Petrology of Serpentinites and Rodingites in the Oceanic Lithosphere

Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften am Fachbereich Geowissenschaften der Universität Bremen

vorgelegt von Frieder Klein

Bremen, 2009

Referent: Prof. Dr. Wolfgang Bach Koreferent/in: Prof. Dr. Cornelia Spiegel Tag der mündlichen Prüfung:…………………… Zum Druck genehmigt: Bremen,...... ……………

Der Dekan

Erklärung

Hiermit versichere ich, dass ich

1. die Arbeit ohne unerlaubte fremde Hilfe angefertigt habe,

2. keine anderen als die von mir angegebenen Quellen und Hilfsmittel benutzt habe und

3. die den benutzten Werken wörtlich oder inhaltlich entnommenen Stellen als solche kenntlich gemacht habe.

Bremen, den

Anmerkungen des Verfassers zur vorliegenden Dissertation

Die vorliegende Arbeit stellt zwar eine monographische Dissertation dar, die einzelnen Kapitel, denen die Einleitung vorangestellt ist, sind jedoch bezüglich ihres Aufbaues so konzip- iert, dass sie unabhängig voneinander publiziert werden können bzw. publiziert sind. Durch diesen Umstand ist es zu erklären, dass jedes Kapitel nochmals eine eigene Einleitung, Diskus- sion und ein Literaturverzeichnis enthält. Auch beim Schreibstil, dem Umfang, der Verwend- ung von Abkürzungen sowie der Formatierung von Abbildungen und Tabellen wurde Bereits den Anforderungen unterschiedlicher Fachzeitschriften Rechnung getragen. Diesen Umstand möge der Leser berücksichtigen.

Bremen, März 2009 Frieder Klein Table of Contents

Zusammenfassung 1 Abstract 3 Prologue 5 Outline 8

1. Introduction 11 1.1. Serpentinized peridotites at mid-ocean ridges 11 1.2. Serpentinized peridotites at active oceanic margins 14 1.3. Hydrothermal systems and serpentinized peridotites 14 1.4. Mineralogical and petrological aspects of serpentinization 16 1.4.1. Serpentinite textures 16 1.4.2. Serpentinization - an isovolumetrical process? 17 1.4.3. Some crystallographic basics concerning serpentine 18 1.4.4. A note on the mineral chemistry of serpentine and its value as a geothermometer 19

1.4.5. The MgO–SiO2–H2O (MSH) system 19 1.4.6. Redox conditions during serpentinization 20 1.5. Rodingitization 24 References 25

Abstract 37 2.1. Introduction 37 2.2. Geological setting 40 2.3. Analytical methods 41 2.3.1. Microscopy and electron microprobe analysis 41 2.3.2. Thermodynamic calculations 42 2.4. Results 46 2.4.1. Petrography 46 2.4.2. Mineral chemistry 51 2.4.3. Phase diagrams 54 2.5. Discussion 59 !"!#"$ \#& ' 2.5.2. Redox conditions during serpentinization 60 2.5.3. Redox conditions during steatitization 61 2.5.4. Implications for a potential H2S,aq buffer in serpentinite-hosted hydrothermal systems 62 2.5.5. Sulfur metasomatism 63

2.5.6. Possible existence of a free H2-rich vapor phase 66 # * *+&/" : References 69

3. Iron Partitioning and Hydrogen Generation During Serpentinization of Abyssal $" <+/*: Abstract 78 3.1. Introduction 78 3.2. Analytical methods 81 3.2.1. Microscopy and electron microprobe analysis 81 3.2.2. Mößbauer spectroscopy and magnetization measurements 82 3.2.3. Geochemical modeling 82 3.3. Results 85 3.3.1. Petrography 85 3.3.2. Mineral compositions 87 <%@#!!B@#&"/C ' 3.3.4. Geochemical reaction path modeling 92 3.4. Discussion 103 3.4.1. Serpentinization at Hole 1274A and geochemical reaction path models 103 E//!"@/#/!C FE 3.4.3. Fe+2+3 exchange equilibria in serpentinites 104 3.4.4. Geochemical reaction path modeling and serpentinization experiments 106 3.4.5. The formation of brucite and serpentine in mesh-rims 108 # +&/" References 113 Appendix 119

EL!/B$\/N/$"/"! modeling 120 Abstract 120 4.1. Introduction 120 4.2. Method 122 4.3. Results 126 4.3.1. Reaction path models 126 4.3.2. Phase diagrams 137 4.4. Discussion 138 4.4.1. Modeling of rodingitization 139 4.4.2. The critical role of aqueous silica 141 4.4.3. Mass transfer by diffusion or advection 142 EEE"[U#"[EE EE+!V$/"!#\# circulation? 145 E# E E+&/" E* References 148

!CW!XYB"[ /N//$XB"\# supporting a unique microbial ecosystem 155 Abstract 155 5.1. Introduction 155 \/@&/# : 5.3. Analytical methods 158 5.4. Petrography 159 5.5. Discussion 161

5.5.1. Origin of the high H2XY\# LM@V" +B@"$XYV\#"B * E"!$" : # ' *+&/" ' References 171

^&/#/ *

_!!$!"!&`^j Zusammenfassung

Die Serpentinisierung von Peridotiten erzeugt große Mengen von Wasserstoff. ^#CVM{/#/#$"#[[ C|+##[@/}WC/_VC# /"_</#!^"!"/_@# von Sauerstoff in Magnetit und Serpentin die Freisetzung von Wasserstoff. Wir haben "B"^VMMMMC#""//#|- f f ziehung in O2,g– S2,g und aH2,aq–aH2S,aq Diagrammen für Temperaturen von 150 bis EFF #"^#&VF</^|C#/#~#""- C#/MMMM##/[#"#/#$$ #$$#/C@$!#/##/V #"[\<&` FR#~[- ^//"`^j[{/F'jVCC#&^!/! Beobachtungen offenbaren eine systematische Abfolge von Mineralvergesellschaftungen +@//& C#" !#// \ ^ /+# Pentlandit + Magnetit bildet sich in partiell serpentinisierten Gesteinen. Die Paragenese <BBB"//@\+# ##%|#C$!@@$$$ #$$#/V/X#&VC#C#//%</}- $$#"C#@#/V+#~#"""_$#/ V#[#"\$"\/CC#$- X#&V#$$#/C#/C# @#/V$#[[C|BB"#${#/ VB#[#$V/#&&!C\C#&- C#$ | |/#/ #[ $[ #/$[CC#"+/$/"\$^ f f _&#/V O2,g und S2[/!#/$/Y2S,aq Iso- !#"!/"##/[$$#/C- YB"\##"[B"/!#$$

^ C#/ V }$$ !#/ " <%V\"/C#""C#/[V"}\# L"!#@//^|C#/@#~#$"V/- "&!$"`""!#!/""_Uj$# und harzburgitische Gesteinszusammensetzungen untersucht und die Modellergebnisse " <& # <%@#!&&!+B V ! @ C# &"!!^##YC@#/^{/F'V/^ \&+@$/V<V/$#/<- #$~<[C# V[@[|#C`

1 #/C/[@@L"!#VF #}&V- #"[{#/VV#/C/|#/V![</- #}$$WX##@/|L"!# }$$&VC#//[#"|#//+# C#"/@L"!##FMF &[|#C@# _#/V}$$[&VVC#![</ #|#C&WX##$^+##/</@- nisse und deren Vergleich mit MgO–FeO–Fe2O3–SiO2–H2O Phasenbeziehungen in Ma- #$|#/"!#!#|#C|#/ *E+VF@F ^@}\V/ C # F ^+##/ !/! # !!/ +BC/[!#/|#C/@@V! @|#C@"@#$XVV"@$V ![@"B"L@&$$|#//! |#C#/%#C/#[&@$/ !#/"^$&["!/!- ["/#!&&!##/"#"$"B- "<#/&"@

"#_&@|#/V/\C\V "[##"[\C#/[@"B"- &!$"#$/L"B"&!$""/ #/#/V#\\/"~#//- #/@VCC#&LB!V/$#//- `\#^!j$L"!#VFF#FF V- //^/$#//#[#&#"V &"\@@@\#<C#"&C!- #/\##\@@V/$#/V\# X!BWC##L"@C_!#L"|#/ C/[|#/V<V/$#/##"- #/@V/C@V/V\- #X#&V/@^ <$VXC#"#^$$#Y+ Spezies "/[XC/!C\CV "[C##"[\//XC#"$[ @V\#^!@/X#&V@//

^#XYB"$`XYj`~&j- XCV}$$#X#[&Y4UY2 #$_"/_&#/$#/~#""C#/ #XY&V<"L&[ des KHF vorhanden sind, dar. Petrographische Untersuchungen offenbaren, dass Olivin @V/!#[#$_#/V}$$ <#/C/[}$$#X#- &CYB"\##$!#/ XY@[L&#%B"+"| #@XY"/C#&C#$

2 Abstract

Serpentinization of peridotite generates large amounts of dihydrogen (H2,aq), in- @B!$MB#$#$#/B#[[/# ![!YB/!#$#V WC@B$B"/!}V"- !V#"B"$MMMM!"!#! f f relations in O2,g– S2,g and aH2,aq–aH2[/"$"!#@F EFF F<}#"!$MMMM! trace changes in oxygen and sulfur fugacities during progressive serpentinization and C$!$"<+/ FR#~ `^//"[{/F'j/!@VB"- /$"#"/!C"/ pentlandite assemblages forming in the early stages of serpentinization to millerite + !B!BB"""@/C&+#W#VB @V@#@/!B!C&+!!B[@#$$/$- VV#@B!$@#$$"$/"# $B/[$"$#+!" #$#C$![#["V$"&#// of serpentinization. In contrast, steatitization indicates increased silica activities and that /#$# $#/B #[[ # !BB" !B V #[ form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under [#[#$#C@#!!#$#$ f f &LV#$ O2,g– S2[/B"$!$ H2S,aq, indicating that H2V\#@#$$

YB//#/!C/B!@#&& "![&"!#LW"- "B"!"`#/_U"!#j#- C@#/&"!L"#"!"!@ B[@#&"/C"#"[<%@#!!B$!B $#B!C#C@#/$"^//"{/F'[< +/ L"!V""V#C- /[/@#`olivine and coeval formation of serpentine, magnetite and dihydrogen requires an external source of silica. At these temperatures, hydrogen fugacities are too $#!@@}"!#!@FMF [@#@"@B//$[@#$ olivine to serpentine, magnetite and brucite requires no external silica. The MgO–FeO– Fe2O3–SiO2–H2O phase relations observed in the mesh rims indicate that serpentine and @#$"Y*E+&B$""!#@FF -

3 &@F_V#/!/!!!/ #@#$"B"@@B[$! #"B"V/$$!$"#$[@C the assemblage serpentine + brucite. Our study indicates that unprecedented details about the reaction sequences during serpentinization may be obtained from merging careful !/!["/[!!B"!V"B" modeling.

L"B"!"!!!V/ $"$/"[U#"[@#L"#!- V/ $ \#M& #@ \# "V $" !#// serpentinization into a gabbroic body. Phase assemblages typical of rodingite (grossular !/j!$"FF FF [@#B \#B#$$@B/@@+\#@"" $$@B/@@[!!!/" replaces clinopyroxene. Our model results support the hypothesis that rodingites form #/!CB\#@B!C- actions are present. Our calculations further indicate that the formation of mineral as- "@/!"/V@B/$"\ "BV@B!VB/!##!\# <$$#"&B@B$$#$Y+![! VB!/"[M#"[@#B/- #"!@#$/@B$!/ activities.

LV\#$XYB"`XYj /V/$B/[Y4UY2 ratio. We sug- / $ V XY[!V !@W!$"!$XY\#/!BV V!B"!B!C[//$Y2. Model calculations predict that high H2$B"\#@ attributed to serpentinization of the troctolites and subsequent hydrothermal reactions @&#XY

4 Prologue

"[&"&#!_R!/!$- !_"/"!$&R#$ "[&$#B#$#`+#[ '*j[!`/[+@[':'+WY![''"[ '_VL""$$['*FB[FF:{#['*{#_[ '*'Y!['::['*Ej[/"/`/[+#- "{#@['*|[FFE|['*E['' [':*['E'j["/`/[+/[''|[':'j $/`/[^R+X[FFEB<['' [FF&[''Fj B#!!#V[B#$[#"[&#- //B@B[/LB$V! `C@#/[C#j["@##"[&["!- ##V#"!VV!"$V !BW@B"@/"@B!/#!"`"[ '[':<B[''<B['*/['*['*' $[':}&}&['**j Serpentinites contain minerals and mineral assemblages that occur almost no- _`|[FF*j!/#!"[ the alteration assemblage consists mainly of magnetite, and brucite or talc (depending on !"!"!#j[""#$"[[ !UB/B#$##["@/[/[- #[[!C[B$#"[- #"#!`+@['*:^&['*E_&['* [':&[''"['Fj L"/!#$!\@B$ /\#!$"W"/"V" _YB"B"\#@B!CC@B/B reducing conditions, caused by the generation of copious amounts of hydrogen during

&YB/#2 to methane, and both gases are extremely !C\#/!Y$"/"!# "!#[VVBV`+@['::+B- $[FF|['*^#V[FF[':|[FF* <"|[FF'/[':j+V!CB" V@$#@`/[/"!`| ['*:jj/`/[{/V`|/V[''*j[@`#- #[''*j[{B`XB[FFj[#""#`<[ 2003)).

