Earth and Planetary Science Letters 280 (2009) 128–136

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Serpentinized troctolites exposed near the Kairei Hydrothermal Field, Central IndianRidge: Insights into the origin of the Kairei hydrothermal fluid supporting a uniquemicrobial ecosystem

Kentaro Nakamura a,b,⁎, Tomoaki Morishita c, Wolfgang Bach d, Frieder Klein d, Kaori Hara e, Kyoko Okino f,Ken Takai b,g, Hidenori Kumagai b,h

a Frontier Research Center for Energy and Resources (FRCER), the University of Tokyo, Tokyo 113-8656, Japanb Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japanc Frontier Science Organization (FSO), Kanazawa University, Kanazawa 920-1192, Japand Department of Geosciences, University of Bremen, PO Box 33440, 28334 Bremen, Germanye Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japanf Ocean Research Institute, the University of Tokyo, Tokyo 164-8639, Japang Subground Animalcule Retrieval (SUGAR) Program, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japanh Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan

Kairei Hydrothermal Field

⁎ Corresponding author. Precambrian Ecosystem LMarine-Earth Science and Technology (JAMSTEC), Yoko

E-mail address: kentaron@jamstec.go.jp (K. Nakamu

0012-821X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.epsl.2009.01.024

a b s t r a c t

a r t i c l e i n f o

Article history:

The vent fluids of the Kairei Received 12 August 2008Received in revised form 13 January 2009Accepted 14 January 2009Available online 20 February 2009

Edited by: R.W. Carlson

Keywords:serpentinizationhydrogentroctolitic rockshybrid alteration modelhydrothermal fluid

Hydrothermal Field (KHF) on the Central Indian Ridge near the Rodriguez TripleJunction are atypical. They have a very high H2 concentration, a relatively high Si concentration, and aremarkably low CH4/H2 ratio. Recently, particular attention has been paid to the KHF, because thehydrothermal fluids are suggested to support a hydrogen-based hyperthermophilic subsurface lithoauto-trophic microbial ecosystem (HyperSLiME), which is considered to be a likely modern analogue for the earlyEarth ecosystems prior to photosynthesis. Despite the increasing interest in the fluid chemistry andassociated biota, the origin of the unusual chemistry of the hydrothermal fluids is still uncertain.Here we suggest interaction of seawater with troctolite recently discovered from small hills near the KHF,provide a possible explanation for the composition of the KHF fluids. Dives with the manned submersibleShinkai 6500 recovered plagioclase dunite, troctolites, and olivine gabbros, which generally constitute thelower oceanic crust. Microscopic observations revealed that olivine crystals in the samples were partly tocompletely replaced by serpentine and magnetite, indicating the generation of H2 by serpentinizationreactions between olivine and hydrothermal fluids. Theoretical model calculations predict that the high H2

and high Si concentrations of the hydrothermal fluids can be attributed to serpentinization of the troctolitesand subsequent hydrothermal reactions with basaltic wall rocks under the KHF. The unique geological settingof the Kairei hydrothermal system, where deep crustal rocks are emplaced in the shallower part of oceaniccrust, is considered to be responsible for the unusual chemistry of the Kairei hydrothermal fluids driving theoccurrence of the HyperSLiME.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of black smoker vents hosting chemosyntheticmacrofaunal communities (Spiess et al., 1980), submarine hydro-thermal systems and associated biota have attracted the interest notonly of geoscientists, but also of chemists and biologists (e.g.,Humphris et al., 1995; Van Dover, 2000; Wilcock et al., 2004). In thepast few decades, it has been revealed that the diverse populations ofthe hydrothermal vent-endemic animal communities are generallydependent on the primary production of symbiotic and free-living,

aboratory, Japan Agency forsuka 237-0061, Japan.ra).

ll rights reserved.

chemolithoautotrophic microorganisms. These obtain energy frominorganic substances, such as H2S, CO2, H2, and CH4, which are derivedfrom hydrothermal vent fluids (e.g., Jannasch and Mottl, 1985). Morerecently, particular attention has been paid to archaeal methanogenssupported by H2-enriched hydrothermal fluids, because this type ofmicrobial ecosystem is considered to be an important modernanalogue to the early ecosystems of the Earth, as well as theextraterrestrial life on other planets and moons (e.g., Kelley et al.,2001, 2005; Nealson et al., 2005; Takai et al., 2006).

