs as anhedral grains of 5–50 lm (locally up to 150 lm) in the matrix or enclosed within magnetite (Fig. 4a). In some samples, the pentlandite grains are altered (e.g., 304-1309D-27R-3 6–9 cm) and are associated with secondary smythite, a sulfide mineral commonly

Table 2 Sulfur geochemistry of bulk-rock and extracted sulfide and sulfate in serpentinized peridotites and olivine-rich troctolites of the central dome of the Atlantis Massif Sample number

Rock type

Depth (mbsf)

Alteration (%)

Monosulfide sulfur content (ppm)

Disulfide sulfur content (ppm)

Total sulfide content (ppm)

Sulfatesulfur content (ppm)

Total sulfur content (ppm)

SO4/ Stotal

d34S monosulfide (&)

d34S disulfide (&)

d34S sulfate (&)

d34S* WR (&)

304-1309D 304-1309D 304-1309D 304-1309D 304-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 305-1309D 304-1309B

27R-3 6–9 cm 31R-2 19–30 cm 42R-1 0–8 cm 58R-1 50–57 cm 65R-2 22–30 cm 83R-1 16–26 cm 137R-1 85–91 cm 150R-3 13–30 cm 169R-1 90–100 cm 189R-4 60–71 cm 194R-2 70–80 cm 227R-3 6–12 cm 232R-1 20–31 cm 233R-1 24–30 cm 234R-2 102–110 cm 237R-2 7–17 cm 241R-2 97–108 cm 248R-1 114–120 cm 292R-2 78–88 cm 11R-1 23–30 cm

Harzburgite Harzburgite Talc-rich harzburgite Harzburgite Cr-rich harzburgite Olivine gabbro Oxide gabbro Anhydrite vein Gabbro Troctolite Olivine gabbro Olivine-rich troctolite Olivine-rich troctolite Olivine gabbro Olivine-rich troctolite Olivine-rich troctolite Olivine-rich troctolite Olivine-rich troctolite Olivine gabbro Harzburgite

155.1 173.2 224.3 300.9 335.7 415.2 675.1 738.7 823.9 923.6 939.6 1095.0 1115.3 1120.1 1127.2 1140.6 1160.7 1193.0 1398.6 57.2

90 90 90 70 50 5 40 n.d. 5 3 1 50 10 30 10 10 10 5 20 90

951 140 4123 299 40 n.d. n.d. n.d. n.d. n.d. n.d. 98 190 349 927 107 87 n.d. n.d. 234

1313 386 4125 472 219 n.d. n.d. n.d. n.d. n.d. n.d. 165 92 301 979 114 148 n.d. n.d. 79

2264 526 8248 771 259 n.d. n.d. n.d. n.d. n.d. n.d. 263 282 651 1906 221 236 n.d. n.d. 313

199 117 195 449 687 n.d. n.d. n.d. n.d. n.d. n.d. 13 3 3 51 53 trace n.d. n.d. 399

2464 642 8443 1220 947 385 678 n.d. 486 386 520 276 284 654 1958 274 236 323 654 1025

0.08 0.18 0.02 0.37 0.73 n.d. n.d. n.d. n.d. n.d. n.d. 0.02 <0.01 <0.01 0.01 0.11 n.d. n.d. n.d. 0.39

8.4 10.5 8.8 11.3 9.3 n.d. n.d. n.d. n.d. n.d. n.d. 0.3 1.3 1.0 1.3 0.8 1.1 n.d. n.d. 13.9

6.3 10.5 6.1 7.7 3.7 n.d. n.d. n.d. n.d. n.d. n.d. 0.5 0.4 1.5 0.8 0.6 0.2 n.d. n.d. 11.3

16.1 19.4 7.9 14.5 6.9 n.d. n.d. 27.5 n.d. n.d. n.d. 16.7 16.9 7.3 10.4 22.8 trace n.d. n.d. 16.3

7.9 12.1 7.4 11.1 6.3 0.7 2.1 n.d. 1.3 0.7 0.5 0.3 0.8 1.2 0.7 3.9 0.5 0.6 0.3 10.4

*

Sulfur mineralogy and geochemistry of IODP Hole 1309D

Hole/Leg

d34S values of the bulk-rock obtained either from direct measurements with the EA or by mass balance calculations for samples extracted for sulfate and sulfide fractions.

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Fig. 2. Examples of cored samples from Hole 1309D. (a) Serpentinized harzburgite 304-1309B-11R-2 23–30 cm showing orthopyroxene replaced by pseudomorphic serpentine and the serpentine mesh texture after olivine. (b) Serpentinized harzburgite 304-1309D-31R-2 19–30 cm showing a high degree of serpentinization and talc/tremolite metasomatism replacing bastite pseudomorphs after orthopyroxene. (c). Olivinerich troctolite 305-1309D-234R-2 102–110 cm showing the foliations formed by serpentine veins. (d). Olivine-rich troctolite 305-1309D-237R2 7–17 cm showing a poikilitic texture of olivine included within plagioclase and clinopyroxene poikiloblasts. Mineral abbreviations: opx, orthopyroxene; serp, serpentine; ol, olivine; plag, plagioclase. S