5 ![@"@/B@/B communities arose in serpentinization because the high H2Y4 concentrations can support microbial communities in surface and subsurface environments of such ultramaf- B"B"`/[+&['':+[FF*XB[ FF<"[FF*[FFL&[FFEjL"/" "#!`B[W/@#/@#$ chemical energy) that thrive independent of photosynthesis and have served as analogue $B"$"BVVV_!`![ FF<"|[FF'<"['''#[FFV <XB[''L&[FFEj Serpentinization also affects global geochemical and geodynamic processes, as it "$#@#/@`X&[FF$j Serpentinization has a large impact on petrophysical characteristics of the oceanic !&$"W!"`_[`''*jL authors propose that the presence of serpentinite can reduce the integrated strength of !@B#!F<V[_`''*j!# C#@@B!C[W!V B$"$#//!///" #B!C!VB@#/"/#!- @B"!!L$`''FjV"B M"/#!@B//[BV#$!C[ and suggested that serpentinization is rather a sequence of mineral reactions. The actual \#&!B$!C!B#| `|[FFj!!![#$!C "[!C$VV!"$B!"W# !@#""[$@B!"$V"@B serpentine, brucite and magnetite. Furthermore, they pointed out that initial serpentiniza- $V!$@#\#\#W$@B/ of serpentinization under more open-system conditions and formation of magnetite by the @&$$@#L#B@"!#$ V#$!!B!!$![W/\#- "$/\#!#@$"- &MB#[L#|`FFj!! ####&B/!!W"$!C general. YV[$BC/$!#@# !/$@!["/@#[B$ pure brucite of serpentinites from mid-ocean ridge settings are available from the literature. In addition, the determination of oxidation state of iron in serpentine and brucite in ##!"!$+3 in serpentine on hydrogen generation dur- 6 ing serpentinization. This could be accomplished by systematic electron microprobe and Mößbauer spectroscopic analyses of brucite and serpentine in pseudomorphic mesh rims. Furthermore, the Fe+3 component of serpentine has never been considered in a geochemical reaction path model. The implementation of the Fe+3-serpentine component (ther- "B"@##/!B#"!!@B"& "[':'j/"!"#/[B"!V !V!//W#/!CB/ $V/@!!#/\#&#@ #/! }!CM@$M" !["@B&V$"""W"! [#!VV!C$@B!"$!- @B`/[|[FFE^RC[FFE^[':jC- ""!VV"/$\##/!- L\#!!@B#$/@@ #"&#!@#$!"W!!/ /`/[X"[FF*jL$"$#$##[[& !B[VV[/#["!B "!BC&"B@!$/@@/`+ [FF*#&[FFj +""""&!C!- / L & "!B VB ""C /@@ V$"V!C$##/!`"['[ '*RYB[''[':'jL"""@ V\#@"/B#/- pentinization (e.g., Honnorez and Kirst, 1975). An equally common – but apparently less !!M$#$/C!`"['[ FF:j$!"`/Vj particular drives rodingitization. !C[C[/C#$ !"L!&!VB !#!B[/"&#!$&/ \![\#\#W["!#}@#B@B- B$"/"$![![\#&#@ "&!B"// "$#"!\#&@ #!B!!$!V! &@#!V//B$!"@B" !$!BYB/$"#/!CY "#$$"!!/BV/#B/$

7 /#\#}B"/@@V/C[ What is causing steatitization? These are basic questions in oceanic petrology that have not been comprehensively addressed. The purpose of this thesis is to change this.

Outline

The primary focus of this thesis is on utilizing phase relations in reconstructing \#&!B#/!C[C/C $@B& ![@#!L! retrospective character, focusing on the research about abyssal serpentinites and leaving #"!VV$!`$$ WW@&@BRYB`''jj ![![@"&$_&`'*j `':j[!$![OMMMM! !MP["!$ MMMM!#//WB/#$#$#/ during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic / FR#~`^//"[{/F'j L#!$"B/@&"B#!V$^ Wolfgang Bach. He introduced me to the construction and interpretation of activity–ac- VB$#/BM$#/B/"$/"!#!#[ !"!$!"@[V#"!"&/ "B" $ # ! @ $ $ - L'`[''j\"R}&@&`\}|&j`|&[''j and did all the petrography in our microscopy laboratory at the University of Bremen `\"Bj

Chapter two is already published as:

8 X[|[}`FF'jMMMM!!M- [#$/B[F[[!/*M'[NFF'U !/BU/F*

![O!/B//#/!- C$@B!$" <+/O$#! relations in the system MgO–SiO2–FeO–Fe2O3–H2O and uses reaction path models to $#B\#"#@/"! V!"$\#"#/!CL!!![ analyses of brucite from a mid-ocean ridge setting. It concentrates on the distribution and redox state of iron in serpentinites and its implication for redox equilibria during serpentinization. Electron microprobe, magnetic and Mößbauer spectroscopic analyses of primary B!V@B!C!#["!- ment, correlate, and improve iteratively our geochemical and phase petrological models. L!$!@#/"BV# "B#!V[$^}$//|L#@B"B$ /#^`V|"j@B^L" <"`VB$[+j!/B"!V#B$ "#!L<%@#!!B"/B#@B ^L"|#$^|#<&C#$&</" (Department of Geology and Geophysics, University of Minnesota, USA).

Chapter three will be submitted to Geochimica et Cosmochimica Acta:

X[[|[}[[[<"[L[<&C[|[|#[L`@ submitted) Iron partitioning and hydrogen generation during serpentinization of @B!$" <+/

L$$#![OL!/B$\/N /$"/"!"/PV!@"$V/ $$/C@BW"/\#M"!@#B C$"[#"[/L"!@B! "["[&\#@#$$B@B!C- tions. L!$!!/!"B@&"B#!V$ Dr. Wolfgang Bach. We discussed almost every topic from the very beginning, I helped

9 calculating the reaction paths, made the illustrations and edited the manuscript.

Chapter four is already published online:

|[}X[`FF:jL!/B$\/N/$" /"!"/{[NFFUFF:FF

L$[$![O!CW! XYB"[/N//$X- B"\##!!/##"@B"P[W!/ $###"B$\#V/$"XYB"`XYj YB$[V!/#"#&[!#! magnetite, similar to serpentinization of peridotite. Alteration of plagioclase buffers aqueous silica at relatively high levels, preventing the formation of brucite. The higher silica VB!"/WB/$#/B[#@#$$#$#$#/B /V[!V/$"$VMBYV[!C $!!B#$[B/#!!B/@B!- "!#@#$#!"@B"#!/# #[B&L"!##$!! !V!#@W!$###"B$V\#XY} !VW!/V@B!V#&"&B

&"#[X[<[L[|[}[X[[Y[X[&[X[L&[X and Kumagai, H. (in press) Serpentinized troctolites near the Kairei Hydrother- "_B{

10 1. Introduction

1.1. Serpentinized peridotites at mid-ocean ridges

! V " $" \& W! !#[ / !&`+/[j[#/["[B!!- $!`$[$&jL"#$# !C"#/@&"FB/`/[Y/[ 1845). All the early and the majority of the more recent serpentinite research has been #!W!<#B@B$Y- /R!!!#@@$VB$@B!C!- `@B // <+ / 'E*j ! `[ 'E'j L"![BV$"@$" `+#"{#@['*jV#/"#"# $V@B!C!@V$"#! /}W!`|#''F\[''j[! V#BW$!/$[!!- @V@$!/`RYB[''jYV[ independent from their provenance all these serpentinite recoveries contributed to the #&/@## L$!!C!/@& `Y['j[!!<Y$" "B/B!B!C!YR"!- C#@B#/[@###"""$O!- "/"P`'*j@#B`'*j the seismic properties of partially serpentinized peridotite did not match those of layer 3 `/#$V$/@@#YV[$@ $!///"!"$$"$ #[//""#B#/'*$- !#`$!['*j|//- "!!/![<@B&##$ the oceanic crust. Beneath a sediment cover (layer 1), the oceanic crust is composed of "/"&{B$@&[W#\ !V#&{B$/@@&BC! LB[#&<Y[["B"!$[ <Y@#&"@\!! @#B[!/#$"#B/"`{BEj/!- tites, residues of partial melting. These features have been a cornerstone of plate-tectonic

11 1. Introduction

B$!*BV!V@#$#!/// "@!$_R#`[''j +/"[&"!$"[&V" #B[!#@W"B\`W!$" $#jL"!"\#"""@#B #&@[#$#""#"$&"!"`#/#$# $#j!!"""!#@[!V "$@##/B"#+[ #!"B&"!# !"_B[!@"W!@B[ /$"$#`</[':j[!V!!#B$ "$!!\ <@B!V$"$/$$$" $#["#!#/`}j[! V"""`^&[':'^&[FFj|B"''F[### //"$#}"#B!- ented orthogonal to the spreading direction, as Penrose-style volcanic rifts are supposed @L//"@B$"$#[/"B required to be parallel to the spreading direction. These oblique rift segments usually con- "!@$###"!+#/B! $@B&$"/"![ !"#$#B@!V/ @##/$@"/"`/[[ '''W[''':'j+$#&B#/$#- "#/V/$"^//# V/$$"$#@#& $"##!!"}$#@#// /["#FF"$@V#V!"[@B" #/@/#!"&$"@$#$#`^&[':' X^&[':j{["!V@B""//@V#/ $#//"B$&"$!"`[''* L#&['':j^//$"#B!//[C$$ $#C[$"#[V##!!"- &[[@#$$"$$$$"`[''j+![ $[V!//["#$## @O"P!" @#!//B "#$"/[@#!@&/#B "!`/[[''< V[FFj<VB#$! @$#/"`^&[':'jL&!

12 1.1. Serpentinized peridotites at mid-ocean ridges development of deep rooted faults (e.g., Schroeder et al., 2007). Serpentinized peridotites $"$#!B!#/WVB VWVB\ }$"&[!/V@B!B@- "/"[B"[!LW!@#$ !C ! / # $" $ VB $ "#B$"$#C@#"!<+`- [''\[FFFX_[':*{/@['':j[ #!}`/[|[FF^&[':'j/ $/@B"[\&&/+`<[FFj #!@B!""!$"B spreading environments. ^//"`^j!B&B gathering variably serpentinized peridotites from ocean crust. Samples dredged from the \##B#V/[V!!C "!!/B$&L"V/$/ VB$#!#!!!B!C !!/![""!!B #&B ':["!"/$["#- /^{/F'[*F$<+"VBE&"#$ X#C`^&['::j+!!W"B"$!C- C@#/#V$"'"!''#$X #~'"$!C!V$"FF"! #/^{/`X{[''*jLB[^{/F' !# F)#~<+`X"[ 2004a). Thirteen holes at six sites along the spreading axis penetrated mantle peridotite /@@&!!#/B*FUF["!V- #B"!\"L"!V/! #B"$"^{/F'[:[*[*ELB"!!B$#B !CC@#/#[`Cj!` chapters 2 and 3) and rodingite (chapter 4). A brief description of the geological setting and the individual drill-sites is given in chapter 2. For a comprehensive description of all //@&/#[$X" `FFE@FF*j|`|[FFEj

13 1. Introduction

1.2. Serpentinized peridotites at active oceanic margins

Serpentinized peridotite from modern active margins, i.e., from trenches asso- #@#C[$W"!V$"#$ #L`/[@&<#['*Ej[$"<#/ C#/$`/[[FFj[L/L`/[|" [ ''j }[L ! $ ! !C $ "/$V/!!\#V$$ #@#@!@BBB`':*jL$$$#@# "@#V/!#!$/" gradient and as a consequence the overriding plate could potentially absorb large amounts $\#[@V/&""!!/$"B!$#@# `BB[':*j$"//!BVB- /"/V!C@#/B![ @#BB/$#W#\[$"/"# V/#$[@W$"FF&"@ W`/[B[':B<[''j

1.3. Hydrothermal systems and serpentinized peridotites

Hydrothermal activity involving serpentinized peridotite has been apparent from #""[#//$<["[3He in hy- "!#"#"VB///"$<+`/[ |[FF|&[FFE#['':_"[FF\- "[''jLVB$#"[{/V[@{ BB"[V$[VB 'F!$#"!V#$#/ #"[&[/""/V#$!#/ !C@/"!V/`+B$[ FF+B$[FFE+"[FF:|[FF#[ FF^V[''*^#V[FF\[FF\@[ FFFXB[FFXB[FF<"|[FF'&#& [FF&#&[FF:#W[FFE"[FF*& [FFEB$[FF*j L{/VB"[V''M''E@#FFF" !$"#$<+#$ FR#~ E Ea `|#V [ ''EjL W! C@#/[ !BW[ C[ [/@@VB//$!|&"&-

14 1.3. Hydrothermal Systems and serpentinized peridotites

V/"!#`F [#[FF^#V[FF" [FF*jB"\#[V["$V/&! "$"!["&/`|/V[''*j diameters up to 10 m. ^#/{_#''*@B"[V ER#$+"/"/$@/ !$@#FF"`##[''*jL@/$#@# /W!!C#"[&#V$#B- "[`|/[''*jL#"#@&"&"W"#" $\#"!#$ `^#V[FFj +@\#$#"[#@"@B![! @#"B$"/V\#"!"[B" B"[V\#$#VB/$VB/ "$##B/#[`#[ FF^#V[FFjL!Y` j{/V: @YB"[YV[B"$\#" are corroborated by experimental and theoretical serpentinization studies (Allen and Sey- $[FF[FFE}C&[FFFjW"![!- $#@B/#@ /V\#<V[#/!C!W"!- ["!#$F /!!@@#B"!#@# "/"#`+B$[FFE|[FF{[ FFj+B@C"!$@"[#"[/!! &B[W!@/"!#\#"B //{BB"[[V''*" &"B$"!/W$<++"$[ /!$*F'FF"`XB[FFj[!C- !!"$"/V\#`&#- &[FFj{BC@B###B`#!F"j- @##["/VB`FM' j[@`!Y'Mj"! \#{&/"!#B"B"[$V B/"V"!"[B"!/- !C$#"[&@"`&#&B[FFjYV[ the International Ocean Drilling Program (IODP) drilled recently into the Atlantis massif, $B$"{BYB"[V"W#VB /@@/`$[FF*j[\#"- ${BL/!/\#&@@B hydrothermal vents, it is important to understand the fundamental petrological controls of !C[@@\BW