The Kairei Hydrothermal Field (KHF) at the Central Indian Ridge(CIR) near the Rodriguez Triple Junction (RTJ) was discovered inAugust 2000 as the first directly observed hydrothermal vent site inthe Indian Ocean (Gamo et al., 2001; Hashimoto et al., 2001). Thechemical composition of the vent fluids, including major dissolved

129K. Nakamura et al. / Earth and Planetary Science Letters 280 (2009) 128–136

species (e.g., Si, Ca, Mg, K, SO4) and gases (CH4, CO2, H2S), was firstreported by Gamo et al. (2001), showing that the Kairei fluids aregenerally similar to the hydrothermal fluids from typical mid-oceanridges in the Pacific and the Atlantic oceans. Subsequent investiga-tions, however, revealed that the Kairei fluids have unusually highconcentrations of H2 (8 mM) despite the similarity of the othermineral and gas element compositions to typical basalt-hosted mid-ocean ridge hydrothermal fluid (Van Dover et al., 2001; Takai et al.,2004; Gallant and Von Damm, 2006; Kumagai et al., 2008). In 2004, itwas suggested that a hydrogen-based hyperthermophilic subsurfacelithoautotrophic microbial ecosystem (HyperSLiME) existed in thesubseafloor environments of the KHF (Takai et al., 2004). Thismicrobial ecosystem is sustained by the primary production ofhydrogenotrophic, hyperthermophilic methanogens, utilizing H2 andCO2 as the primary energy and carbon sources. The H2 and CO2 arecompletely photosynthesis-independent substances, provided only bygeological (hydrothermal) processes, implying that the HyperSLiME isa likely modern analogue for the early Earth ecosystems prior tophotosynthesis (Takai et al., 2006).

Hydrogen-enriched high-temperature hydrothermal vent fluidssimilar to the Kairei fluids have also been reported from severalhydrothermal vent fields of the Rainbow, Logatchev I and II, Ashaze Iand II, and Nibelungen located along the Mid-Atlantic Ridge (MAR)(Charlou et al., 2002, 2008; Melchert et al., 2008). These MAR H2-richhydrothermal vent fluids have generally been attributed to theserpentinization of abyssal peridotite that has been tectonicallyuplifted and emplaced near the hydrothermal vent fields (Charlouet al., 2002; Douville et al., 2002). In contrast, there has been nogeological evidence indicating the involvement of peridotite in thegeneration of H2-rich hydrothermal vent fluids at the KHF (Van Doveret al., 2001; Gallant and Von Damm, 2006). In addition, although theperidotite-hosted hydrothermal fluids generally exhibit higher CH4

and lower Si concentrations compared to basalt-hosted ones (e.g.,Charlou et al., 2002), the Kairei hydrothermal fluids have CH4 and Siconcentrations similar to typical mid-ocean ridge hydrothermal fluids(Gallant and Von Damm, 2006). This suggests that a basaltic host rockis required to explain most aspects the fluid composition, and that thereason for the unusually high H2 concentrations of the Kaireihydrothermal fluids is still uncertain.

In 2006, we discovered serpentinized troctolitic rocks (not typicalmantle peridotite) near the KHF (Kumagai et al., 2008), which couldbe related to the unusual composition of the Kairei hydrothermalfluids. Here, we show that seawater interaction with troctolitic rocksfollowed by recharge through basalt can cause the distinct chemistryof the hydrothermal fluids at the KHF.

2. Geological background

The KHF is located at the first segment of the CIR immediatelynorth of the RTJ (25°19.23′S, 70°02.42′E) with water depth of 2415–2460 m (Gamo et al., 2001). The hydrothermal vents are situated onthewestern slope of the Hakuho Knoll, which is an off-axis knoll of theCIR, ~6 km north-east of the ridge axis (Fig.1A, B). In January 2006, weperformed direct seafloor observations and rock sampling using themanned submersible Shinkai 6500 to develop a more comprehensiveunderstanding of the geological background of the hydrothermalactivity at the KHF (Kumagai et al., 2008).