S

py

py pd

po

pd po

ml pn, cpy

hz

Serpentinized peridotites

a Fe

hz

Olivine-rich troctolites

304-1309B 11R-1 23-30 cm 304-1309D 27R-3 6-9 cm 304-1309D 31R-2 19-30 cm 304-1309D 42R-1 0-8 cm 304-1309D 58R-1 50-57 cm 304-1309D 65R-2 22-30 cm

aw

ml pn, cpy

b Cu, Ni

Fe

305-1309D 227R-3 6-12 cm 305-1309D 232R-2 20-31 cm 305-1309D 233R-1 24-30 cm 305-1309D 234R-2 102-110 cm 305-1309D 237R-2 7-17 cm 305-1309D 241R-1 97-108 cm

aw

Cu, Ni

Fig. 3. Fe–S–(Cu,Ni) ternary plots showing the chemical compositions (based on atomic proportions) of the sulfide minerals analyzed by electron microprobe of (a) the serpentinized peridotites and (b) the olivine-rich troctolites drilled at Site U1309. Due to the low contents of Pb and Zn in the sulfides compared to Cu and Ni contents, these two elements are not plotted on the diagrams. Cu and Ni are plotted together at the same apex, thus pentlandite and chalcopyrite plot in the same area of the diagrams. The serpentinized peridotites are dominated by pentlandite with minor millerite and pyrite. Sample 304-1309B-11R-1 23–30 cm differs from the serpentinized peridotites of Hole 1309D and is composed of millerite, violarite and heazlewoodite. The olivine-rich troctolites are composed of an assemblage of mackinawite, pyrrhotite, pentlandite, and rare occurrences of chalcopyrite and heazlewoodite (Table 2). Mineral abbreviations are aw, awaruite; pn, pentlandite; po, pyrrhotite; py, pyrite; ml, millerite; cpy, chalcopyrite; pd, polydymite; hz, heazlewoodite. The sulfide mineral assemblages of the serpentinized peridotites of the southern wall of the Atlantis Massif are reported for comparison (shaded area; data from Delacour et al., 2008).

formed during low temperature and relatively oxidizing conditions (Krupp, 1994). In sample 304-1309D-65R-2 22–30 cm, pentlandite is associated with an unknown Ni– Fe sulfide (41.6 wt.% S, 8.0 wt.% Fe, and 49.9 wt.% Ni) of 20–50 lm in size (Fig. 4b), which is enclosed within a magnetite grain and appears to have recrystallized after pentlandite. Pyrite and millerite are associated with pentlandite in sample 304-1309D-42R-1 0–8 cm, which is enriched in both mono- and disulfides (Table 2 and Fig. 5). Pyrite and millerite (2–10 lm in diameter) occur as interstitial phases in the matrix or enclosed within magnetite. The serpentinized peridotite of Hole 1309B (304-1309B-11R-1

23–30 cm) shows a distinct sulfide mineral assemblage composed of millerite, violarite [FeNi2S4] and heazlewoodite [Ni3S2] (Table 1). Millerite is the predominant sulfide phase and forms euhedral grains of 2–10 lm enclosed in orthopyroxene or magnetite, or as an interstitial phase. Heazlewoodite occurs as a 10 lm-size interstitial phase, whereas violarite is present as 10 lm grains enclosed in orthopyroxene and magnetite. Pentlandite compositions are similar for all the serpentinized peridotites and show low contents in Cu and Zn (trace to 0.2 wt.% and 0.004–0.02 wt.%, respectively). Pyrite in sample 304-1309D-42R-1 0–8 cm is characterized by 2

Sulfur mineralogy and geochemistry of IODP Hole 1309D

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Fig. 4. Backscatter electron microprobe images of sulfide minerals in the serpentinized peridotites and olivine-rich troctolites from Site U1309. (a) Serpentinized harzburgite 304-1309D-58R-1 50–57 cm showing magmatic pentlandite enclosed within magnetite. (b) Serpentinized harzburgite 304-1309D-65R-2 22–30 cm showing magmatic pentlandite mantled by magnetite and altered to Ni–Fe sulfide phase formed after pentlandite. (c) Olivine-rich troctolite 305-1309D-234R-2 102–110 cm showing secondary mackinawite enclosed within magnetite and associated with serpentinized olivine. (d) Olivine-rich troctolite 305-1309D-234R-2 102–110 cm showing a mackinawite grain, associated with native Cu and mantled by magnetite, within the serpentine mesh texture. (e) Olivine-rich troctolite 305-1309D-241R-2 97–108 cm showing the association of pyrrhotite and mackinawite mantled by magnetite and within a matrix formed by olivine grains. (f) Olivine-rich troctolite 3051309D-227R-3 6–12 cm with pentlandite and pyrrhotite grains enclosed within magnetite mineral. Mineral abbreviations: Pn, pentlandite; Mt, magnetite; Ma, mackinawite; Ol, olivine; Po, pyrrhotite; Cu, cupper.

wt.% of Ni, whereas Zn and Cu are low (<0.01 and 0.04 wt.%, respectively). Millerite contains 1.5–3.4 wt.% Fe, no Cu and trace amounts of Zn (<0.02 wt.%). Chemical extractions indicate that the serpentinite samples contain sulfates (Table 2); however, we were unable to detect and identify crystalline sulfate phases by microscopy, by EDS mapping and punctual analyses, by Raman spectroscopy, or by X-ray diffraction. Alt and Shanks (1998, 2003) reported similar results and were unable to identify

sulfate phases in serpentinites with high sulfate contents recovered at Iberian Margin, Hess Deep and MARK area. 4.1.2. Olivine-rich troctolites and olivine gabbros A 140 m-thick layer of olivine-rich troctolites intercalated with predominantly olivine gabbros was recovered between 1095 and 1236 mbsf. Primary minerals consist of olivine (up to 70 modal %) and chromium spinel included within plagioclase and clinopyroxene poikiloblasts