15 1. Introduction

1.4. Mineralogical and petrological aspects of serpentinization

1.4.1. Serpentinite textures

Textural characteristics of serpentinites are fundamental for the interpretation of !#@`#@jL@W!!@B}& }&`'**j"!VW@&@BRYB`''j !C#"[&W@@VB$##W# $#!L"O!P"!& are only slightly serpentinized, completely transformed massive forms of serpentinite, B"BC![@$""$- !UB"["B!`"# volcanism). LW#$!@V@VB!N`j pseudomorphic (preserving important features of the protolith, e.g. plastic deformation, and pre-serpentine alteration assemblages) textures formed after olivine and pyroxene (to a much lesser extent after amphibole, talc and chlorite), (2) non-pseudomorphic textures formed either from the same primary minerals or from pseudomorphic serpentine textures, and (3) textures formed by serpentine veins. Pseudomorphic textures form through !C$""#"[&L$!#- morphism varies from excellent to indistinct, and the latter grade into non-pseudomorphic W#<B"V!!"B!#"!W#< $$![V["!$!#"! W##$/ Olivine alters along fractures and grain boundaries to form easily recognized pseu- "!"!$"W#"@["RL"" !$"!"!B@ $L#/$"/$" /$#"/$!jU#/ W#`""W#[@#@""" not possible). This texture is related to fractures in the mineral grains of the protolith and is hence not strictly a pseudomorphic texture. Both types of textures consist mainly of !`![@/$j"!`![@/ j[@#["/ Serpentine pseudomorphs after pyroxenes are generally called bastites, a term coined by (Haidinger, 1845). The term has also been applied to serpentine pseudomorphs after amphiboles (Weigand, 1875). Hostetler (1966) has pointed out that once serpentinization is complete, it is often impossible to distinguish a pyroxene bastite from an am-

16 1.4. Mineralogical and petrological aspects of serpentinization

!@@}&}&`'**jV$#!C$ chlorite also produces bastites indistinguishable from those after chain silicates. There- fore, it seems preferable to use the term for a serpentine pseudomorph after chain or sheet silicates. !#"!W#$"#/BC$!#- morphic serpentine textures or, less frequently, directly through the serpentinization of !"BV[!BW["!@!#"!W#@V !/`/[@$"!!!#"!!j &/B!`"#!/U! veins replace pseudomorphic serpentine). The orientation of the elongate grains may vary $""!B"["V&!#[#@!- [$&!#+!#"!W#["/ and brucite are common accessory minerals in non-pseudomorphic textures. Veins of serpentine along fractures, shears and joint planes can be found to a greater /"VB!L/B!$V@- guished - paragranular and transgranular veins. A paragranular vein is an anastomizing VV&$!!#!!B+- /#VVV&B/$!`$V!j #!!BU/BL[VB!$VW- #N"VV`"/#"!B[j[/V[[@ V`[@!!#Vj[![@V`[@- "!Vj[V#//BV`"!B[Vj

1.4.2. Serpentinization - an isovolumetrical process?

+ # "B ! W# !C- V#"!"$"!B V#"LV#"/#/`Dj$V! @#`//jN

2Mg2SiO4 + 3H2

L$!C$!BWN

3MgSiO3 + 2H2

!C$C@#/N"$V!BW`: VVNEV!BWj#V#"$E

Mg2SiO4 + MgSiO3 + 2H2

V#"!C"!"V$

+ 2+ 6Mg2SiO4 + 2MgSiO3 + H2O + 10H

Thayer (1966) suggested that the preservation of primary textures such as euhedral olivine pseudomorphs and relict primary chromite layering indicated volume-for-volume replacement of peridotite by serpentine. Hostetler et al. (1966) and Page (1967) argued that large-scale removal of MgO ##!!@B[V/["#$""V #/!C{#`/[RYB[''j#" !C&!@B@BB$B#![ /V#"/$\@B@#$# shear zones in serpentinites. The expected volume increase during serpentinization has "`/[{['*Ej$!C#!"@B $/""{`'*Ej#$"$#/B- "#"#@[/##!$! L[V[@V/W$!VV!- C`EFFFVj@V@B!`+#"{#@['* ^&[':'L"!<['*j"$"!`/[ Harper et al., 1988).

1.4.3. Some crystallographic basics concerning serpentine

Serpentine is a layered mineral and its principal polymorphs are antigorite, lizard- [BL##$!"&[- B/B[@#B`! NBj+""[/#$BV

18 1.4. Mineralogical and petrological aspects of serpentinization

#/@#V#$B`RYB[ ''j{C"!$!B[B"!$ B[$"B/NB!BV[- #/[B

1.4.4. A note on the mineral chemistry of serpentine and its value as a geothermometer

_V`'**j!V[!/"$!!B"![ i.e. lizardite, chrysotile and antigorite. The general accord is that lizardite and chrysotile $""!#"!/`_V['**_V[FFE<B[ '*jYV[!"/B"@V#@"!- ture indicator since differences in free energy among serpentine polymorphs are minimal and serpentine does not occur as a pure Mg-endmember. Element substitution and in- $/B"#/!"##$[VC$ !!B"!@B["!$/B$$$ !#

1.4.5. The MgO–SiO2–H2O (MSH) system

|"!#@/![!"B"! principally composed of MgO and SiO2"#@#$$

19 1. Introduction of SiO2"V$

1.4.6. Redox conditions during serpentinization

!C&/W"B#/`_&[ '*[':<"|[FF'&[''![FFEjL- V$VB/"#\#$"#"[ B"[VB<+`#[FF^#V[ FF[&#&[FF"[FF*jB" 1.3. The conjunction of serpentinites and extremely reducing conditions is even more evi- dent from active continental serpentinization settings, that emanate hydrogen- and meth- /`+@['::|['*:VB['*VB[ ':*#[':'§#&V[':j<&#W $$#V!BW@B$"/!@ $/$B/#/!C[##BW!N

3Fe2SiO4 + 2H23O4 + 3SiO2(aq) + 2H2(aq) fayalite magnetite regarding hydration of olivine and 20 1.4. Mineralogical and petrological aspects of serpentinization

Fig. 1. MgO-SiO2-H2O (MSH) chemography plot shows that hydration of olivine will yield serpentine and brucite, while hydration of orthopyroxene will yield serpentine and talc. Only rocks with a 1:1 molar ratio of olivine and orthopyroxene will have neither brucite nor talc. Talc rocks (steatites) require addition of

SiO2 (or removal of MgO) to form.

3FeSiO3 + H23O4 + 3SiO2(aq) + H2(aq), ferrrosilite magnetite

//B$!BW[WB/B$$- "$"/W$""!$@B-

21 1. Introduction

/LVB$B//#\#WB/$#/B of the system via the Knallgas equilibrium H2`j¤F2(g) + H2(aq). Partially ser- !C! #/ B ""B &M B

+#`3j"""B!B[@##`j[ `j!#V`"@['~#['':jV been reported. Frost and Beard (2007) noted that although iron alloys are reported from !["/"/@#$$N

Fe3O4¤ 2(aq) magnetite iron

"#WB/$#/B!+V""B"& #!/[V#"$!["@##" redox buffer – they merely react to redox conditions superimposed by other mineral – \##@|`FF*j$#WB/VN

4Fe3Si2O5(OH)4 3O4 + 4SiO2(aq) + 8H2O in serpentine iron magnetite

#!#/["#$W$"[!#- B$"!LB#!"#&! VB!/$"!#/@#"/- tite and traces of native iron. Possibly native iron and magnetite form at the expense of

Fe(OH)2@#[/$/N

6Fe(OH)2 + O2`j3O4 + 6H2O in brucite magnetite

2Fe(OH)2 Y2O + O2(aq). in brucite iron

LVV[@#$"$"/V iron. +#/ " B #$#! #[ !!B @#$$ W equilibria during serpentinization, they can be used as a redox monitor. Frost (1985) noted !$MB!/!WB/$#/B $#[V/#@<`$B"/#Cj@#$$[- /!#@WB/$#/WV/# @<""["@"B"$#[ !`|CV&[FFY[FFjV@[

22 1.4. Mineralogical and petrological aspects of serpentinization fugacity–fugacity diagrams for O2 and S2 can be recalculated to better constrain redox #/!C![V$!- VBMVB$#/BM$#/B/"$MMMMMM! #!@"MMMM! !$#["//W$"B /!C[B#@#C Based on petrographic and mineral chemical results, Bach et al. (2006) pointed #@#$@!["/@##"B- "W#/!C<"|`FF'j"!#- ed the most elaborate serpentinization model so far, as they explicitly accounted for Fe+2- !/@[!@#$!V## that used reaction path models to emulate serpentinization did account for solid solutions $!@#L/#!&#V"![ #/[B@B$!W/[$# incorporated into serpentine and brucite is not available for the generation of dihydrogen and models that account for Fe-partitioning appear to predict much more reliable amounts $B/$!!@#[$ "/@/"V@#B[! "[/#!&!@#[V!"#$ dihydrogen generated during serpentinization. +"B"#@[#/@#$[@\# ![@#"//B!"!#[& "! $ !+/ " ! @B <" |`FF'jC@#/#//!CFFM / VB&#//"#$"/ thus the greatest amounts of dihydrogen. B[B$&`FF*j#!CW!" FF F<!#":F""&/`"

23 1. Introduction

A:freshrock(plg+cpx±ol) B:rodingite(grsdi±chl)

SiO SiO2 An 2 An Zoi,Prh Zoi,Prh Tlc Tlc Trm Grs Trm Grs Di Di Parg Opx Parg Opx Serp Serp Chl Chl Ol Ol

0.75*CaO 0.75*MgO 0.75*CaO 0.75*MgO An An Al2O3 Al2O3 Prh Prh Zoi Zoi Grs Grs Chl Chl Parg Parg Serp, Serp, Fo, En Fo, En CaO Di Trm MgO CaO Di Trm MgO

Fig. 2. CaO-MgO-SiO2 and CaO-MgO-Al2O3 chemography plots indicating that advanced rodingitization is associated with a loss of SiO2 and a gain of CaO.

1.5. Rodingitization

/[""B$#/B!C![V#- dergone intensive metasomatism as a consequence of the serpentinization of surrounding ! `"[ ' [ '* RYB [ '' [ ':'j LB!!$//@@`@j&["B"!$!- /[!BWV`/j"!![/@@ contains much more SiO2[+2O3#!C$ peridotite are rich in calcium, as serpentine and brucite usually contain no (or only trace "#$j#"\#!C! (if the protolith is olivine-rich, i.e., orthopyroxene-poor, see chapter three), buffered by !@##@+/@@##@B![ @B!C\#[V@B/#"[`/j[#- //"/B`"['*jB""@/ / "! / $ M+ [ #/ C[ ![ /#- UB/#[V#V!##B!+ "!/#[#\#!"!#V /C[/VBVBL#"!VB- dressed in chapter four.

24 References

References

+@[L+[[^|#[\`':'j~"@![!![ I. Geology and petrology of the critical zone of the Acoje massif. Tectonophysics 168, 65-100. +@[L+[#[[|&[X[{B[\{[[V[ <`'::j<B//![~"@![!!N^! /"\/B*[ +/[\`j^<YV[YYV[Y['F[_/ [^V#@[§& +/[[[\|[<`''j</WB/! $#$!V$"U@- +@BN}"[|`j/$^/ /"[#[/[^//"[E +@[[[\|X#[<`'*:j##$ "#`{j*[EEE +W[Y![\^`''jL!!N+/# $!//N[{<[<#[||/[`j !+L<+/#{N\/B Special Publication, 3-38. +[^_B$[}_`FFj"!V\#$"#- "[B"B""/N+W!"#B EFF [FF@\"""+*[E +[^_B$[}_`FFEj!C/N- $"{B@B"B"\""- chimica Acta 68, 1347-1354. +[ &[} `'':j #$# !C !N - !C!"@#$##$\!B Research 103, 9917-9929. +[[&[}[[|[}[#&[Y[\[|#[\ (2007). Hydrothermal alteration and microbial sulfate reduction in peridotite and /@@W!@B"$#/<+/[ FR`^ {/F'jN+#$#WB/!#B\"B[\!B[\B- ":[F:FF[NF:FFFF'UFFF*\FF* +"[<[_#[+[|"[[C&[[|[}[\@@/[^[- [<[|&[|[{&C[XY#$$[`FF:j#"! `44U40j$/B"!B[{/V[`<+- /[E ERj\"""+*[EF*E +#[`'*jL#$B@##+$N +V_"\/B*[' 25 1. Introduction