Dive observations on the western slope of the Hakuho Knollrevealed that the knoll is entirely made up of pillowed and sheetedbasaltic lavas; no exposures of peridotitic rocks were observed(Kumagai et al., 2008). This result is consistent with that of previousinvestigations performed around the KHF (Van Dover et al., 2001;Gallant and Von Damm, 2006). Examination of seafloor morphologyon a larger scale, based on the SeaBeam bathymetry data, indicatessmall topographic highs (hereafter referred to as Uraniwa-Hills)~15 km east of the KFH (25°19.23′S, 70°02.42′E; Kumagai et al., 2008)

(Fig. 1B). The Uraniwa-Hills are two small hills, North Hill and SouthHill, which are divided by a 200 m deep NE–SW trending valley(Fig. 1C). These hills display distinct morphology from surroundingabyssal hills extending in ridge-parallel directions (NNW–SSE)(Fig. 1A). Two corrugation structures perpendicular to the ridge-axisare developed on the South Hill (Fig.1C). The North Hill is elongated inthe same direction as the South Hill, although corrugation structuresare less obvious (Fig. 1C). During our dives at the Uraniwa-Hills, werecovered troctolitic rocks which potentially represent exhumedlower oceanic crust. Although the uplift and emplacement processesof the deep-seated rocks are still unclear, the characteristic topo-graphy and exposure of deep-seated rocks suggest that the Uraniwa-Hills represents an Ocean Core Complex (OCC) (e.g., Blackman et al.,1998).

3. Methods

Chemical compositions of minerals were analyzed by an ElectronProbe Micro Analyzer (EPMA: JEOL JXA-8800 Superprobe) at Kana-zawa University. The analyses were performed under an acceleratingvoltage of 15 kV and beam current of 20 nA, using a 3 μm diameterbeam. Natural and synthetic mineral standards with JEOL softwareusing ZAF corrections were employed for all minerals. Further detailsof EPMA analysis were described in Morishita et al. (2003a,b).

Mineral-fluid reactions were calculated using the SUPCRT92database (Johnson et al., 1992), updated as outlined in McCollomand Shock (1998) andWolery et al. (2004). Reaction path calculationswere carried out with the aid of the computer program EQ3/6(Wolery, 1992). In the calculations, thermodynamic data for mineralsand aqueous species are also from the SUPCRT92 database, and log Kvalues were calculated by the SUPCRT92 program for temperaturesfrom 0 to 400 °C and a pressure of 500 bar. Activity coefficients foraqueous species were calculated using the B-dot equation (Helgeson,1969).

4. Petrography

We studied 24 olivine-rich gabbroic rocks dominated by troctolitesfrom the Uraniwa-Hills; one plagioclase dunite, 19 troctolites, and fourolivine gabbros. None of the rocks are typical residual peridotites,which would be expected from partial melting in upper mantleconditions, but they likely are constituents of the deeper parts of theoceanic crust and/or at the crust–mantle boundary (e.g., Dick et al.,2000). Similar lithologies have recently been reported from OCCs atthe Atlantic Ocean (along the 15°20′N Fracture Zone; Takazawa et al.,2007 and Atlantis Massif; Frost et al., 2008), suggesting that this kindof deep-seated rocks is major constituents of OCCs.

Both the plagioclase dunite and troctolite are mostly composed ofsubhedral to euhedral olivine (N90 and 75–40 modal %, respectively)and anhedral plagioclase with small amounts of clinopyroxene (b2modal %) and spinel (b1 modal %). All the samples are intensivelyaltered to serpentinites. Thin-section observations reveal that thesesamples clearlyexhibit a pseudomorphicmesh-rim texture (Fig. 2A, B),which is a characteristic microtexture of oceanic serpentinizationunder static conditions (e.g., Wicks and Whittaker, 1977). Olivine isextensively replaced by serpentine and magnetite, although olivinecores are preserved in some parts (Fig. 2A, B). The Mg-number(=100 Mg/(Mg+Fetotal) atomic ratio) of the olivine cores in theplagioclase dunite and troctolites ranges from 91 to 88 with mostvalues around 88, while that of serpentine is more variable rangingfrom 80 to 97withmost values around 95. Brucite, which is a commonmineral in oceanic serpentinites, is not observed, whereas trace talcreplacing olivine is present. The Cr-spinels are subhedral to rounded inshape and relatively unaltered compared to other igneous minerals.The plagioclase in the troctolites has experienced varying degrees ofalteration. The alteration phases occurred in plagioclase are mainly

130 K. Nakamura et al. / Earth and Planetary Science Letters 280 (2009) 128–136

Table 1Selected chemical and thermodynamic parameter.