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Fig. 5. Histogram showing the gain and/or loss in sulfide-sulfur and sulfate-sulfur in the serpentinized peridotites and olivine-rich troctolites of the central dome of the AM, compared to sulfur content of fertile mantle of 250 ppm. The serpentinized peridotites have gained variable amounts of sulfates and sulfides, which are interpreted as a mixture of locally variable sources: sulfate uptake from seawater, sulfate derived from sulfide oxidation, and sulfide added through a multistage reaction involving hydrothermal alteration of gabbroic rocks (see text for discussion). The olivine-rich troctolites have only trace amounts of sulfates and two samples show high sulfide-sulfur contents that are related to magmatic variability.

(Fig. 2c–d). A recent petrostructural and geochemical study of these rocks suggests a complex crystallization history in an open-system with impregnation of MORB-type melts (Drouin et al., 2007). The degree of alteration varies from 5% to 50% and is dominated by the alteration of olivine to serpentine and magnetite forming a ribbon to mesh texture. Serpentinization of olivine is accompanied by volume expansion and formation of microfractures in the neighboring plagioclase, which is altered to chlorite, prehnite, hydrogarnet and minor actinolite. Development of coronas of tremolite and chlorite is common at the contacts between plagioclase and olivine and generally reflects static alteration under greenschist facies conditions. Clinopyroxenes are commonly rimmed by tremolite. Multiple veins of serpentine, chlorite, tremolite, and minor calcite cut the olivine-rich troctolites (Fig. 2d). The primary mineral assemblage of the olivine-gabbros consists of subhedral or interstitial olivine, euhedral to anhedral plagioclase, anhedral clinopyroxene and rare orthopyroxene. Textures and alteration phases of the olivine and plagioclase are similar to those observed in the olivine-rich troctolites and are characteristic of greenschist facies conditions. Olivine is replaced by mesh-serpentine and form corona alteration at contacts with plagioclase that is altered to chlorite, prehnite and hydrogarnet. Clinopyroxene is replaced by tremolite and actinolite, primarily along the rim, and by minor brown amphibole; orthopyroxene is altered to talc when in contact with plagioclase (Expedition Scientific Party, 2005a,b; Blackman et al., 2006).

The sulfide assemblages in the olivine-rich troctolites and the olivine gabbros (Table 1 and Fig. 3b) consist of mackinawite [(Fe,Ni)S0.9], pyrrhotite [Fe1  xS, x = 0– 0.2], pentlandite, and rare chalcopyrite [CuFeS2] and heazlewoodite [Ni3S2]. Secondary mackinawite is the predominant sulfide phase and occurs as anhedral grains of 15–100 lm diameter, and reaches up to 600 lm in sample 305-1309D-234R-2 102–110 cm. Mackinawite generally occurs in the matrix (Fig. 4c) or is enclosed within magnetite (Fig. 4d–e). Magmatic pyrrhotite is the second most common phase. It forms subhedral grains of 10– 200 lm as an interstitial phase or within pyroxene, magnetite or olivine and is associated with mackinawite or pentlandite (305-1309D-227R-3 6–12 cm). Primary pentlandite occurs as grains of 10–50 lm as an interstitial phase or within magnetite and pyroxene, and is also observed as secondary sulfide associated with serpentine and magnetite veins in sample 305-1309D-234R-2 102–110 cm. Heazlewoodite (2 lm grains) was observed only within the matrix in the olivine gabbro 305-1309D-233R-1 24– 30 cm, and chalcopyrite occurs as interstitial grains of 5–15 lm only in sample 305-1309D-234R-2 102–110 cm. Sample 305-1309D-227R-3 6–12 cm is markedly different and is composed of pentlandite, wurtzite [(Zn,Fe)S] and pyrrhotite (Table 1). Magmatic pentlandite is the most abundant phase and occurs either as grains of 5– 100 lm enclosed within magnetite and pyroxene, as an interstitial phase in the matrix, or associated with pyrrhotite (Fig. 4f). Wurtzite was found only as 10–30 lm interstitial grain.

Sulfur mineralogy and geochemistry of IODP Hole 1309D

Fig. 6. Sulfur contents and sulfur isotope compositions of the serpentinized peridotites and olivine-rich troctolites (including the olivine-gabbro) of the central dome of the Atlantis Massif, compared with the range of compositions of the serpentinites and metagabbros of the southern wall of the Atlantis Massif (shaded region, Delacour et al., 2008). (a) d34Ssulfate plotted against SO4/ Stotal indicate that in general sulfate is not directly precipitated from seawater but must be derived from a mixture between seawater sulfate and sulfate produced by sulfide oxidation. (b) d34Ssulfate against d34Ssulfide shows the variability of the analyzed samples and the positive d34S values for both sulfate and sulfide of the serpentinized peridotites. In contrast, the olivine-rich troctolites have uniform mantle-like d34Ssulfide values and variable d34Ssulfate values. (c) d34Ssulfide plotted against sulfide-sulfur content indicates an enrichment in sulfide-sulfur and an increase in 34S of the sulfides in the serpentinized peridotites compared to a mantle reference value of 250 ppm and +0.1 ± 0.5& (Sakai et al., 1984; Alt et al., 1989), but shows no correlation between the two. In contrast, the majority of the olivine-rich troctolites lie within a narrow range reflecting primary igneous sulfur compositions. Schematic evolutions of the d34Ssulfide and sulfide-sulfur contents are shown for several processes. Melting or leaching of sulfides leads to a decrease in sulfide-sulfur contents with no isotope fractionation. Oxidation of sulfides also leads to a decrease in sulfide-sulfur contents with kinetic isotope fractionation producing low d34Ssulfate values and high d34Ssulfide values. Hydrothermal sulfide addition, via leaching of igneous sulfides during high-temperature alteration of the gabbroic rocks, inorganic sulfate reduction of seawater sulfate, and precipitation of sulfides in serpentinites during low-temperature serpentinization, leads to an increase in sulfide-sulfur contents with enrichment in 34S. Microbial sulfate reduction produces enrichment in sulfide-sulfur contents with low d34Ssulfide values for an open-system and high d34Ssulfide values for closed-system. Curves are from Alt et al. (2007).