+#"[{#@[Y`'*jL<+/E ©!- C#"[##$_¤#- dien des Sciences de la Terre 8, 631-663. |[}[|[[^&[Y||&[_L`FFj^VB$ VB"!/#!/#/ F _\"B[\!B[\B"[FF'UFF\FFF*' |[}[\[[YVB[[#&[Y[<`FFEj@- !M/$"^{/F'[<+:\- "B[\!B[\B"[F'[NFF'UFFE\FFF*EE |[}X[`FF:jL!/B$\/N/$"/- "!"/{[NFFUFF:FF |[ }[ #&[ Y[ \[ [ $[ |[ <#[ } Y#"![ _ `FFj V/ # $ !C N !- trography, mineral chemistry, and petrophyscis of serpentinites from MAR : `^ {/ F'[ *Ej \!B { [ {F[ NFF'UFF\{F: |&[_L[_"[Y[<[[|[}[^&[Y|[[_[}&[ {[|[{/"#[Y`FFEjYB"V/"/" NL#!/\&&#/\"- B\!B\B"[F:FF[NF:FFFF'UFFFE\FFF* |[[{"[Y""@/[\`'*j\"V$!- ent-day serpentinization. Science 156, 830-832. |[[R[L[`'*:jB!C ["§#/V\"""+E[EE 145. |/[+[[<+[V[<[@[+[##[§[[ Y[[{B[`''*jL@!!- #!&&`<+/[+<+/"jN!"B !${_#_L+"\!B 78, 832-833. |#V[ | [ XV[+ \[ <&V[ [ &V[ \+[ XV[ \ {C[§^`''Ej<V#[!VE Ea[<+- /|^\_[F |CV&[\+[^@#&[+XV&[L+`FFj{"!# heat capacity of pentlandite. American Mineralogist 86, 1312-1313. |&[<`''j\"</§&NW$VB Press. |#[^[Y@[[Y&[[<`''j<"!"$!- &$"\$L$"`_[ Rj

26 References

#$\!B'[F[F*'FF[F'' |"[Y[}/[^[<{[[L!![^[$[[[L[[ {[\[X[[L[X"[<[<[[X[[Y}[<`''j \/B$L/$N+#@!#@#C!_L- tions American Geophysical Union 77, 325. |/V[§+[|&V[[&BV[[\#V[_\/V[+ < `''*j+ B! $ " "$"/ B"N |& "& $ B"[E ER#[<+/\/B$ Deposits 39, 58-78. |[\[ #[\[[<\#[`':'j#/@B! @$"/#E[ |[_[_"[[[\[YC[B[Y`'*Ej"[ carbonate breccias from the equatorial Mid-Atlantic Ridge. Marine Geology 16, 83-102. [[|&"[^X["[^X[<+[_[[|[<[[+V- /[_[[+_[`''*j#/!#$$" /$"<+/#:[' [<`''j_"!"$"&\"/ #$\!B':[EE* [<[|#[||#/#[Y`''j!C!/@@ <+/WVB *R R_B {F'[:*F [ <[ |[+[ ^!#[ [ _[ [ \/[ [ {[ [ <#V[ [ <BC[[<#[<[##[\[@[+V[`'''j< +/+C!N/W"/$!- rived event of enhanced magmatism 10 to 4 Ma ago. Earth and Planetary Science {*[*' [<[< V[[<[<[^!#[[^#[[\[[+/[[|#- [+[^#@#[\[Y#"[_B[`''jL#[#"[ W!#[ #// $#/ ! <+ / `)E)j Geology 23, 49-52. "@[+[<{[[L[{[\`'jV"- <#&W##$_¤#- dien des Sciences de la Terre 2, 188-215. ![Y[R[X[|B[<[< [|+[#$[+[X@[{{ {VB[^`FFj+B/@#@#$"@""#B"- @B"/#E[ #[{[^V[[##[§[|![Y"[`FFj\-

chemistry of high H2Y4V\##/$"#"[& @B"[`:ER[<+j"\/B'

27 1. Introduction

#[{[##[§[|#/#[Y[^V[[_#@#[[|![[

^!/B[+[+!!#[[+`'':jY4 plumes generated @B!C$#"[&$ FR$# C<+/\"""+[ 2333. "&[ + "[ ^ `':'j _"/ "B" !! `D\ $DY $j$"':X$"#"$!B- butions. American Mineralogist 74, 1023-1031. [`'*jL@#$!##$ Geology 80, 709-719. [@#B[<Y`'*j##"!$ #V\!B!B[*: "[\`'j![/[#!B! mountain chains. Geological Society of America Special Paper 73. "[\`'j~!""& ~\/#VB[|#*[F "[\`'*j{"!#C!&$$[ Oregon, and Washington. US Geological Survey Bulletin 1247. $!`'*j$N!\"*[E 25. VB[<`'*jYB/!C$/ <[+/B[$_"\/B|#- letin of the Society of Economic Geologists 66, 1265-1266. VB[<[\@[_|[~[_[^$$[\+<+/[__ (1987). Serpentinization and the origin of hydrogen gas in Kansas. AAPG Bul- letin 71, 39-48. ^R+[<X[<|`FFEj!@#$#"[ $" # " "# ` ^/ /" {/ '[ FFjN$$!C$<$"<- ogical Magazine 68, 887-904. ^RC[<[|[|#[^`FFEjLB"&$" #$#C+_#!#$ Mineralogy 16, 73-83. ^&[[W[[#C[[&B[+[X/[{[<B[{B[} |`'::j\//$<+X[/FUF' Report, Proceedings of the Ocean Drilling Programs, 15-22. ^&[Y|`'*EjL&$"![// #[[/_B{ ^&[Y|`':'j+@B![VB!/// "/""N#[+^B[<`j</""

28 References

|W$N|&[*F ^&[Y|[{[#[Y`FFj+#!/$ /#E[EFE ^[<`':j@#$"#/!C carbonate alteration of some Archean dunites, Western Australia. Economic Geol- ogy 76, 1698-1713. ^V[[#[{[^#V[_[XB[[##[§[V[_[

|![[V[<\"[`''*jY/Y2Y4 content in B"\#$"@B"! ER+<+ /"[<+/`V/{_#[#B''*j"! <+_L+"\!BE*[: ^#V[_[#[{[&[_Y[|V#[[V[[^V[[ ##[§[#[^+!!#[`FFjL@V\#` ER[ <+jN\#$#"[&!!"- <+/B"\#"\/B:E[*E: _&[`'*jL^#"!N+"$$&$# !#""@/@B#"[&_"- ic Geology 70, 183-201. _"[Y[<[[|&[_L[B[^[[[{/"#[Y[ ^&[Y|[<[[\"[\"[^}`FFj^VB$ @#B"V/#!/\&&/ +#E[FE _[[Y[\_V[|`''*j_$$$!C! /B$"$#/!//_- B{[::' _V[|}`'**j<"!"$!!!+#V of Earth and Planetary Sciences Sci. 5, 398-447. _V[|}`FFEjL!"#B"VNB"@- \/BVE[E*'F _V[|}L""$$[`'*Fj/""!"$#"[&

+!N/B"

29 1. Introduction

#$/B[ [||[`FF*jVB!C#$ Petrology 48, 1351-1368. [|[|[[</[+$$[_`FF:jL$"$" /$"^YF'^NXB#/!$!- C#$/BE'[':: \[\{[XB[^[|[<[X[+[{#/[X+[|#- [[^+[|[&#&[\`FFjF[FFFB$B- "VB{BV[F[E'E': \[\{[[+{#B[`''j/@!- B"!C$_"- Y^![:'N<V[[\[X<[+[<B[ `j/$^//"[#E*- /N^/!/"[F' B[[+"@[_{Y#/[^<`':j/"!"$< Forearc seamounts. Geology 13, 774-777. B[ B[ \ `':*j / $ V "# $ V- "NX/[|Y[B[[|C[|[\}`j "#+[</!N+"\!B[ 61-69. B[<[<`''j{/B["/B[/$!"#- V$"L"$"#N#${/- //$^//"[#[E \@[[<C/V[[|V[§[!V[L[&V[\+ R[<`FFFj!!#[+$ {/VYB"`<+/[E ERj\/B$ Deposits 42, 296-316. \"[[X&""[\#&[<^`''jL@B- "!#"[ R[<+\!B{['*'': \[_[#[{[$XB[[{<`FFFj$" $$/<+/#$+C`::E:jN#"[ W!#/$B"V_B{ 177, 89-103. Y/[}`:Ej#$+@#/} Y![\^[|"[X#[`'::j+[["[@! #B$#@\""!"$!![$- /#$\!B'[EE Y[YY`'jYB$@[/#N+V#"+ F. Buddington. 599-620. YC[X[`'*j/B$/$"#<+-

30 References

$#C//[@#</B and Petrology 49, 233-257. Y[|["[\[<#"![+_V[|}`'j|# alpine serpentinites. American Mineralogist 51, 75-98. Y[+`FFjL"B"$#N+V$ !/"<#/<L+E[*'*' $[|[|&"[^[[|_[[§[<[^[<{[ /^//"_W!FEUFB`FF*j "!W#!//\/B 35, 623-626. B[X[+#"[Y[[L"V[|`FF:j!C$- !"/"#N$"{& !"!W[B"\/BE'['F @&[Y<#[+`'*Ej|!$"#L[ 2. Rare-earth geochemistry. Marine Geology 16, 205-211.

[}`':j_W!"V/$$Y2¤- !@#@#</B/B'[F' X[+^&[Y|`':jL$/$" Kane Fracture Zone. Marine Geophysical Researches 6, 51-98. X[+_[^`':*j_V$V"/"!#/ !/N+!!\/B[* X[+{[<`''*jL/$!W!# "VB$<+XVB$'FN X[+[[<[<[^_[^`j/$ ^//"[#/N^/- gram, 5-21. X"[[X&[_[<[^{/F'!@[B`FFEj ^{/F'"!/<+/$" E ^_#F[EF X"[|[X&[_[<[^[+@[[|[}[[{[B[ ["@[{<[[<[![+[^&[Y|[#[[\[ <[\[[\[[\[<<[\"[^}[\$[[^}[Y- VB[[$[|[#[\[$[[<#[}[#&[Y[[ <[[L[B[<L&C[_`FFE@j{/F'#""B- /$^//"!/"! /<+/$"E//V/{/F' $#$/V^_#[|C[ \/[|"#:*[<B#BFF/'NLW +¬<VB^//"/L©

31 1. Introduction

X"[|[X&[_[<[^B[`FF*j{/F'#""BN !F&"&#V@#BB@<+ /[E M NX"[|[X&[_<[^`j ^[#/[L©N^//"[ XB[^[X[+[|&"[^X[\[\{[|#[[^+[{- B[<^[[_[&[<[[XX[{@[\L[VCC/[ B[+`FFj+$$WB"V[<+ /F #E[*: XB[^[X[+[#\[\{[§/[^[&[L<[|#[[ ^+[YB[<[&[<[[_[&#&[\[&#@[<[ |B[+[{[|[{#/[X[\&[^[|#&"[X[|B[+[ |C[}[[X[_[<{[^#[+[|[<[{B[ <^[|[+[#""[_BV[`FFj+! B"NL{BYB"F*[E:EE X&[^`FFj!#@#':[EEE X[|[}`FF'j!!- #$/BF[*' {/@[§[|#[^[[<[X[+< V[`'':j"[ "[!#&#W!/<+/`F)F)j B""B"" @# "! $ \ !/ !N|#&[}[^B[L[X[+{/@[§`j #/"/"""/}/[^N+"\- physical Union Mongraph, 153-176. {#[`'*j/B$!B!!$+@L$ <[#@C</#/!<#- gen 55, 431-455. W[+[^&[Y|}&[L`''j/$"#X <|$"#/ R_E :R_- butions to Mineralogy and Petrology 110, 253-268. {#[\_[}\`'*'j/""!"$_L !@#</B/B:[E: {[|`'*Ej!$&\!B# Astrological Society 39, 465-509. {[[+`FFj\B"B"V@B- !C$!\!B{'[E <"[L<`'''j</!#$"/B$ primary biomass production by autotrophic organisms in hydrothermal systems _#!#$\!BFE[F*'F*E <"[L<`FF*j\"#$<@_/B$ "#!B "[Y ^! YB" B"

32 References

Astrobiology 7, 933. <"[L<|[}`FF'jL"B"B//- #/!C$#"[&\""" +NFFU/FF:FF < V[`FFj!C$@B!"/\- science 335, 825-852. <[[{/"#[Y[^&[Y|[[_[\[{[\"[^ }[{[X[X#[\[&[}[<#[_"[Y`FFj </""/"\/#!/\&& /[+#E <B[+[[_/[<`''j"!/$! $"<+/EF#@# Mineralogy and Petrology 23, 117-127. <B[|`'*j+W!"#B!C$@/V- ines. American Mineralogist 14, 462-478. </[}`':j[[/$##@&#$\- physical Research 73, 1959-1982. <[ < [ X"[ [ B[ <B[ { `FFj ^!@ \# $# W"!+ < $ ! "# VN ^/ /" {/ ' \"B \!B \B" E[ N FF'UFF\FFF:: [/[\`':jYB//$""#&" _B{[F [XY[/&[L&[X`FFjYB/V#@#$#- !"@B"`{<_jNBWB# Trends in Microbiology 13, 405-410. &[_Y`''jL#$V&!&$ _L!$#@V</[F*' [+`''jL</N<#|{VN!/- lag. "[[}"[+_[<[[Y`FFjYB" $V\#/#VN#/$- pentine phases. American Mineralogist 87, 1699-1709. [\LY![\^`'::j"[$#&¬" $#/!![/#"L:*[! ¬{! RYB[^`''j#V#"!@"!C\/BF[ 705-708. RYB[^`''j!N$!/B §&NW$VB

33 1. Introduction

RYB[^^B[<^`''jL"!$CL$"- tion of magnetite in serpentinites. American Mineralogist 78. RYB[^[[_}&[`''jL/$/$"