Kairei 6 Edmond 12 Comment

Temperature (°C) 365 370 aFe2+ (mmol/kg) 6.0 13.1 alog aFe2+ −6.19 −6.17 blog aH2S −2.41 −2.33 apH (at 25 °C) 3.44 3.13 ain situ pH 4.42 4.17 bγH2 1.214 1.301 cObserved H2,aq 7.9 0.25 a, dLog K (reaction 1) −2.17 −2.20 ePredicted H2,aq 0.055 0.027 b, dLog K (PPM) −3.68 −3.64 fPredicted H2,aq 0.86 0.99 b, dLog K (methan) 8.59 8.35 gΔGmethanogenesis −20.0 57.3 b, h

a: From Gallant and Von Damm (2006).b: Calculated from data in Gallant and Von Damm (2006).c: Calculated (see text)d: In mmol/kge: =γ⁎10^[log K+2⁎pH+2⁎ log aH2S+log aFe2+]f: =γ⁎10^[(log K+2⁎ log aPo)/(4/3)]g: CO2,aq+4H2,aq=CH4,aq+2H2Oh: In kJ/mol

Fig. 2. Photomicrographs of the olivine-rich gabbroic rocks from the Uraniwa-Hills.(A) Mesh texture composed of serpentine and magnetite in a plagioclase dunite. Relictolivine crystals are partly recognizable. (B) Mesh texture composed of serpentine andmagnetite in a troctolite. Relict olivine crystals, as well as serpentine+magnetite vein,are also identifiable. (C) Olivine crystal partly replaced by serpentine and magnetite inan olivine gabbro. Clinopyroxene and plagioclase are relatively unaltered. Mineralabbreviations: Serp=serpentine, Mt=magnetite, Ol=olivine, Cpx=clinopyroxene,Pl=plagioclase, Vn=serpentine+magnetite vein. Scale bars represent 0.5 mm.

131K. Nakamura et al. / Earth and Planetary Science Letters 280 (2009) 128–136

composed of very fine-grained chlorite, prehnite with minor amountsof grossular/hydrogrossular and AlO(OH). Small veins consisting ofprehnite and chlorite commonly cut highly altered parts.

Fig. 1. (A) Bathymetric map, based on SeaBeam data, of the Central Indian Ridge (CIR), SouthwJunction (RTJ). The location of the Kairei Hydrothermal Field (KHF) is indicated by the star symbthe first segment of the CIR (CIR-S1). (B) Bathymetric map showing the Hakuho Knoll and Urolivine gabbros were discovered. The location of the KHF at thewestern slope of the Hakuho Knplagioclase dunite, troctolite, and olivine gabbro. Note that both the North and South Hills are

The olivine gabbros are also affected by serpentinization, althoughthe degree of alteration is less than in the plagioclase dunite andtroctolites. Most of the olivine (85–82 for Mg#) is partly replaced byserpentine+magnetite, with rare occurrences of chlorite adjacent toplagioclase.Microfractures in the olivine crystals are lined by serpentinetogether with fine-grained magnetite (Fig. 2C). Clinopyroxene andplagioclase are generallywell preserved in theolivinegabbroswith traceamounts of tremolite replacing clinopyroxene rims and plagioclase.

Paragranular serpentine±magnetite veins are also abundant in thesamples, most of which have magnetite (Fig. 2). This type of veining iscommonly observed in serpentinized abyssal peridotites (e.g., Andreaniet al., 2007) and olivine-rich gabbroic rocks (Frost et al., 2008), andlikely formed when much of the olivine was already serpentinized(Bach et al., 2006). In addition to serpentinization, the plagioclasedunite is subjected to later stage seafloor weathering, resulting in thedevelopment of brownish halos with Fe-oxyhydroxide.

5. Discussion

5.1. Origin of the high H2 concentrations in KHF fluids

The exceptionally high H2 concentrations in the KHF fluids areremarkable as other basalt-hosted vent fluids with similarly low totalgas contents have an order of magnitude lower H2 concentrations.Realizing that the major element chemistry of the KHF fluids requiresreactions with basalt, Gallant and Von Damm (2006) proposed amodelin which basalt–water interactions are essentially responsible for thefluid chemistry, while the unusually high concentration of H2 isgenerated by precipitation reactions of metal sulfide minerals, such as:

Fe2þ þ 2H2S→FeS2 þ 2H

þ þ H2 ð1Þ

Cuþ þ Fe

2þ þ 2H2S→CuFeS2 þ 0:5H2 þ 3Hþ: ð2Þ

We calculated the equilibrium concentration of H2 predicted forreaction (1) to assess ifmetal sulfideprecipitation is a viable explanationfor the observed H2 concentrations in the Kairei vent fluids. The H2

est Indian Ridge (SWIR), and Southeast Indian Ridge (SEIR) around the Rodriguez Tripleol. Note that the abyssal hills, usually parallel to the ridge axis, are largely disturbed aroundaniwa-Hills. At the Uraniwa-Hills, olivine-rich rocks of plagioclase dunite, troctolites, andoll is also shown. (C) Bathymetricmap of the Uraniwa-Hills, showing sampling localities ofelongated perpendicular to the trend of the surrounding abyssal hills.