20 304-1309B-11R-1 23-30 cm

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Total sulfur contents of the serpentinized peridotites range from 642 to 8443 ppm with 34S-enriched isotopic compositions (+6.3& to +12.1&; Table 2) compared to mantle values (+0.1 ± 0.5&; Sakai et al., 1984; Alt et al., 1989). The olivine-rich troctolites and olivine gabbros exhibit a narrower range of total sulfur contents (236 to 1958 ppm) and d34S values from 0.8& to +3.9& (Table 2).In the basement rocks of the central dome, most of the sulfur is present as sulfide as indicated by low SO4/ Stotal ratios (<0.01–0.11; Fig. 6a) except in some serpentinized peridotites, which have SO4/Stotal ratios of up to 0.73 indicating a significant amount of sulfate (Fig. 5). Sulfate-sulfur contents of the serpentinized peridotites range from 117 to 687 ppm with d34S values from +6.9& to +19.4&. The olivine-rich troctolites show lower sulfate-sulfur contents, from trace to 53 ppm, with d34S values from +10.4& to +22.8& (Fig. 6b). Monosulfide- and disulfidesulfur contents in the serpentinized peridotites are highly variable, ranging from 40 ppm to 4123 ppm and from 79

Seawater isotope composition

δ34Ssulfate (‰ VCDT)

4.2. Sulfur geochemistry

30

δ34Ssulfide (‰ VCDT)

Mackinawite compositions are similar for all the olivinerich troctolites and Cu contents vary from 1.2 to 5.3 wt.%, whereas Zn contents are lower than 0.04 wt.%. Pyrrhotite and pentlandite are characterized by low Cu and Zn contents (<0.9 and <0.1 wt.%, respectively).

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to 4125 ppm, respectively. The mono- and disulfides are enriched in 34S, with d34S ranging from +8.4& to +13.9& and from +3.7& to +11.3&, respectively (Fig. 6c). The olivine-rich troctolites have variable amounts of monosulfide- and disulfide-sulfur, ranging from 87 to 927 ppm and

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and sulfur (fS2) and oxygen (fO2) fugacities during interaction of seawater-modified hydrothermal fluids with the oceanic crust. In oceanic basement rocks, sulfur may originate from two major sources: seawater sulfate (d34S = +21&; Rees et al., 1978) and mantle/basaltic sulfides (d34S = +0.1 ± 0.5&). A number of processes, such as sulfate reduction, sulfide oxidation, leaching of igneous sulfides, seawater sulfate precipitation and biological activity, can further affect the sulfur geochemistry and lead to compositional variations in oceanic basement rocks (Figs. 5–7).

-0

Log ΣS Fig. 7. Log fO2-log RS diagram showing the stabilities of oxide and sulfide minerals during serpentinization (from Alt and Shanks, 1998 as modified after Frost 1985). Heavy solid lines show mineral reaction boundaries at 300 °C and 2 kbars, calculated from thermodynamic data and inferred from petrologic data. Light gray solid lines represent the boundaries between dominant sulfur species, and dashed lines show contours of log fS2 in the fluid. Shaded area is the stability field of pentlandite. The arrow shows the evolution from highly reducing conditions (pyrrhotite + pentlandite + Fe–Ni alloys) that commonly prevail during early stages of serpentinization at low water–rock ratios (such as at Hess Deep, Alt and Shanks, 1998), to an assemblage of pentlandite + millerite + heazlewoodite, which characterizes the reducing conditions at the central dome, and finally to an assemblage of magnetite + pyrite dominant during oxidizing conditions at high water– rock ratios as documented at the southern AM-LCHF (Delacour et al., 2008). Mineral abbreviations: Py, pyrite; Po, pyrrhotite; Pd, polydymite; Mt, magnetite; Mi, millerite; Hz, heazlewoodite; Aw, awaruite; Hm, hematite; Va, valleriite; Ta, taenite; Km, kamacite; I, iron.

from 92 to 979 ppm, respectively, whereas their d34S values lie in a narrow range close to mantle-like values, from 1.3& to 0.3& and from 0.8 to 0.2&, respectively (Fig. 6c). 5. DISCUSSION In mid-ocean ridge environments, sulfur geochemistry is sensitive to fluid fluxes, fluid–rock reactions, temperatures,