[|#"@[#"L`Y2O) during serpen- C\"""+['*F: [§[[[[L[§#"[Y§"C&[L`FFj$" <L#/N[&"@V@&@ @#</B/BE[: /[`'*j!C|#<#[$@#<- eralogy and Petrology 14, 321-342. [{[<Y`FFEj\""$"""#- "[B"N!C[/C[\@"B !!\"""+:[ [ [ { `':'j+ #VB $ L! # +"!$#V#< Marine Geophysical Research 11, 89-100. #&[Y[|[}[\[<[Y/[<[#[\YVB[`FFj \"B $ @B ! `<+ /[ FR[ ^ {/ F'j "! $ \#& !/ V" "\/BE[*'F [[W[@[_`'*Ej$#!/- /B$B#E*['E' Prichard, H. M. (1979). A petrographic study of the process of serpentinization in ophiol- #@#</B/B:[E &#&[\[{B[<^[XB[^[_`FFj{"!# V!#{BB"[[V$"B/ @!/"""\/B'[E &#&[\[{B[<^[[[\[\[[_[{#![ E., Sylva, S. P. and Kelley, D. S. (2008). Abiogenic Hydrocarbon Production at {BYB"'[FEF* "[`'Fj@![+#[#[_/$[_#/ und Paragensis. Mineralogical Magazine 29, 374-394. [+[}$&[{|"[X`':*j!C#"[B- "VB<+/:#$\!B- cal Research 92, 1417-1427. [~#[`'':j#$V[B !$"|/@"`}+!j[ 13, 43-56. #W[ [ ##[§ {#[ `FFEj !! ! B" $ {#&B&[@[{/V\B"[<

34 References

Atlantic Ridge. Economic Geology 99, 585-600. &[ [ X#&@[ <[ [ < [L `''Fj / $ !@ $!C$C#/<$"#V$ B/ WB/ ! _ B { FF[ 291-303. Sanford, R. F. (1981). Mineralogical and chemical effects of hydration reactions and ap- plications to serpentinization. American Mineralogist 66, 290-297. [ _ [ RYB[ ^ }&[ `':'j / !C "[&$+@@\@[</ 27, 579-591. "[X[X&B[+[\@[^[V[{<$[`FF*j \"B$B"\#$"#"[{/VB- "[[/<+/"!! V/"\/BE[ &[<[XB[^[|[+|[+`FFEj{V- B&#@\/"!{BYB" Field, Mid-Atlantic Ridge. Environmental Microbiology 6, 1086-1095. [L[[<[^&[Y|[#[[B[X"[| `FF*jV\!/\"@B W$#/ <+/N+#B- $^{/F'\"B\!B\B": #[<[|&[^[Y[L<"[L`FFj!C "!${$_B_<+@/B[E B$[}_[[##&[^#[`FF*jWV#"- $#/!CW!"#BFF [FF @"!$#"[B"B"" /\"""+*[:*:: [`'E'j&$<+/#$\/B*[:''

![Y[<@"[+[&[L["[\|[^X`FFEjY2-rich \#$"!CN/"@"!/$ +"B$FE[::: V[L<XB[`''j{#!"@B"! basalt aquifers. Science 270, 450-454. #[[+@[L+[[<#&[|<#@[X`':'j !C$+<$[~"@![!!B/ and oxygen isotope geochemistry. Ophiolites and crustal genesis in the Philip- !+""[N_V[FF* L&[X[\"[L[L#/[[&B"[[YB"[Y[[XY

35 1. Introduction

Y&[X`FFEj\""@/V$B- gen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem `YB!{<_j@V!B"[_W"!:[ 269-282. Thayer, T. P. (1966). Serpentinization considered as a constant-volume metasomatic process. American Mineralogist 51, 585-710. Thompson, G. and Melson, W. G. (1972). The petrology of oceanic crust across fracture C+N_V$&$\!/#- nal of Geology 80, 526-538. L$[|[+&Y"[Y//B[_`''FjL$$$!C- B"/#!@BN!!B"B$ Earth and Planetory Interiors 65, 137-157. L#&[|_[{[X&[<`'':j</"#"### [/""!"!W<+/#- nal of Geophysical Research 103, 9857-9866. }/[ Y `:*j ^ ! /@# && \/ - sanstalt 25, 197. }C[{&[_{`FFFj^/#/#"[$"@#@- "B"B"@B"!/#V\#"! #$\!BF[:':EF }&[[+\`'*'j_"!@WB"@"# $!W#<*[*::F }&[}&[_}`'**j!W#!C </[E'E:: §#&V[<["&B[<[^BB&[|+^[+`':jYB/ "/B$$!B!$"&XB& <#^&B+&"&#&[EFE

36 Interactions

Abstract !C$@B!&!#W"B- ducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O. We have compiled and V#"B"$!"!#! in fO2[/fS2,g and aH2[Y2[/"$"!#@FEFF F<}#"!$! changes in oxygen and sulfur fugacities during progressive serpentinization and steatitiza- $!$"<+/ FQ#~` ^//"{/F'j/!@V#//B"/$" #"/!C"/!"@/ $"/B/$!C"!B!BB"" "@/C&+#@V@#@/!B!- C&+!!B[@#$$/$VV#@B!$ @#$$"$/"#$B/[$"- $#+!"#$#C$![#[ "V $" & #/ / $ !C [ - CV/#$#$#/B#[[# !BB" !BV #[ $" #/ !B $ peridotite is exhausted and H2V![#[ #$#C@#!!#$#$&LV# of f[/f[/B"$!$Y2S,aq, indicating that H2S in V\#@#$$[Y2V\#@#$$@B !["B"V#$Y2V\##$ !/V&L#$!#"/- cates H2[V/\#@B"$L!- $B//!#BC/!B$## facilitate the abiotic formation of organic compounds.

2.1. Introduction

< ! ""B W! \ $ # spreading mid-ocean ridges by detachment faulting that initiated close to the spreading W`/[''*L#&['':j/""!B $&#$`!Cj! /!C/B\#/B$- !`_[''*j[/"@#/$`L"!¬<- '*F¬^&[''j["@![[@V- \`+¬&['':R|['':XB[FFj+"&@$# $!C/B#/#$/\#[ @B#$V"B!`&[''"@

37 2. FeNiCoOS phase relations

['{[':+@¬[':'j##/$"#"[ "$[ $ / " ![ W@ W!B high concentrations of dissolved dihydrogen (H2[j`LB['|['* ¬ /[ ':j ! \ B" B" high H2[$F""U&/`#[FF^#V[FF XB[FF&#&[FFjL/Y2,aq concentrations are due to the oxidation of Fe+2&@B+3 in magnetite that forms along !@##/!CB$`FF*jV! experimental data suggesting that incorporation of Fe+3 in serpentine may also generate considerable amounts of hydrogen. L"@$$$\#&$#$&"![- &["!#@W"@B!! "`/}C¬&[FFF¬[FFE<"¬|[FF:j[ @# &/ $ "\# #@ /V \# "! #_W!"#`/B$¬^/[''B$ [FFE[FF*j!["\#@#$$Y2,aq and also H2[B"#V/\@B"[$[ !B!B"/`

Table 1: Idealized formulae of opaque minerals in serpentinized peridotites Mineral Chemical Formula

awaruite Ni3Fe tetrataenite NiFe pentlandite (FeNi)9S8 godlevskite Ni9S8 heazlewoodite Ni3S2 millerite NiS polydymite Ni3S4 violarite FeNi2S4 magnetite Fe3O4 pyrrhotite FeS pyrite FeS2 linnaeite Co3S4 cattierite CoS2 jaipurite CoS wairauite CoFe chalcopyrite CuFeS2

+# "B ![ V[ !V# - #/W!B\B"B"@#$B $"B"LV"$[#["B" $"#!$B"L'` [''j@_U`}B[''j@`F<j"- B"@$"!#$"FEFF "#$ #`/Xj$!$!B#V- @"#/VRY$$#!!#"$#B !!$"^//" `^j{/F'[<+/`<+j ["!@ Y[Y[!/`':j[#!- #V#!$!#/"Y2,aq and H2[\#"@/@V"- !!"#[W!"# `&B¬B$[':<"¬[FF#[FF^#V [FF+¬B$[FF&#&[FFB$[FF*j

39 2. Fe

1275 1274 1273

15°20'N FZ 1272 1271 Fig. 1. Location of the study area in the vi- 1269 cinity of the 15°20’N Fracture Zone. Inves- 1268 tigated samples are from Sites 1268, 1270, 1271, and 1274; redrawn from Kelemen et 1270 al. (2004c). LHF

2.2. Geological setting

L$ FR#~`~[/j!/`±"U year, full rate) MAR has been explored in detail by numerous surveys (e.g. Rona et al., ':*|#/#['::[''*B['':_¬[ '''_[FF#[FFj|V"!$ E /V&"BB!"/@ `_<|j`^¬|#/#[':^[''jL"/"E /V"|#/#/VB"[/ @/VB@#RB/VB/!" $"&\`/[''*_¬['''#- [FFj_WV#!$!C!/@@& @$\&V$/#![/$ magmatic extension, crustal thinning and the formation of oceanic core complexes along /V/"$#`_¬['''j^{/F' 19 holes at eight sites north and south of the 15°20‘ FZ into variably serpentinized peri- #@B/@@&`X"[FFE[FF*jL$/@$- !$&@&$X"`FFEj |`FFEj[$#$""!$ this study. Y:+$VB#$ FQ~W E*""!BC@#/[#[/B"/"& "BCL&$$@B!VV!C- B#!"!!VV!"$!@B`Cj#[ @#!#$$ *F #" $ [ \& $

40 2.3. Analytical methods

<+VB${/VB"[`E EQj "VB$"Y*F!VVB!C- C/@@V+#/$!$"Y*F *F^"B&!#["!B /B$"[&/@@#V@"!B "[Y*F*F^"@ and oxide veins. Site 1271 is located on the inside corner high of the MAR spreading segment south of the 15°20‘ FZ. Drill core 1271A is mainly composed of completely serpentinized dunite. Drill core 1271B comprises variably serpentinized dunite and harzburgite. Steatitization "& *E&"$ FQ~$VB 'EF"!*FF"$"$"$#Y 1274A penetrates 156 m into the basement and recovered 35 m of core that comprises 77 % harzburgite, 20 % dunite, and 3 % gabbro. Peridotite from this hole represents the &$"{/F'#!$/"!V a comprehensive description of all the drill sites and a more detailed description of the "/B"B$X"`FFE[FF*j[ |`FFE[FFj#&`FFj

2.3. Analytical methods

2.3.1. Microscopy and electron microprobe analysis

L!BV/"\/#/ {^<©Y"""!<"!BC Q_{#!!@©+:'FFR"!@VB$X`\- "Bj[#!![VV/!V!"<BC /V/$F&$@"#$F+$#B$# )"@""|B#"#L #/~+"<""!!/@& "/$!"&$#!#""@/ #"!"!/!@V['/!B$&- $#!#"@+#/"B$B @#$"/C$"$"[V# !##//!!/V!C

41 2. FeNiCoOS phase relations

2.3.2. Thermodynamic calculations

L"B"###/L'` [''j"!#L@$L'$`': K and 105j"B"!"[<XB$[[#$ state parameters for pure minerals, aqueous species and gases for the calculation of equi- @#"`/XV#j$"!#!##!FFF FF MPa. The database used for this study combines all upgrades from the slop98.dat and the !F@`}B¬V[FFEj/"##- D D /\"R}&@ (GWB jV*F`|&[FF*j+"B" D database for GWB "@$!#$F<"!#$F[[ FF[FF[F[FF[F[EFF {/XV#@"!# @BL'##/VRY$$"!#W![/- /$$$!#`@j{/XV#$#$" /VL@+VB$[$Y2[#$/^#"" `':j$2[[VB$[$Y2[#"@#B "!#`Y/['*Fj+#$#/B$ H2S and H2$"X"`':'jX"¬&`':Ej#//"V $"@VYV[/XV#$#@#"@ V/#!!![@#`j$#/B- #V@$L±FF `j//@`±F/#j @FFEFF $/ #@[ @ "B" $ ![!$L'"- ties in these data and their propagation in the calculation of phase boundaries are hard to quantify. Standard state thermodynamic data for minerals, aqueous and gaseous species, /"!#!B$"!" $#$!B&#B["#@ preliminary.

|CV&`FFj#"!#!B"#"$ synthetic pentlandite (Fe4.604.54S8) and reported a standard entropy (S°j$E*E'U" per K and H298.15Y0$*:F&U"/[#- ° !B$$"`DYfj$:E*F&U"$"!`4.54.5S8), #/!$$"$`j"`j$"@¬ $ ° Y"/B`''j" ¬X!!`':*j!DYf$:**/E'&U"[ ° /"##+!!\@@/B$$"`D\f ) of :&U"V#/"!$[/V@B ° @¬Y"/B`''jL#"@D\f¤:&U"$" 42 2.3. Analytical methods

Table 2: Equilibrium constants for dissolution of selected opaque minerals (P = 50 MPa)

Reaction Mineral log K

n25o. 0 °C °C 100 °C 200 °C 250 °C 300 °C 350 °C 400 °C

1 awaruite 231 70 196 80 161 91 120 06 104 74 91 77 80 46 72 21 2 tetrataenite 11943 103 67 83 38 61 90 54 05 47 41 41 63 37 38 3 pentlandite 57 71 56 73 56 35 59 09 61 67 65 15 70 08 71 77 4 heazlewoodite 3068 26 61 16 94 7 73 3 93 0 32 3 23 5 40 5 godlevskite 87 45 84 42 79 62 78 71 80 01 82 44 86 50 87 29 6 millerite 9 13 8 83 8 42 8 49 8 73 9 09 9 64 9 84 7 polydymite 121 00 113 88 99 44 89 24 86 54 85 08 84 96 83 59 8 violarite 118 14 110 42 94 59 83 35 80 31 78 57 78 21 76 67 9 vaesite 15 48 14 63 13 44 13 45 13 90 14 62 15 72 16 65 10 wairauite 120 48 109 25 84 27 62 91 55 11 48 63 42 79 38 57 11 cobaltpentlandite 82 22 79 11 73 66 71 80 72 65 74 64 78 29 78 83 12 jaipurite 8 25 8 00 7 64 7 72 7 94 8 29 8 83 9 03 13 linnaeite 112 41 10566 9184 8196 7930 7780 7756 7574 14 cattierite 17 94 16 81 14 98 14 41 14 64 15 17 16 11 16 91

15 H2S,aq 7 28 6 86 6 37 6 53 6 82 7 22 7 78 8 49

16 H2O 50 66 4630363527582431215319091684

Reaction no.