Fig. 3. (A) Mineral-fluid phase diagram in the MgO–SiO2–H2O system as a function oftemperature and activity of aqueous SiO2 at 500 bars. Quartz saturation curve is shownas a dotted line. Composition of the Kairei hydrothermal solution is shown as white star,and that of the hydrothermal solution in equilibrium with troctolite is denoted by thedark gray star. (B) Change in concentrations of H2 and SiO2 (aq), as well as pH, in thehydrothermal solution as a function of weight of basalt encountered and reacted withthe solution. Calculations were performed at the P–T condition of 400 °C and 500 bar.

132 K. Nakamura et al. / Earth and Planetary Science Letters 280 (2009) 128–136

concentrations controlled by the PPM (pyrrhotite–pyrite–magnetite)buffer (2pyrrhotite+4/3H2O=1/3 magnetite+pyrite+4/3H2) werealso calculated. Calculationswere conducted for two vent fluids from theKairei and Edmond field that have similar temperatures (Table 1).Predicted H2 concentrations for reaction (1) are b0.1 mM in both fluids,while the PPM buffer is predicted to give H2 concentrations of almost1 mM. These concentrations are similar to experimental findings andtheoretical predictions for basalt–seawater interactions at 400 to 425 °C(Seyfried et al., 2002). Neither of the reactions can explain thehigh levelsof H2 in Kairei fluids (8 mM), and reaction (1) falls short of supplyingenoughH2bya factorof 140. These simple calculations suggest, therefore,that metal sulfide precipitation is very unlikely responsible for theunusually high concentration of H2 in the Kairei hydrothermal fluids.

Having ruled out the possibility that hydrogen-producing reactionswithin a basalt–seawater system can produce 8 mM H2, we nextexamine if seawater reaction with troctolitic rocks exposed in thevicinity of the KHF provides a viable explanation for the high H2

concentration measured in the Kairei hydrothermal fluids. It is wellknown that the serpentinization of mantle peridotites, which aredominated by olivine, generates extremely H2-rich hydrothermalfluids (e.g., Berndt et al., 1996; Wetzel and Shock, 2000; Allen andSeyfried, 2003). Although the troctolitic rocks from the Uraniwa-Hillsare not typical mantle peridotites, this process could also provide theH2 to the Kairei vent fluids, if these rocks contained enough olivine toproduce H2 during their hydrothermal alteration. To investigate thehydrogen-generation capacity of troctolite, we conducted geochem-ical reaction path model calculations of troctolite–seawater interac-tions at 400 °C and 500 bar, using a bulk chemistry of typical Uraniwatroctolite calculated based on typical primary modes of 75% olivine,b25% plagioclase, and b5% clinopyroxene (Table 2). Because allplagioclase grains were altered in the troctolite samples, the anorthitecontent of the original plagioclase was assumed to be An80(plagioclase phenocryst composition of primitive CIR basaltic rocks).Contributions from other phases (spinel, orthopyroxene) wereneglected from the calculation because of their low abundances. Themodel result shows that serpentinization of the Uraniwa troctolite at awater-to-rock ratio of unity can produce more than 16 mM of H2,which is similar to H2 concentrations in vent fluids issuing frombasement dominated by peridotite (Charlou et al., 2002; Schmidtet al., 2007).

5.2. Troctolite–seawater interactions — observations and models

The serpentinization of olivine producing hydrogen is usuallywritten as the reaction:

ðMg0:9Fe0:1Þ2SiO4þ41=30H2O¼ 1=2Mg3Si2O5ðOHÞ4þ3=10MgðOHÞ2þ1=15Fe3O4þ1=15H2 ð3Þ

olivine þ water ¼ serpentine þ brucite þ magnetite þ hydrogen

In our samples, however, olivine crystals in the troctolitic rocks arepartly to completely replaced by serpentine+magnetite, whereasbrucite is essentially absent. Brucite is unstable at high-silica

Table 2Composition of rocks used in reaction path models.

wt.% Troctolite Basalt

SiO2 42.3 51.5Al2O3 6.4 16.1FeO 8.3 8.4MgO 39.0 8.5CaO 3.6 11.4Na2O 0.4 3.0

activities, such as imposed by an external source of silica, accordingto the reaction:

3MgðOHÞ2 þ 2SiO2ðaqÞ→Mg3Si2O5ðOHÞ4 þ H2O ð4Þ

brucite þ aqueous silica→serpentine þ water:

The absence of brucite in our samples may therefore imply that it wasreacted out by high-SiO2 activity fluids produced during plagioclasereaction. Taking into consideration the absence of brucite as well aschemical compositions of olivine and serpentine, the serpentinizationreaction of the troctolitic rocks could consequently be considered tobe:

2:85ðMg0:88Fe0:12Þ2SiO4 þ 0:67SiO2ðaqÞ þ 3:66H2O

¼ 1:76ðMg0:95Fe0:05Þ3Si2O5ðOHÞ4 þ 0:14Fe3O4 þ 0:14H2 ð5Þ

olivine þ aqueous silica þ water ¼ serpentineþ magnetiteþ hydrogen:

The presence of small amounts of talc replacing olivine is consistentwith such a scenario, because talc is stable only at relatively high-SiO2

activity conditions (Fig. 3A).The presence of plagioclase could cause relatively high SiO2

activity in the reaction fluid during alteration of the troctolitic rocks,as well as formation of observed chlorite-rich coronas at the olivine–

Fig. 4. NiO vs. forsterite content of olivine in plagioclase dunite, troctolite, and olivinegabbro samples from the Uraniwa Hills. Mantle olivine array (Takahashi, 1986) andcompositional range of mid-ocean ridge basalts from FAMOUS segment on the MAR (leRoex et al., 1981) are shown for comparison. Note that olivine Ni contents of the olivine-rich gabbroic rocks are significantly lower than those of typical mantle peridotite, butsimilar to those of mid-ocean ridge basalts.

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plagioclase contacts. The replacement of plagioclase by prehnite takesplace at silica activities where talc and not serpentine is stable (Bachand Klein, 2008; Frost et al., 2008). The observed breakdown ofplagioclase and olivine to prehnite and chlorite

CaAl2Si2O8 þ 1:25Mg2SiO4 þ 2:5H2O¼ 0:5Ca2Al2Si3O10ðOHÞ2 þ 0:5Mg5Al2Si3O10ðOHÞ8 þ 0:25SiO2ðaqÞ ð6Þ

anorthiteþ forsterite þ water ¼ prehniteþ clinochloreþ apueous silica

is expected to proceed at low silica activities, while it acts as a sourceof silica. The equilibrium silica activities of this reaction are muchhigher than that of forsterite–talc equilibrium.

Mg3Si4O10ðOHÞ2 ¼ 1:5Mg2SiO4 þ 2:5SiO2ðaqÞ þ H2O ð7Þ

talc ¼ forsterite þ apueous silica þ water

It is plausible to assume, hence, that this reaction will cause localreplacement of olivine by talc such as observed in Uraniwa Hilltroctolites. The presence of grossular, however, strongly indicates thatlow silica activities also played a role during serpentinization.Grossular forms only at very low silica activities (much lower thanforsterite–talc equilibrium), such as when plagioclase-dominatedlithologies interact with serpentinization fluids in which silicaactivities are buffered to very low values by brucite–serpentineequilibrium (Frost and Beard, 2007; Bach and Klein, 2008). Theserelations indicate that a range of SiO2 concentration in the reactionfluid affected the Uraniwa Hill troctolites.

The replacement of plagioclase by grossular is best explained iftemperatures were below 320–340 °C, because brucite is not stable athigher temperatures and thus the aqueous silica activity of theolivine–serpentine buffer increases strongly with temperature(McCollom and Bach, 2008). Brucite was probably reacted out (ordissolved by seawater) subsequent to the grossular formation.Temperatures are higher in the reaction zone of the KHF fluids, sothat a direct relation between the secondary mineralogy of thetroctolites and the vent fluids is not apparent. With reconstructedfresh rock compositions, however, we can use model calculations toexaminewhat the equilibrium phase relations in a seawater–troctolitesystem are at 400 °C and how the model fluid composition compareswith that of the vent fluid. The model results suggest that there is~1 mM aqueous SiO2 in the reacting fluids, which is slightly higherthan those in the peridotite–seawater calculations (0.5 mM onaverage; Wetzel and Shock, 2000). Note, however, that the SiO2

concentrations observed in the Kairei hydrothermal fluids (~17 mM),which almost reach quartz saturation, are much higher than theconcentrations predicted in the troctolite–seawater reaction model(Fig. 3A). So while observed and predicted troctolite–seawaterreactions can explain the high H2 concentrations in the KHF fluid,they fail to account for the high SiO2 concentration. This implies thatafter the interaction with the troctolites, SiO2 must have been addedsomewhere along the fluid flow path of the Kairei hydrothermalsystem.