The sulfide mineral assemblages provide constraints on fO2 and fS2 prevailing at the central dome. Serpentinization at low water-rock ratios commonly leads to reducing conditions related to the oxidation of ferrous iron in olivine and favors the formation of low-sulfur sulfide assemblages, in particular pyrrhotite and heazlewoodite, Fe–Ni alloys (e.g., taenite, awaruite), native metals and hydrogen (Fig. 7; Alt and Shanks, 1998, 2003; Fru¨h-Green et al., 2004; Palandri and Reed, 2004). Complete serpentinization of mantle peridotites at higher water-rock ratios, in contrast, leads to higher fO2 with an assemblage of pyrite, valleriite [4(Fe,Cu,Ni)S  3(Fe,Mg,Al)(OH)2], hematite, millerite, and precipitation of seawater sulfate (Fig. 7; Eckstrand, 1975; Frost, 1985; Alt and Shanks, 1998; Palandri and Reed, 2004). In gabbroic rocks, primary igneous sulfides commonly consist of pyrrhotite, pentlandite and chalcopyrite. Under the P-T conditions of the uppermost mantle, pyrrhotite and a monosulfide solid solution (Mss) phase are the two stable sulfide phases. With decreasing temperature and fractional crystallization of the Mss, the residual sulfide liquid will be enriched in Ni and Cu (Ebel and Naldrett, 1996, 1997) and will form the magmatic assemblage pyrrhotite, pentlandite and chalcopyrite (Bishop et al., 1975). The mantle peridotites at the central dome show a moderate to high degree of alteration (50–90%; Table 2 and Fig. 8) related to pervasive circulation of seawater and serpentinization in the upper part of Hole 1309D. Strontium isotope compositions range from 0.70687 to 0.70904 and indicate that serpentinization occurred at low to moderate water–rock ratios (Delacour et al., 2008). Low to moderate fluid fluxes allow reducing conditions during serpentinization to be maintained and lead to the formation of low-sulfur sulfide assemblages (Figs. 3a and 7) and precipitation of native Cu (Table 1). The serpentinized peridotites are dominated by low-sulfur and Ni-rich sulfides (pentlandite, violarite, millerite, heazlewoodite), and rare pyrite (Figs. 3a and 4a–b). Alt and Shanks (1998) document similar low-sulfur sulfide assemblages in serpentinites from Hess Deep (pentlandite, marcasite, millerite, valleriite), which are associated with Fe–Ni alloys (e.g., taenite and/or awaruite; Grobe´ty et al., 1997; Alt and Shanks, 1998; Fru¨h-Green et al., 2004) and reflect lower fO2 conditions during serpentinization (Fig. 7). Slightly higher conditions of fO2 and fS2 are recorded in serpentinites of the MARK area with an assemblage of brucite, millerite, pentlandite, pyrrhotite, and rare

Sulfur mineralogy and geochemistry of IODP Hole 1309D

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Fig. 8. Total sulfur contents, bulk d34S values and alteration degree versus depth of the selected gabbroic and ultramafic samples from IODP Hole 1309D. High total sulfur contents and high bulk d34S values of the serpentinized peridotites correlate with the degree of alteration/ serpentinization and reflect a multistage reaction history. Relatively homogeneous d34S values are recorded downhole in the gabbroic rocks and olivine-rich troctolites. Enrichment in 34S in some samples is likely related to fluid circulation in fault zones, whereas enrichment in sulfide content is linked to magmatic processes or melt-rock reaction (see text for discussion). Thick line in the plot of alteration represents the downhole alteration profile with a 5 m interval (after Blackman et al., 2006).

heazlewoodite and awaruite, interpreted by Alt and Shanks (2003) as relict from an early stage of serpentinization at low water–rock ratios. At the southern wall of the AM, the sulfide assemblages in the serpentinites are dominated by secondary pyrite with only rare occurrences of pentlandite (Fig. 7). This reflects higher fO2 during long-lived serpentinization related to the high fluid fluxes associated with the Lost City hydrothermal system (Delacour et al., 2008). The serpentinites of the central dome show reducing conditions intermediate between the highly reducing conditions of early stages of serpentinization, such as in serpentinites at Hess Deep (Alt and Shanks, 1998), and the relatively oxidizing conditions of the southern wall (Fig. 7). These differences reflect the regional variations in fluid fluxes at the Atlantis Massif. The predominance of Ni-rich sulfides associated with heazlewoodite in the olivine-rich troctolites indicates low fO2 and is different from the typical igneous sulfide assemblages (pyrrhotite + pentlandite + chalcopyrite) reported for gabbros from Hole 735B (Alt and Anderson, 1991), Hess Deep (Puchelt et al., 1996), and 15°200 N (Alt et al., 2007). The presence of pentlandite and pyrrhotite reflect primary igneous sulfide formation and fractional crystallization of Mss with Ni-enrichment (Ebel and Naldrett,

1996, 1997). However, the occurrence of mackinawite reflects secondary processes. Mackinawite has been only reported in serpentinized ultramafic rocks, in association with pentlandite, pyrrhotite and native Cu (e.g., Muskox intrusion, Coolac ultramafic belt, Beni Bousara and Ronda massifs; Chamberlain and Dellabio, 1965; Ashley, 1973; Lorand, 1985 and reference therein). It is generally believed that mackinawite occurs as replacement of pentlandite during serpentinization processes at low temperature (Chamberlain and Dellabio, 1965; Harris and Vaughan, 1972). This is supported by experimental studies (Takeno et al., 1970; Sweeney and Kaplan, 1973; Zoka et al., 1973; Power and Fine, 1976), which indicate that mackinawite crystallization is favored under low temperature (<200 °C), low fO2, increasing fS2 and hydrous conditions. Textural relationships of mackinawite with magnetite and serpentine in the olivine-rich troctolites of the central dome indicate that mackinawite formed during the relatively early stage of serpentinization, most likely by replacement of pentlandite or desulfurization of pyrrhotite. Mackinawite has also been observed in few intervals along the hole, but only in association with serpentine alteration of olivine-rich samples (e.g., 305-1309D-100R-1 42–46 cm and 305-1309D-136R-2 60–71 cm). The occurrence of mackinawite in the olivinerich troctolites is thus likely linked to serpentinization of