+ 2+ 2+ 1Ni3Fe + 8H + 2O2,aq = 3Ni + Fe + 4H2O + 2+ 2+ 2 NiFe + 4H + O2,aq = Fe þ Ni + 2H2O + 2+ 2+ 3Fe45Ni45S8 + 10 H = 45 Ni + 45 Fe + 8HS + H2,aq + 2+ 4Ni3S 2 + 4H + 05 O2,aq = 3Ni + 2HS + H2O + 2+ 5Ni9S 8 + 10 H + 9Ni + 8HS + H2,aq 6 NiS + H+ + Ni2+ + HS + 2+ 3+ 7Ni3S 4 + 4H = Ni + 2Ni + 4HS + 2+ 3+ 8 FeNi2S 4 + 4H + Fe + 2Ni + 4HS 2+ 9 NiS 2 + H2,aq + Ni + 2HS + 2+ 2+ 10 CoFe + 4H + O2,aq = Fe + Co + 2H2O + 2+ 11 Co9S 8 + 10 H + 9Co + 8HS + H2,aq 12 CoS + H+ = Co2+ + HS + 2+ 3+ 13 Co3S 4 + 4H = Co + 2Co + 4HS 2+ 14 CoS 2 + H2,aq = Co + 2HS + 15 H2S,aq = HS + H = + 16 H2O H2(aq) 0.5 O2,aq

/¬`'*j|#/"!#!B&/[ #VRY$$W!L'$["!#/ K values for dissolution of pentlandite. A standard molar volume (V°) of 153.3 cm3U" #$#!$"/V@BX#V`''j

Heazlewoodite ° ° ° D\f[DYf and S $C`3S2j&$"@¬Y"/B (1995). We used high-temperature heat capacity data from Stølen et al. (1991) to calculate <XB$[L° (40.655 cm3U"j$C# using cell constants given by Parise (1980).

Awaruite ° ° ° Y`FFj!D\f [DYf and S $#`3Fe).We calculated log K V#$#@B"$VRY$$W!L'$ [@#/"!#"#V@L° (26.96 cm3U 43 2. FeNiCoOS phase relations

"j#$"/V@B+B`''Fj

Tetrataenite ° ° ° Y`FFj!D\f[DYf and S $`j}# /XV#$@B"$VRY$$W!L' $[@#/"!#"&/L° (13.84 cm3U"j#$"/V@B+@`'*:j

L"B"!!$/V&`9S8j@$"!B ° "#"@B=`''Ej$7S63S2+DYf$:F'&U" calculated from H298.15Y0`*E'FEU"j#/!B$$" ° ° $/V@B@¬Y"/B`''jD\fV$"DYf and standard "!$/V@B@¬Y"/B`''jL° for a natural /V&`E:*"U"j#$"/V@B`'::j

Millerite L"B"!!$"`j&$"@ ¬Y"/B`''jL$!"*' "& #$#$#@#"EFF

Vaesite ° ° DYf , S !B$V`2j&$"L`['':j ° ° D\f`E:&U"jV#/DYf"!$ The V°&$""B¬<"&`''j

Violarite ° ° ° }#D\f and S $V`2S4j!@B/`'*jDYf& $""6¬X!!`':*j+!BV@[#/X V#$#$V#/VRY$$W!L' $[L°&$""B¬<"&`''j

° ° DYf , S !B$!BB"`3S4j&$"L`[ ° '':jD\f`':&U"jV#/"!$/V @B@¬Y"/B`''jL°&$""B¬<"&`''j

Cobaltpentlandite ° ° DYf`:E*'&U"j `E*U"!Xj$""@`9S8) ° $!#&$"V`'EjD\f (-836.43 44 2.3. Analytical methods

° &U"j"!##/DYf"!$/V@B @¬Y"/B`''jY/"!#!B!$" Kelley (1949). The V° (147.102 cm3U"j # #/ !" $" "¬`'*j

Wairauite ° ° D\fDYf$#`j[/#!"@B compounds database (Dinsdale, 1991). We calculated dissolution constants by means of VRY$$W!L'$L° (14.09 cm3U"j #$"/V@B|B`''Fj

° ° DYf , S /"!#!B$!&$" ° ° <`'*EjLD\f$"##/DYf and standard molar !$/V@B@¬Y"/B`''jL°$`3S4) `2j&$"@¬Y"/B`''j[$!# `j$"#"V`'*Ej

45 2. FeNiCoOS phase relations

2.4. Results

2.4.1. Petrography

} /# B! $ & N !C $ ! - atitization of serpentinite. At Site 1274, peridotites are partially to fully serpentinized, *F[*:[!B$#B!C!V undergone additional steatitization to variable degrees (see Bach et al., 2004). Microtex- tures of the serpentinized peridotites range from pseudomorphic mesh and hourglass tex- #$V@@W#!#"!&/ W#LB!!B!C&`':j"W# $V"!U@#""</ B$"V&/$"/@#- $"!!C`/|[FFj"!B!C& V !U@#U [ $ !! & " / Most samples are extensively veined by paragranular and transgranular serpentine veins. /#V$""/&##B$! !#!!B[/#V#!!B`X- "[FFEj{!!V/#B V#"W#VB!#"!&/W#!W"B to gabbroic intrusions steatitization is strongest and often invades adjacent serpentinite @B!/$"/#!V_V/BC& original serpentine micro-texture is commonly preserved, indicating alteration under stat- `|[FFEj!!@#B"/" ["@B_&`'*j[VB##BV/ $"/!$!@BV/$ grains by a thin (5-10 μm) ferrit-chromite rim. L $/ !/! ! $# !# " "@/ variably serpentinized and steatized peridotites. Because of the small grain size of most &$# !# "[ [ @B \ / "!B $"!@LV"!@"#"@// !!/V!CBC" compositions of opaque phases by electron microprobe and used the compositional data $![

$"^{/F'/B"!@B" ""!`#&[FFB[FF*j#"##

46 2.4. Results

#[WQ@@R$"/"/[@@B_&`'*j$ ^#"![#[#@BV!BW[@@B {`':'j[/C"!V/"B#[ ""B"!B!@BB#[B$"#/!- CB`FF*j!#$"/"#[`!B @@$![@[!BV/@#j !@@B##/""!/$!"!B- $#B"!$":##[!" #"@$"!V/$"*F[*[*E#[/# "/#"[&$$!"!B+#/ pyrrhotite occurs in many serpentinized peridotites described in the literature (e.g. Shiga, ':*+@¬[':'{[':'j[@"!- V/$"{/F'<`FF*j!#$!B/ !"!$"Y:+L!/!@@B$"/" //@@#$#B$#!$ #!/V/`FF3""j[ $#"!$"*`/j["B@!"BLB##B occur in porphyroclasts of former orthopyroxene (bastite), but no pentlandite inclusions $#$!BW

Secondary opaque phases LV/!±FV&$#!#"L principal opaque minerals in partly serpentinized peridotites include, in order of decreas- /@#["/[@![![C`L@j +#""@##B""#/[&$#!# "!![B"/!"WL/C ranges from < 1 to 50 μm. By far the most abundant mineral assemblages are pentlandite #"/!C"/`/@$j

Mesh rims In pseudomorphic serpentine mesh rims, disseminated opaque phases are generally < 3""#"/B#@"@B\/ immersion microscopy or conventional quantitative electron microprobe analysis. Semi- quantitative micro-scale element mapping revealed the presence of magnetite, pentland- [C["#`!$##B@# @B/"!#$#j"!B!C magnetite forms threads along former olivine grain boundaries or pre-serpentinization /&

47 2. FeNiCoOS phase relations

(a) (b) (c)

(d) (e) (f) (g)

(h) (i) Polydymite-ss YC

\V& Magnetite Magnetite

(j) Pentlandite (k) Millerite Polydymite-ss e Magnetite Millerite e Magnetite Pyrite \V& YC e

Fig. 2. in variably serpentinized and steatized peridotite samples from ODP Leg 209. (a) Pentlandite in bastite !!"#:%#'*'+ pentlandite (medium grey) located in a paragranular vein, partly altered to awaruite (light grey) and mag- <'!=>#"?@:=%?'*'J- landite (medium grey) intergrown with and rimmed by awaruite (light grey); located in a transgranular vein !=>!":X%Y'*'[ grey) intergrown with awaruite (light grey) and mantled by magnetite (dark grey) located in a transgranular !=>?"Y:??'*'J T!=>]"]Y:XY%Y mm). (f) Pentlandite (medium grey) and heazlewoodite (light grey) rimmed by magnetite (dark grey) in !!"X]:??'*'^_<' <' !=>?":@ ? '* ' + _- woodite (light grey), which is partly replaced by godlevskite (medium grey) and mantled by magnetite <'!!"#:%?'*'[<<:< !!"#:%#?'*`'+ heazlewoodite (light grey), godlevskite (light to medium grey), pentlandite (medium grey), millerite (medium dark grey) and magnetite (dark grey); heazlewoodite has replaced pentlandite during serpentinization. Godlevskite probably replaced heazlewoodite as serpentinization neared completion, whereas the initiation of transformation of heazlewoodite and godlevskite to millerite is most probably related to steatitization !?"Y?:Y#%Y?'*'{|< serpentine along pseudomorphic cleavage plane. Magnetite (dark grey) is in sharp contact with pyrite (me- <'@]>?"]:%??'

48 2.4. Results

Table 3. Opaque phase assemblages and 18O isotope data* for the studied samples

Hole 1274 A 1274 A 1274 A 1274 A 1274 A 1274 A 1274 A 1274 A 1274 A Core 10 15 15 16 17 18 20 22 27 Section 1 1 2 1 1 1 1 1 2 Depth (cm) 3-10 106-114 39-46 44-52 121-129 83-93 121-126 24-32 5-11 Depth (mbsf) 49.33 75.06 75.86 84.14 89.51 94.13 104.11 122.34 147.65 Rock type Du Hz Hz Hz HZ HZ Du Hz Hz Lab code AP-88 AP-92 AP-93 AP-94 AP-95 AP-96 AP-98 AP-99 AP-103 Pentlandite ++ + + Co-Pentlandite +++ +++ +++ + ++ +++ +++ + + Awaruite +++ +++ +++ + ++ ++ + + Heazlewoodite +++ + +++ + Godlevskite +++ Millerite Polydymite-ss Magnetite +++ +++ +++ +++ +++ +++ +++ ++ + Pyrite Chalcopyrite Serpentine +++ +++ +++ +++ +++ +++ +++ +++ +++ Brucite +++ + + + + ++ +++ + + Talc + + vein 18O 7.4 6 4.8 5.4 5.7 5.4

Hole 1268 A 1268A 1268A 1268 A 1268 A 1271 A 1271 B 1271 B 1271 B Core 2 2 4 13 20 4 7 10 17 Section 1 2 3 1 1 1 1 1 1 Depth (cm) 10-16 108-115 26-35 46-55 8-12 105-110 15-22 30-35 98-102 Depth (mbsf) 14.10 16.48 28.04 68.74 103.65 29.55 36.35 50.8 85.49 Rock type Hz Hz Hz Hz Hz Du Du Du Hz Lab code AP-02 AP-03 AP-08 13R1 none AP-55 AP-61 AP-63 AP-67 Pentlandite + ++ + Co-Pentlandite + +++ ++ ++ Awaruite + Heazlewoodite +++ +++ +++ Godlevskite ++ ++ Millerite +++ +++ + Polydymite-ss ++ ++ ++ Magnetite + + + ++ ++ +++ +++ +++ Pyrite +++ +++ ++ +++ +++ +++ Chalcopyrite + Serpentine + +++ + ++ ++ +++ +++ +++ +++ Brucite Talc +++ + +++ ++ ++ + vein + + + 18O 5.9 3.7 4.8 4.1 * 18O isotope data are from Alt et al. (2007); + scarce; ++ abundant; +++ very abundant, Du = Dunite, Hz = Harzburgite

Veins !/#!V[@#&B#@"/ //&$$$""&[#!V # "" / [ C[ #[ /V& !$B!$V/"/! !#"/@!#"/- cur in the same vein. In larger transgranular (isotropic picrolite) veins, typically 0.5-1 mm &["/#!B/&#@&##B

49 2. FeNiCoOS phase relations

/@$V/#V[&$#!#$$- !"!#B[[@#/CB!B/ (up to 50 μm in diameter) than in meshes or paragranular veins. In transgranular veins of !B!C!!$/#" @B"/[#///!#!!/#@#" #@#B"@B"/`/j[!B !@B#"/`/@jL@BV $"/`@j["/!"&@B[# #!@!$#B CU"/`/$/j"!!B!#C + magnetite assemblages in transgranular veins of almost fully serpentinized peridotites micro-scale element mapping revealed the presence of relic cobaltian pentlandite. It oc- ##B!B"3"VB/C- @ C "/[ #/// C "//W!$@!YC#/ /V&"/"B"@!#[ &//V&L/V&B/ W!$@!|#/V&W#VB$#$#B- !C&["&B/V&!C`/j [/$!C""!B!C!$"Y 1268A magnetite in veins is partially replaced by pyrite.