5.3. A hybrid model of KHF vent fluid chemistry

We have discussed that basalt–seawater interactions can explainthe concentration of major dissolved components of the vent fluids,while troctolite–seawater interactions can explain the remarkablehigh H2 concentrations of KHF fluids. The Kairei hydrothermal field islocated within basalt. It is therefore likely that the vent fluids interactwith basalt on their way to the seafloor. Perhaps the fluids rechargedthrough troctolite and discharged through basalt and the vent fluidcomposition is a witness of both stages. To test if such a scenario canexplain the fluid chemistry, a geochemical reaction path model, using

the whole-rock chemical compositions of troctolite and basalt(Table 2) was computed. First seawater was reacted with troctolite,while increasing the temperature to 400 °C (water-to-rock mass ratioof 1), then the resulting fluid was progressively reacted with basalt(final water-to-rock mass ratio of 1). The results of these modelcalculations (Fig. 3B) show that reactions with only small amounts ofbasalt (b5 g basalt per kg fluid) can increase SiO2 in hydrothermalfluids that previously reacted with troctolite close to quartz saturation(Fig. 3B). This suggests that even limited interaction between thefluids and the basaltic wall rock of the KHF could produce the high-SiO2 content of the Kairei hydrothermal fluids. H2 is not predicted todecrease notably during interactions with small amounts of basalt in ahydrothermal upflow zone. The proposed hybrid model can henceexplain both high SiO2 and high H2 concentrations.

5.4. Implications for low methane concentrations

One further peculiarity of the KHF fluids remains to be explainedtheir low CH4 concentrations. Typical peridotite-hosted hydrothermalsystems are known to exhibit high CH4 concentrations (around 2–3 mM) along with the H2-enrichment (Donval et al., 1997; Charlouet al., 2002). In striking contrast, CH4 concentrations of the KHF fluidsare just as low as in the nearby basalt-hosted Edmonds HF fluids, thathas a factor of 50 less H2 (Van Dover et al., 2001; Takai et al., 2004;Gallant and Von Damm, 2006; Kumagai et al., 2008). In H2-richperidotite-hosted hydrothermal systems, CH4 is considered to beproduced by reduction of CO2:

CO2 þ 4H2→CH4 þ 2H2O: ð8Þ

It is known that the reaction kinetics for the reduction of CO2 to CH4

are sluggish under typical hydrothermal conditions (Seewald et al.,2006). Therefore, the Fischer–Tropsch type (FTT) or Sabbatiersynthesis with metal alloys and/or oxides may play a critical role incatalyzing of the formation of CH4 in hydrothermal fluids (e.g., Horitaand Berndt, 1999; Foustoukos and Seyfried, 2004). Ni–Fe alloys, suchas awaruite, appear to be an excellent catalyst for CO2 conversion toCH4 during serpentinization of peridotite (e.g., Horita and Berndt,1999).

Typical mantle peridotite is known to contain considerableamounts of Ni, mainly residing in olivine crystals. In contrast, Niconcentrations of olivine in troctolite and olivine gabbro samples aresignificantly lower (Fig. 4). Because the amounts of Ni–Fe alloy directly

Fig. 5. Phase diagram for the Fe–Ni–S–O system at 400 °C and 500 bars (Klein and Bach,2008). Fe phases are hematite (Hem, Fe2O3), magnetite (Mt, Fe3O4), pyrite (Py, FeS2),and pyrrhotite (Po, Fe1−XS). Ni- and Ni–Fe phases are awaruite (Aw, Ni3Fe), bunsenite(Bu, NiO), millerite (Mi, NiS), heazlewoodite (Hz, Ni3S2), and pentlandite (Pn, (Fe,Ni)9S8). Light and dark gray circles define the compositions of hydrothermal solutions inKairei and Rainbow hydrothermal fields, respectively. Possible aH2–aH2S conditions fortroctolite–seawater and peridotite–seawater reactions in early stage of the serpenti-nization are shown as light and dark gray dotted stars, respectively. Dotted arrows arethe inferred evolution paths of Kairei and Rainbow fluids.