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olivine and provides an upper temperature limit of approximately 200 °C for the fluids circulating at depth. This is consistent with shipboard borehole temperature measurements of 120 °C at the bottom of the hole (Expedition Scientific Party, 2005a,b; Blackman et al., 2006). The olivine-rich troctolite sample 305-1309D-227R-3 6–12 cm contains a distinct sulfide assemblage with the predominance of pentlandite associated with wurtzite, pyrrhotite and mackinawite (Fig. 3b). The moderate degree of alteration and serpentinization (50%; Fig. 8) of this sample suggests that precipitation of wurtzite could be related to local Zn-rich fluids. However, low-sulfur and Ni-rich sulfide assemblages dominate the olivine-rich troctolites and reflect the low fO2 and fS2 conditions prevailing during alteration by moderately low-temperature fluids (200 °C). The distribution of igneous sulfides in the gabbros of Hole 1309D may result from accumulation of immiscible sulfide liquid between the cumulate minerals, as suggested for sulfides in gabbroic rocks at ODP Hole 735B (Alt and Anderson, 1991). The studies of Naldrett (1981) demonstrated that in the system Fe–Ni–S, the first crystallizing phase is Fe-rich and Ni-poor relative to the coexisting mafic silicate melt. Further fractional crystallization leads to Ni-enrichment in the sulfide liquid that would consequently precipitate Ni-rich sulfides. Therefore, the increase in Ni associated with low iron contents observed in the olivinerich gabbros may reflect accumulation of sulfide melt between the silicates (olivine, pyroxene and plagioclase). 5.2. Downhole variations in sulfur contents and sulfur isotope compositions The variations in sulfur geochemistry with depth in Hole 1309D are presented in Fig. 8. Fertile mantle peridotites generally contain about 250–300 ppm total sulfur, present as sulfide and with d34S values of +0.1 ± 0.5& (Ringwood, 1966; Sakai et al., 1984; Alt et al., 1989; Chaussidon et al., 1989; Lorand, 1991; Hartmann and Wedepohl, 1993; Shanks, 2003). Compared to these reference values, the serpentinites of the upper part of Hole 1309D have high total sulfur contents and high bulk d34S values (+6.3& to +12.1&; Table 2), indicating a significant addition of 34 S-enriched sulfur during serpentinization (Figs. 5 and 8). We rule out direct precipitation of sulfate from seawater as a major process for sulfur enrichment in the serpentinites because d34Ssulfate values are generally lower than that of seawater sulfate and do not correlate with sulfate concentrations (Fig. 6a), and because they are dominated by sulfides with high d34S values (Fig. 6b–c and Table 2). The total sulfur contents and bulk d34S compositions of our sample set increase with the degree of alteration in the upper part of the hole, and show a broad decrease with depth, toward mantle-like values (Fig. 8). This suggests that seawater circulation and fluid–rock interaction in the upper part of the Hole and along fault zones are likely responsible for the addition of sulfur and enrichment in 34S (Fig. 8). However, the olivine-rich lithologies, preferentially selected in our study, show a higher alteration degree (up to 90%) than that of the gabbroic rocks at Hole 1309D (average  27%; Fig. 8), and therefore may not truly be representative

of processes in the more gabbro-dominated portions in the upper part of the Hole. One serpentinite sample, 3041309D-42R-1 0–8 cm, shows very high sulfur contents (Fig. 8a), with 97% of sulfide-sulfur, suggesting local sulfide metasomatism. Two mafic samples, the plastically deformed oxide gabbro 305-1309D-137R-1 85–91 cm and the gabbro 305-1309D-169R-1 90–100 cm, located near a fault zone between 695 and 785 mbsf (Blackman et al., 2006), show a slight enrichment in d34S values compared to mantle composition (Fig. 8). Their moderate and low alteration degree and the slightly enriched d34S values are likely related to circulation of seawater-derived fluids along the fault zone. Relatively high total sulfur content (678 ppm) and high primary Fe2O3 content (>25 wt.%) in sample 305-1309D137R-1 85–91 cm probably reflects increased solubility of sulfide in mafic melts with increasing iron contents (Haughton et al., 1974). Because of its retrograde solubility (Blount and Dickson, 1969), anhydrite precipitates when seawater is heated above 150 °C. Thus, the presence of the anhydrite vein at 739 mbsf suggests seawater circulation at temperatures above 150 °C within the fault zone. The d34S value of anhydrite of +27.5& (Table 2) is higher than seawater composition and, as discussed below, points to the influence of closed-system sulfate reduction. With the exception of two samples, the olivine-rich troctolites deeper in the section show uniform total sulfur contents (Fig. 8) and homogeneous bulk sulfur isotope compositions, ranging from 1.2& to +0.6& (average of +0.2&), typical of magmatic rocks and upper mantle material. One olivine-rich troctolite, sample 305-1309D-234R-2 102–110 cm, is enriched in total sulfide-sulfur and total sulfur contents (1958 ppm, Figs. 5 and 8), with no correlated enrichment in 34S. Percolation of mafic or MORB-type melts (Drouin et al., 2007) through the ultramafic rocks may have added sulfides with magmatic d34S values, as it has been suggested for peridotites sampled at Hess Deep and recovered during Leg 209 (Alt and Shanks, 1998; Alt et al., 2007). Another olivine-rich troctolite sample, 3051309D-237R-2 7–17 cm, has a bulk d34S value of +3.9& higher than the mantle-like sulfur isotope compositions and contains sulfates with a high d34S value (+22.8&). This is likely related to minor sulfate reduction, in a closed-system, during limited circulation of seawater, a similar process as the one discussed below for the anhydrite vein below. 5.3. Hydrothermal sulfide addition during serpentinization The high d34Ssulfides values in the serpentinized peridotites in the upper 225 m of Hole 1309D (Figs. 5, 6 and 8) may result from various processes: (a) partial to complete reduction of seawater sulfate in a (semi-)closed-system; (b) a multistage reaction history involving precipitation of sulfides leached from hydrothermal alteration of gabbroic rocks at high temperature; and (c) oxidation of sulfides by circulating seawater. Sulfide oxidation and conversion of pyrrhotite to pyrite can be caused by loss of iron, sulfidization by H2S, oxida-