Bastite Serpentine veins crosscutting bastite (serpentine pseudomorphic after pyroxene) are V$"/!#/#U"/- tite, the pentlandite occurring as a solitary phase in bastite exhibits a distinct octahedral cleavage (Fig. 2a). Where serpentinization is advanced, pentlandite in veins crosscutting @""@BU/#`/j[#///# !!+#@W#VB$#!- landite.

!V"//#B$"!B- //$C`/&j"!BC&!B[ B!@B"U/`+[FF*j+#[![ C[/V&$!C!B- C&`//j}![B"@B"/[!/ from reaction to millerite or other higher sulfur-fugacity phases. Relics of the assemblage !#"/$#!B!C!V #/C[$"@/!C

50 2.4. Results

"/!/V&"/$#$#B!C peridotites that have undergone steatitization. With increasing degree of steatitization, #$#!#[!/VB!@B#$##[`/&j <"@##[!BC"![! C/V&}CV["$V- !BB" # `!BB"j / W! $ ! ""/`/&j"!BC!"/ "!B!@B!B!B@"!$"*F[* *E#B$C"!$"Y:+/ !B<`FF*j$#!B!B"$" "#[#"!$"$$Y:+[@##- rence of these minerals is clearly related to gabbroic intrusions.

!! L "@/ B! / / W $ !C `L@ j #"/W#VB$#!B!C![ !C"/##B$#$#B!C !L"@/!#`@j["/C- [C/V&"/`$#B!C&j"- "[@#@#`/[/j{B[!#B! `/j+W""!&@"@! `!B@jL#$##[V!W#VB C&"[V`!B@j["$!BB- "B!/"/V$#"$#B!C"! $"Y:+L!B@/![/$C- [!$B$@L"@/@V//W$ !C$"!#"/!C "/C/V&"/[#/ progressive steatitization manifested in Hole 1268A to magnetite + pyrite + millerite `"/!B[$B&/j!B"!BB"!B !BB"`!BV@B"B@j

2.4.2. Mineral chemistry

Awaruite !B!C!$"Y*E+$# VB@F*"'"[!VB`#!!"- B^L@+j!B""#[##B" "!$"Y*E+#/V!!$:" 51 2. FeNiCoOS phase relations

#/!!@"$ "!@`FF!!"j+#Y:+ //$"EF*"LV@F*'" [#//B"#[/#!" +#@"!$"Y*|"!V@B$# "@!$!"![#$" U"$#W@B"!"- !}#`j@@B"@`'j+@¬ Pasteris (1989) in serpentinites, but could not be found in the samples investigated here. L$"$@!##//@B[$ #"#/$#

!B "! / "B # @9S8`j!!!W"B#!!$@"- `#!!"B^L@+/jL@$!V$" V#B$`±F"j`E"jL""U#$# $"!/'U:`jL$#/[V[@ 1.06 and 1.65. Rather than real variations in pentlandite composition, the elevated ratios [B/#C!L/B ""U#$#$!$"U#$#/$ #!!@BY¬&`'*jB!- !@BX`':j/! "!$"Y:+*+B![ "!$"*FB$!Y*|*E+ $!$#/"V same serpentine vein.

Heazlewoodite L""U#$#$C"B"V @EF'@"#$`±F"j""# $`±F"j`#!!"B^L@+jC $"Y*E+YV[!$"##$!C- ["@V[#!![@## ""U#$#B"B"!$"Y*|[- V[V@#C@"#$`F*"j" "#$`±F"j/Y// #//""/#B[ B!!FF/"U#$#!VB /[!@B/"/#

52 2.4. Results

Co9S8

1268 400°C 1270 300°C 200°C 1271

1274

Fe S Ni S 9 8 Atomic percent 9 8 Fig. 3. Ternary pentlandite diagram redrawn from Kaneda et al. (1986). Pentlandite forms a continuous Co9S8:}4.5Ni4.5S8 solid solution at temperatures above 300 °C. Bimodal Co distribution in pentlandites from Sites 1271 and 1274 indicates temperatures below 200 °C, whereas those from Site 1268 indicate temperatures > 300 °C.

L""U#$#$/V&/@F:`#!!- "B^L@+Ej<B"B$9S8 pro- !@B`'::j}/V&!C["U#$# /BV["U#$#!/V& """"!#/V&$"{/F'[// @F*:"FFF:"[!VB!V [#///""/# analyses.

Millerite L " "U#$# $ " `#!!"B ^ L@+j Y :+/@F'*F}"!/V&"U#- $#/BVLV$"FF"@" @#!/"<`±" jB!B!BB"["/ `"j#"/#B"!B 53 2. FeNiCoOS phase relations

!BB""[@#"@F""&- @B["@!"!$"Y*| V$±"[/@"/ expense of cobaltian pentlandite.

%"%# !BB"$"Y:+""U#$#V@F* F:F[!BB"$"Y*|/"U#$# #$/"/`#!!"B^L@+j"! "@!BB"V[//$"* "BB"/"V"$** "[#!BV/"L "B@F"[@#/"* "|#$!#W#$!BB"`/"/- "!$"Y*|j""!@BV (Supplementary Data Table A6).

Magnetite </#$""!""#$`#!!- "B^L@+*±'"j`±F"j</!/- @!/B"!"/ @!!!@"$ "!@`FF!!"j"/B$&$"Y*E+#Y 1268A magnetite analyses reveal slightly elevated copper and zinc contents (< 0.05 mol. %).

L""U#$#$!BV@FE:FE`#!!"B ^L@+:j&!B/$"FF*E"L$ !B$"`/!"/- j[$!B/[$#[# "!BB"L"B$!B/V/!!$ V"!`E:"jLB!±F"[@#@/ "`#V:E"j$!!!!B @"$FF!!"

Chalcopyrite !B&$"Y:+W@""!- `#!!"B^L@+'j""#$`±FFE"j detected.

54 2.4. Results

2.4.3. Phase diagrams

}#/"#/!/fO2 vs. log fS2 and log aH2,aq vs. log aH2[!@##$ !#$F<["!#@FEFF Y2¤`/Ej<- @V#B&/#"!`@#[V&[V[ j"$"/"""$"@#@#" $&B$V" B$`FFEj#VBVB/"[!! Y2Y2YB"EFF F<- @B[$"@BB$`FFEj[@B[ $V[!BB"[/V&[@/![# !"@!"[/V&[!BB" V[$!!/"L@B/ $"#//@BB$`FFEj/[B#C !V\V&W!@"@V /Y2,aq and H2[V#W#"@! <""[V["/V&V#/ !#/}$!@#$/V& $W"$!!"@!L@B[$ #/$V&@BB$`FFEj$ !!$C/Y2[V"&@B[@B[ $#//##$""@B$"!|# &/"!V/[!@#$ tetrataenite as grey continuous lines to account for the manifested coexistence of pent- #"/{/VBVB/"$B- tem in the H2[Y2[!$"!#@FEFF F <`/j|#$"!B$@!"!$ ![C$!@B[/B!- L/$!"[W! H2,aq and H2[V+##!@![ #@W!@B[$#/$# L@V!!#@!`/jY- V[/@#$VB"!"@B $&$#!V@!@![ W/!BB!#"! !#"@V@![$ project.

55 2. FeNiCoOS phase relations

0 0 150 °C 200 °C –1 Pyrite –1 Pyrite

–2 –2 Vaesite Vaesite Pyrrhotite –3 –3 Pyrrhotite Pentlandite S,aq 2 –4 Pentlandite –4 Millerite Tetrataenite

Millerite –5 –5 Godlevskite

log a H Tetrataenite Magnetite –6 Godlevskite –6

–7 ruite –7 Hematite

Hematite Magnetite Heazlewoodite Awaruite

Heazlewoodite Awa –8 –8 –8 –7 –6 –5 –4 –3 –2 –1 0 1 –8 –7 –6 –5 –4 –3 –2 –1 0 1 0 0 250 °C 300 °C

–1 Pyrite Pyrite –1 Pyrrhotite Pyrrhotite Vaesite –2 Vaesite –2 Polydymite Pentlandite Pentlandite –3 S,aq 2 Tetrataenite –3 Tetrataenite –4 Magnetite Godlevskite Godlevskite log a H erite –4 Millerite –5 Mill Magnetite

–5 –6 Hematite ruite Hematite Heazlewoodite Heazlewoodite

Awaruite

Awa –7 –6 –7 –6 –5 –4 –3 –2 –1 0 1 –6 –5 –4 –3 –2 –1 0 1 0 0 350 °C 400 °C

Pyrite Pyrite Pyrrhotite –1 te –1 Vaesite Pyrrhotite Vaesite ydymi Pentlandite Pol Pentlandite Tetrataenite –2 –2

S,aq Tetrataenite 2

Millerite Magnetite Magnetite –3 RHF –3 erite Polydymitell LHF Mi Godlevskite log a H Godlevskite –4 –4

Hematite aruite

Hematite Heazlewoodite Heazlewoodite

–5 Awaruite –5 –5 –4 –3 –2 –1 0 1 –5 –4 –3 –2 –1 0 1

log a H2,aq log a H2,aq Fig. 4.><:<|}::{:<#? 400 °C at 50 MPa. Dashed lines are the boundaries of the magnetite, hematite, pyrrhotite, and pyrite stabil- <'%_ <<<*< <<<'< phase boundaries as dotted and grey lines. Phase boundaries represent equal activities of the minerals in `*Y#?<^2 and H2T ^}'""^}'<*??%* 2002).

56 2.4. Results

Py Vs main

0 , a [ Py Pd

main Mi , a [ Vaesite Pyrite –5 Polydymite

Pn Mi Millerite Hz –10 Pentlandite ,g 2

S log aH2S,aq = -1

Pyrrhotite f Mt

log –15 Hematite Heazlewoodite Pn Pn Magnetite Hz –20

Awaruite Mt Mt Aw 350°C 50 MPa –25 –35 –30 –25 –20 f log O2,g

Fig. 5.}<:<|}::{:<Y#? #?[*<<< labels in italics); continuous lines are boundaries of awaruite, pentlandite, heazlewoodite, millerite, poly- <<*^2S isopotential is for an activity of 1 mmol/kg. It is calculated for the equilibrium S2,g + H2O,l = O2,g + H2S,aq using SUPCRT92 and assuming unity activity of water. The }::{:*>- <<: interaction. It follows the H2S isopotential, suggesting that H2T< buffered to values around 1 mM.

57 2. FeNiCoOS phase relations

0 0 150°C 200°C –1 –1

Pyrite –2 –2 Pyrite Cattierite Cattierite –3 –3 te Pyrrhotite S,aq

2 Linnaei –4 aeite Pyrrhotite –4 Linn –5 –5 log a H Cobaltpentlandite Cobaltpentlandite –6 –6 Magnetite Magnetite

–7 e uite –7 Cobalt

Hematite Wairauite

–8 Hematit Waira –8 –8 –7 –6 –5 –4 –3 –2 –1 0 1 –8 –7 –6 –5 –4 –3 –2 –1 0 1 0 0 250°C 300°C

–1 –1

Pyrite Pyrite –2 Cattierite –2 Pyrrhotite Cattierite Pyrrhotite

–3 Linnaeite S,aq 2 –3 –4 Hematite Linnaeite

log a H Cobaltpentlandite –4 Cobaltpentlandite –5 Magnetite Magnetite

te –5 –6 Cobalt Cobalt

Hematite Wairaui

Wairauite –7 –6 –7 –6 –5 –4 –3 –2 –1 0 1 –6 –5 –4 –3 –2 –1 0 1 0 0 350°C 400°C

Pyrite –1 –1 Pyrite Pyrrhotite Cattierite Pyrrhotite Cattierite

te –2 –2 S,aq

2 Linnaei

Linnaeite –3 Cobaltpentlandite –3 Cobaltpentlandite log a H Magnetite Magnetite –4 –4

Cobalt irauite

Hematite Wa Wairauite Cobalt

Hematite –5 –5 –5 –4 –3 –2 –1 0 1 –5 –4 –3 –2 –1 0 1 log a H2,aq log a H2,aq Fig. 6.><:<|}::{:<#? =??#?[*<<<% continuous lines are boundaries of cobalt, wairauite, cobaltian pentlandite, linnaeite, and cattierite stabil- <*`*

58 2.5. Discussion

2.5. Discussion

"#$#%#\ rock interaction

The phase diagrams displayed in Figs. 4-6 indicate considerable temperature de- pendences of the positions of invariant points and univariant reaction lines in the H2[ H2S,aq activity plane. Hence, before the H2 and H2$/\#- @"!V/"!#$\#& are required. These can be estimated using phase relationships (Bach et al., 2004) or oxy- /!"!`+[FF*j[![B!" of olivine by serpentine, brucite and magnetite in the presence of fresh clinopyroxene $"#!!$$Y*E+[V@!"!# $!C`±FFF |[FFEj/&WB/- tope data, Alt et al. (2007) estimated variable serpentinization temperatures of peridotites $"{/F'!"[&!! "!#`±F j@/18O (up to 8.1 ‰) of samples $"Y*E+[/"!#`FF j @B18&V#`EEaj:L! B"V$"@#$""!- #`/['*X[':+¬&[FFX&C¬#/&[FFEj }V/$"""![$18& V@`+[FF*L@jL/18"!@ &$"Y:+*E+\B"$$ !""!|@#V/#/ estimates of alteration temperature. In particular, the compositions of pentlandite and !BB""BV"!#$"X`':j! !$""!#@`[j'/WS8'/WS8 in the FFFF "!#/+FF [!!@V## @!W""@!`/j@ and non-cobaltian endmember pentlandite indeed co-occur in veins in some samples from Y*E+[/$""!#$FF `#!!"B ^L@+j}&18V#$"!`E:*Ea[+[FF*j @"!#+/"!$"Y*| `FFj$#@![#"- "!#F #@B+`FF*j@18$&$" Y:+"&$"*#[34S. $" "! *E+FE " ! $" Y

59 2. FeNiCoOS phase relations

:+$##FF //{B["!#"B VWFF VY*E+$#B[18O data exist for that "!+!&$"Y:+@"!- #!!BFF [18O values of those samples (Alt [FF*jBB"V$"###FF `/[ '*jL"!$!BB"/&$"Y:+- `[j3S4!@EFF [@#BV$" EF L!BB""!$Y:+"! /"!#$F /#$"WB/! data.