134 K. Nakamura et al. / Earth and Planetary Science Letters 280 (2009) 128–136

affect the reaction kinetics of the reduction of CO2 to CH4 (Horita andBerndt, 1999), the lower concentrations of Ni would reduce thecatalytic role for the FTT synthesis in the Uraniwa-Hills rocks, resultingin the lower concentration of CH4 relative to H2 in the KHF vent fluids.In addition, Ni–Fe alloy is stable only at the restricted conditions of verylow fO2 and total activities of the sulfur species (Frost, 1985; Klein andBach, 2008). Assuming that fO2 is mainly controlled by SiO2 activityduring serpentinization (Frost and Beard, 2007), relatively high-SiO2

activity of serpentinization of the troctolitic rocks (as stated above)could result in relatively high fO2 conditions compared to serpentini-zation of typical abyssal peridotite. The troctolitic rocks are alsoexpected to have higher S content than typicalmantle peridotite due toits high incompatibility (Puchelt et al., 1996). Both high fO2 and high Sconcentrationswill lead to relatively high sulfur activities predicted for

Fig. 6. Schematic representation of the hydrothermal system at the KHF. (1) Hydrothermal reunusually H2-rich, but CH4-poor, hydrothermal fluid. (2) Subsequent reaction with basalthydrothermal fluid.

the alteration of troctolitic rocks. Ni–Fe alloys cannot develop underthese conditions and instead, Ni-sulfides (heazlewoodite and mill-erite) become stable (Fig. 5). This can explain the absence of Ni–Fealloy in the serpentinized Uraniwa troctolites. It is possible thatawaruite, although an ultratrace accessory in metaperidotites, plays apivotal role in catalyzing methane formation from CO2 reduction, sothat methane did not form in significant amounts during fluidinteractions with troctolite, in which awaruite is apparently notpresent. Alternatively, the residence time of the fluids in the hightemperature reaction zone was too short to allow for significantmethanogenesis.

The petrological and theoretical considerations lead us to proposea fluid path model to explain how the Kairei hydrothermal fluidsevolved (Fig. 6). Seawater is recharged into the oceanic crust at aroundthe Uraniwa-Hills, and the subsequent hydrothermal reactionwith thetroctolitic rocks produces hydrothermal fluids with high H2 but lowCH4 concentrations. The hydrothermal fluid reacts further withbasaltic rocks underlying the KHF while discharging, and becomesenriched in SiO2. These processes in the Kairei hydrothermal systemproduce the distinct chemistry of the hydrothermal fluids.

6. Conclusions

Serpentinized troctolitic rocks discovered from the Uraniwa-Hills,~15 km east of the KHF, provide key insights into the origin of unusualchemistry of the Kairei hydrothermal fluids. High concentrations of H2

in the Kairei fluids are attributed to serpentinization of olivine in thetroctolitic rocks. Alteration of plagioclase in the rocks release SiO2 intofluids during serpentinization, which could result in relatively highfO2 condition compared to serpentinization of typical abyssalperidotite. The higher fO2 condition, as well as high sulfur activitydue to high sulfur content of the troctolitic rocks, could in turn hinderthe occurrence of Ni–Fe alloys that is known as excellent catalysts forthe FTT or Sabbatier synthesis. This could lead to a drastic suppressionof CH4 formation, resulting in the unusually low CH4/H2 ratio of theKairei hydrothermal fluids. The hydrothermal fluid produced by thereaction of the troctolitic rocks further reacts with basaltic wall rocksunder the KHF, becoming highly enriched in SiO2. It is concluded thatcombination of the troctolite-alteration at recharge and reactionzones near the Uraniwa-Hills and subsequent basalt-alteration atdischarge zone under the KHF causes the distinct chemistry of thehydrothermal fluids that support the unique microbial ecosystem.

action of circulated seawater with the troctolitic rocks in the Uraniwa-Hills results in theic rocks underlying the KHF could cause the high concentration of SiO2 in the Kairei

135K. Nakamura et al. / Earth and Planetary Science Letters 280 (2009) 128–136

Acknowledgements

We are very much indebted to A. R. L. Nichols for his careful reviewof the manuscript. We also would like to thank Y. Imai and the Shinkai6500 operation team and M. Ishiwata and the crew of the R/VYokosuka for their skillful support, and the shipboard scientific partyfor their invaluable collaboration during the YK05-16 Cruise. Con-structive comments by Jean-Luc Charlou and B. Ronald Frost greatlyimproved the quality of this manuscript. This study was financiallysupported by the Grants-in-Aid for Scientific Research from theMinistry of Education, Culture, Sports, Science and Technology ofJapan (MEXT) (No. 14253003), JAMSTEC multi-disciplinary researchpromotion grant entitled “An interaction of mantle and life throughEarth history”, and the Program for Improvement of ResearchEnvironment for Young Researchers from Special Coordination Fundsfor Promoting Science and Technology, commissioned by the MEXT.

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