Sulfur mineralogy and geochemistry of IODP Hole 1309D

tion by O2, or oxidation/sulfidization by SO4 2 (Gitlin, 1985; Schoonen and Barnes, 1991; Shanks et al., 1995; Shanks, 2003). Sulfide oxidation, however, results in a decrease in sulfide concentrations and thus is inconsistent with the high sulfide-sulfur concentrations. Enrichment in sulfide-sulfur and in 34S can occur during abiotic or biotic reduction of seawater sulfate. Microbial sulfate reduction takes place at low temperatures (up to 110 °C; Stetter, 1996) and is typically associated with a large sulfur isotope fractionation of 20& to 70& (Goldhaber and Kaplan, 1980; Brunner and Bernasconi, 2005). In contrast, inorganic sulfate reduction only occurs at temperatures above 150 °C (Andrews, 1979; Ohmoto and Lasaga, 1982; Goldhaber and Orr, 1995; Machel et al., 1995) and is dominated by kinetic isotopic effects that produce variable sulfur isotope fractionations depending on the temperature conditions and the reaction progress, i.e. rate of the overall reaction (Mottl et al., 1979; Shanks and Seyfried, 1987; Machel et al., 1995; Ohmoto and Goldhaber, 1997). Sulfates may be reduced through conversion of organic matter (thermochemical sulfate reduction), oxidation of ferrous iron, or through conversion of igneous pyrrhotite to secondary pyrite (Mottl et al., 1979; Shanks and Seyfried, 1987). Because organic matter contents are low and pyrite is not observed in our samples, except in sample 304-1309D-42R-1 0–8 cm, sulfate reduction through oxidation of ferrous iron is probably the more likely process. The range of observed d34S values for mono- and disulfides may be explained by partial reduction of seawater sulfate in a closed-system. However, the high sulfide-sulfur contents in the serpentinized samples would require higher fluid fluxes and water–rock ratios than those calculated by Delacour et al. (2008). This suggests that a further process must be responsible for the increase of sulfide in the olivine-rich portions of the section. High sulfide contents and moderately high d34Ssulfide values at the central dome can be attributed to a multistage reaction history involving interaction of seawater with gabbroic rocks, as suggested for serpentinites at MARK and at 15°200 N (Alt and Shanks, 2003; Alt et al., 2007). Hydrothermal alteration of gabbroic rocks at high temperature (300–400 °C) may produce leaching and mobilization of the igneous sulfides to the fluids. The H2S-rich hydrothermal fluids can subsequently react with the serpentinites at lower temperatures (300 °C) and precipitate Ni-rich sulfides (heazlewoodite, millerite, and pentlandite) under reducing conditions and with d34S values between 4& and 13& (Alt and Shanks, 2003). The serpentinites in the upper part of the central dome are indeed characterized by Ni-rich sulfides (Fig. 3a) and have d34S values (3.7–10.5&, Table 2) similar to the MARK serpentinites. This model is consistent with the predominance of gabbroic rocks at the central dome and with their alteration under greenschist facies conditions (400 °C). The occurrence of millerite and late pyrite (Fig. 3a) in one serpentinite sample, 304-1309D-42R-1 0–8 cm, with high total sulfide-sulfur content (0.08 wt.%; Figs. 5 and 8) is similar to the assemblage of MARK serpentinites (Alt and Shanks, 2003). As mentioned earlier, the d34S value of +27.1& of the anhydrite vein at 739 mbsf can be explained by a closed-system sulfate reduction process, and not by direct precipita-

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tion from seawater. In a closed-system, sulfate reduction can be modeled as a Rayleigh distillation process: as the sulfate reduction progresses, the sulfates remaining in solution are progressively enriched in 34S, the light 32S isotopes being preferentially removed by the newly formed sulfides. Consequently, the sulfides further produced by sulfate reduction will similarly and progressively be enriched in 34 S. Assuming a fractionation factor (Dsulfate–sulfide) of about 29–35& for a temperature range of 150–200 °C (Ohmoto and Rye, 1979; Ohmoto and Lasaga, 1982), a d34S value similar to that measured for the anhydrite vein will be attained when 15–20% of sulfate is converted to sulfide. Petrographic observations indicate that the anhydrite is associated with pyrite, which provides further evidence for a process of closed-system sulfate reduction during anhydrite formation. 5.4. Sulfide oxidation The sulfates in the serpentinized peridotites have variable d34S values (Table 2), which are not linearly correlated with the sulfate contents and SO4/Stotal ratios (Fig. 6a). This suggests that another component with low d34S value is required in addition to seawater sulfate. In addition, the serpentinized peridotites show very high mono- and disulfide-sulfur concentrations whereby the monosulfides have higher d34S values than the coexisting disulfides. In two samples (304-1309D-42R-1 0–8 cm and 304-1309D65R-2 22–30 cm), the d34S values of the sulfides are higher than those of the coexisting sulfates. These signatures are the reverse of those predicted by equilibrium fractionation (Ohmoto and Rye, 1979). A possible mechanism to explain these isotopic signatures is provided by the experiments of Grinenko and Mineyev (1992), which have shown that sulfide leaching from a gabbro and an olivine-gabbro at 60– 90 °C is associated with partial oxidation of sulfide and leads to the production of light sulfate and heavy sulfides with a fractionation of up to 10&. In their experimental columns the leached sulfide was redeposited downflow and led to the formation of a zone with high sulfide concentrations with high d34S values. Kinetic isotope fractionations of similar magnitude during abiotic sulfide oxidation were also reported by Fry et al. (1988) and Toran and Harris (1989). The sulfates produced by this process, would have low to moderate d34S values and may remain in the system or be continuously removed by circulating fluids as the reaction progresses (Andrews, 1979). Their mixing with seawater sulfates can lead to the observed d34Ssulfate values for the two samples that show higher d34Ssulfide values than the coexisting sulfates. 6. SUMMARY AND CONCLUSIONS Our studies show that uplifted portions of oceanic crust may record vertically and laterally large variations in sulfur contents and isotope compositions that reflect variations in fluid fluxes, temperature, fS2, fO2 and/or differences in fluid- or melt-rock interaction. During uplift and exhumation of the Atlantis Massif, fluid flow was more limited in the gabbroic-dominated domains of the central dome, in

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contrast to high fluid fluxes recorded in the peridotite-dominated southern wall. In general the gabbroic sequences at the central dome show homogeneous and mantle-like total sulfur contents and bulk sulfur isotope compositions, whereby local variations from mantle compositions (Fig. 8) are likely related to melt-rock reactions or to interaction with 34S-enriched seawater-derived fluids along local brittle fault zones. Gabbro–seawater interaction may have a significant impact on the sulfur geochemistry of narrow intercalated zones of ultramafic and olivine-rich lithologies. In the upper part of Hole 1309D, high concentrations in sulfide with high d34S values in the serpentinites reflect a multistage reaction history involving precipitation of sulfide leached from high temperature hydrothermal alteration of the subjacent gabbroic rocks, and subsequent minor oxidation of the sulfide minerals. The ultramafic lithologies represent more reactive domains with higher seawater fluxes under greenschist facies conditions, resulting in higher degrees of alteration during serpentinization. At the southern wall of the Atlantis Massif, 5 km south of Hole 1309D, high fluid–rock ratios and hydrothermal activity at Lost City (Delacour et al., 2005; Delacour et al., 2008) resulted in major changes in sulfur mineralogy and sulfur geochemistry of the basement rocks. The serpentinites at the southern wall are characterized by seawaterlike d34S values, low total sulfur and sulfide-sulfur contents with high d34S values, and a predominance of pyrite reflecting high fO2 conditions during serpentinization (Fig. 7; Delacour et al., 2005, 2008). In addition, microbial activity in the basement below LCHF is suggested by d34S values of both sulfates and sulfides of samples located beneath the active hydrothermal chimneys (Fig. 6c). These sulfur compositions contrast with those of the central dome and reflect clear regional variations in fluid fluxes, fluid–rock interaction and temperature conditions and indicate that the central dome has remained relatively unaffected by the extensive hydrothermal circulation that has been documented at Lost City (Kelley et al., 2005; Boschi et al., 2008; Delacour et al., 2008). Comparison of Site U1309 with the southern part of the Atlantis Massif and with published studies from other localities documents how sulfur speciation in oceanic peridotites evolves over time and how it can be used to trace local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity. In concert, these studies show that long-lived histories of uplift and alteration of mantle peridotites on the seafloor have important consequences for the speciation and cycling of sulfur (Alt and Shanks, 2003). Serpentinization of oceanic peridotites and the alteration of serpentinites represent fundamental processes that provide a sink for sulfur from seawater. These processes can also release mantle-derived sulfur to the oceans and can be closely linked to microbial activity on and below the seafloor. ACKNOWLEDGMENTS We thank the co-chiefs and the IODP scientific parties of Expeditions 304 and 305 for their significant contributions to the data collection and discussions at sea. We also acknowledge Swiss IODP

and Maria Coray-Strasser for assistance with the analyses. We are grateful to previous Associate Editor Martin Goldhaber, Mike Mottl, and an anonymous reviewer for their constructive comments. We would particularly like to thank Jeff Alt for his thorough review and thoughtful input, which significantly improved this manuscript. This work was supported by Swiss SNF Grants 2100-068055 and 200020-107620 to Fru¨h-Green and Bernasconi.

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