2.5.2. Redox conditions during serpentinization

V##V#[[WBB" V$W#/!C`/_&['*[ ':+¬&['':j#!/!V/V!C $@B!$"^{/F'"!@B// ! "@/ !B !C ![ ! # "/- !C"/""@/ "!$"^{/F'V#"B/[`± 0.1 vol.%) and hence incapable of buffering H2[/"@/ mineralogy apparently monitor changes in H2,aq activity superimposed by reactions be- V\#!

`j 4.5Fe4.5S8 + 4H2,aq + 4H2O 3Fe + Fe3O4 + 8H2S,aq.

LW"BWB/#$#$#/B"- #"/#@@$C"!BB/ #@B$B/[!#@ FFF `/Ej+#CV#"@/[ although they may co-occur in the same thin section. The assemblage pentlandite + hea- C"/Y2[V#@B/#$ \#[#///!@&@BN

`j 4.5Fe4.5S8 + 6H2O 3S2 + 1.5Fe3O4 + H2,aq + 5H2S,aq

60 2.5. Discussion

#@&@BN

`j 3Fe + 6H2S,aq + 4H2O Fe3O43S2 + 10H2,aq

|/WB/$#/#/// #$#$#/"!#/@&$!- #+#/"!V$!@&C "/`/$j[##@& !["@/[#$`j&!+EFF # @B [ W! !B [ Y[ "@/ !#"/@

2.5.3. Redox conditions during steatitization

L!#!"@/$#C!"!B different from those found in partly to fully serpentinized peridotites. With increasing /$C"/!@B!B#$#!#[ !/VB!@B#$##[L/ WB/ #$# $#/ ` _&[ '* [ ':j ^#/ C $!C!"/W!$#$#!#[ C/V&`/jL!"$!- #@B"@V[#/"B&!$fO2!BN

`Ej 3S2 + H2S,aq H2[

`j 9S8 + H2S,aq H2['

L!"$"/@B!B!@BN

(6) Fe3O4 + 6H2S,aq 2H2,aq + 4H2O + 3FeS2

`Ej`j/Y2,aq and increasing H2S,aq activities. With pro- /VC!"$"@B!BB"`/&j[ a further decrease in H2,aq and an increase in H2[VN

`*j Y2S,aq H2[3S4

61 2. FeNiCoOS phase relations

`:j 3O4 + 6H2S,aq 2H2,aq + 4H22S4

!BC!"/"!B/!BB""- nant assemblage (Fig. 2i). Although this is not an equilibrium assemblage per se, those phases do represent a small range in H2[Y2[VF `/EjL #@B$V!B"!#±EFF @"[ "W"#"#@B$V!B*""!##*FF `&¬X##['jL/!B$"Y:+`#! *EE"j"@"!##F #// #$!B"@L[#["!Y2[Y2S,aq activities @B/$VL!BY:+! !//#$#$#/Y2[U/Y2S,aq conditions `/EjLB!$"C!!/B&$"$ veins and steatitization of serpentinite. In addition to forming vaesite from polydymite in the course of increasing sulfur fugacities,

`'j 3S4 + S2,g 2

V!B/""B!!BB"N

(10) FexWS4 FexWS2

#"B"B$V@/"- peratures (Fig. 4).

2.5.4. Implications for a potential H2S,aq buffer in serpentinite-hosted hydrothermal systems

}WW"$#["@/@VV`#- Vj Y2S,aq concentrations measured in high-temperature vent \#$"#"[B"B"#V/@ {/VB"[V#$"Y2$#""U&/ `"

(11) S2,g + 2H2[¤Y2S,aq + O2,g

62 2.5. Discussion

`[':jL"!#@@W\# "<Y2F [!Y2[!$"</"&- @B[F F<"<Y2[!$fS2UfO2 evolu- V@B #$ !@V thus be suggested that H2S,aq in serpentinite-hosted hydrothermal systems is buffered @B#@@!Y2& "!#{BB"V\#[@#W!B `XB[FF^#V[FFj#//V$Y2S activities that are in /$V3"U&/#Y2VB@#$$- /@B!@&"C"!#`+¬ B$[FFEj$FF `/Ej+VW!!V@BB$ `FFEj[B!C/Y2[Y2S,aq concentrations found B"#//"/@\##@EFF F<+#/!B"B" [VB@V"/@"@/- tered peridotite. Perhaps the serpentinites and soapstones drilled from the area around {/V#&&U#!\C#{/VV [#!$![V[Y2S,aq is set by pentlandite desulfur- C<#["!!`!# ##[j[!!#&BB@#$$"\#!#V H2[W!!!"C#!\C#@#!! the levels of dissolved H2{/V@B"\#B !CEFF #!"B# there is no unique H2[Y2S,aq buffer in peridotite-hosted systems, but H2S,aq should @@B!#@V##"<"!##FEFF YV["V"!$&!C$ /"!#V\#&$"Y*E+V"!#[ &$"Y:+V! H2[Y2[B"$V\#L!@"$ unique H2[Y2[@#$$@#$$"/@#$#W"

2.5.5. Sulfur metasomatism

L#$#$&$"^{/F'/$"FFF `#&[FF+[FF*jLBV@B!"! !#!!"`FF¬&[FFEj+#!- graphic observations reveal, main-stage serpentinization results in desulfurization of pri- "B#[`+¬&['':j#B[#$##@$"& during serpentinization. Indeed, sulfur concentrations in many serpentinite samples are

63 2. FeNiCoOS phase relations

@FF`/*j!$2V5!B!C! "@#`2±EFjVB5"! "!B!CC&LV#$#$ $}#$#$#B#[@" the course of steatitization? Hydrogen produced in copious amounts during serpentiniza- &!#$#$#/!##$##$!"B#[V H2N

(12) S2,g + 2H2[¤Y2S,aq

"B#[$!#$#C@"@#/- !CN

`j `!"B#[jY2[¤Y2[`#j

When serpentinization nears completion the conditions become less reducing and reac- `j!$[/"C/#$#$#/B"- blages such as observed in Hole 1268A to develop. One possible explanation for the sulfur enrichment in completely serpentinized peridotites is a moving serpentinization front. Sulfur is leached from the peridotite during active serpentinization, removed by !C$!!&!C"! !!$B"#/V!C/ !/\#!&#!Y2[B#@#B#"!#[ #$#WB/$#/!VLY2[$[V[# V@$B#@#[V#$#$#/B!#@#$$Y2S,aq V#$$"<`@VjL#[#"#- @VY:+[#\#W#@"#$!- C \# #/ C WB / #$# WB/ $#/#C#!B@!!!$B"#!\ C[#!/#\#"WB# !B"/$\#"W/@#V#[!!- [@##$##W#$#$#[# @#"/B@BB/V#!/\#[ #$#!"!$B"#[V$"Y:+`32¤ a+[FF*j##$/[#$ #$#@#[$"@""B[|¬Y!&`FFFj $#"#"#$VYF:`{/*@- </j##[!!$"/ #/\#V/$"!!!@#$#- """"YV["&BC!

64 2.5. Discussion

2.0 1268A 1270A 1270B 1270C 1270D 1.5 1271A 1271B 1272A 1274A 1.0 S wt.%

0.5

0.0 30 35 40 45 50 55 60 65 70

SiO2 wt.% Fig. 7. Whole-rock concentrations of sulfur vs silica for samples from ODP Leg 209. Partly serpentinized peridotites (< 40 wt. % SiO2) have sulfur concentration that are slightly enriched or markedly depleted relative to depleted mantle peridotites (0.012 wt. % S). Sulfur is strongly enriched in silica-metasomatized (i.e. brucite-free) serpentinites (> 40 wt. % SiO2) and steatites from Hole 1268A. Data plotted are from the literature (Kelemen et al., 2004b; Paulick et al., 2006; Alt et al., 2007). (See text for details.)

VW/B/#$#`/*#&[FFj/ the sulfur-metasomatism preceded the silica-metasomatism. In those samples, steatitiza- !V!!#"!!BW`@j contrast, serpentine replacing olivine is apparently unaffected by steatitization. Because bastite and vein serpentine are usually devoid of brucite they can be readily transformed [@#/!"W#/#" #&#!@$!@$"+!!B[- duction of silica to the system leads to increased oxygen and sulfur fugacities that, in turn, !"#[!!+##!B/WB/$#/[ B/!#"/$"@$V $W"!N

(14) 3Fe2SiO4 + 2H2O 2Fe3O4 + 3SiO2,aq + 2H2,aq or

(15) Fe3Si2O5(OH)4 Fe3O4 + SiO2,aq + H2O + H2,aq

+/@#![&!V[@#- !@#$$[E$"/#@#C#`/¬ Beard, 2007). As silica activity goes up, reaction (15) may be reversed and Fe-rich serpen-

65 2. FeNiCoOS phase relations

$"L#!V&$Y2,aq, required to pyritize magnetite [see reaction (5)]. Because talc does discriminate against Fe much more than serpentine, #["!/!&#/"""""B$@# reacted out, but before replacement of serpentine by talc is complete. The source of silica "!@@B/@@#`|[FFEj##V@ proposed to explain the sulfur and S isotopes systematics (Alt et al., 2007). Both silica- #$#"&$"Y:+[$[@@W!@B VV"$/@@/[$#B$#Y:+ FR#~`X"[FF*j

2.5.6. Possible existence of a free H2-rich vapor phase

+"@V[!#"/#@"!BB/ #@B$B/[!#@ FFF `/j|#"!#"$$&$"^ {/F'/BV!"!#/[$Y2-rich vapor phase may exist in abyssal serpentinization systems. In continental settings active serpentinization produces H2/"`/LB['VB['*|['*: §#&V[':VB[':*+@['::#[':'j +#/B!#!#@B$B- drogen, serpentinization of abyssal peridotites may produce a free H2-rich vapor phase. L@!!!V#B[@W!"&`<"¬[ 2001, 2006) and theoretical considerations (Sleep et al., 2004). Our calculations provide additional support for the idea that H2 concentrations close to or exceeding hydrogen solubilities may develop during serpentinization. Figure 8 compares the H2 concentra- !/#!"/C#@#"N

1 1 1 1 `j 4.5Fe4.5S8 + 93– H2O + 22–3¤'3– H2,aq + 23– Fe3O4E3S2

$ #@#" Y2[/ ¤ Y2,aq. As indicated in the phase diagrams presented @V[ #@/ "@/ ! W"B / Y2,aq concentrations #[!#FFFF "!#/L$$$ pressure is also considered in the calculations and illustrated in Fig. 8. Hydrogen concen- #B"#\#V/$"!B"B" `/#[FFj$"/#"W"#" V#W!YV[{B\#B/$"<`&#- &[FFj##B/!!#$*<`"@ !#{Bj@BB$$[V+#@/ "/L!B!B$/"!#B- 66 2.6. Conclusions

1.5

1 0.5 ,aq 2 0 3D 0 Log aH -0.5

H2 ,aq solubility -1 Aw-Hz-Pn-Mt control -1.5 100 200 300 400 500 Temperature °C

Fig. 8. Comparison of the amount of H2,aq corresponding to H2{::_:: magnetite equilibria (compare invariant point in Fig. 4) and the amounts of H2,aq soluble at pressures indicated by the numbers in italics. It is assumed that the partial pressure of hydrogen in the gas phase cor- responds to the total hydrostatic pressure. Solution curves terminate just short of the two-phase boundary. It should be noted that a hydrogen gas phase could potentially develop at pressures below 50 MPa.

"B"`Y¬|['''<"¬[FFj}#// !$#&/\#"BVWV a H2/![#"&VB$[B$/B- `<"¬[FFj}B//!$" during serpentinization depends on the pressure (Fig. 8). Our calculation results suggest that a free hydrogen gas phase could potentially develop at pressures < 50 MPa. Dihydro- /"\##@V!/@@&$" #`/XB[''*XB¬\[FFj"B!VV$ V!"$\#W##W"$B!B"- !V"$!#"!#$!\#

2.6. Conclusions

}!!!!VVB#$#" for the evolution of temperature and the fugacities of sulfur and oxygen during perido- ##"!$"Y*E+ !C#$"B#/VB"!#$±FF

67 2. FeNiCoOS phase relations

C #!"! !C # / WB/ sulfur fugacities. Sulfur-metasomatism affecting fully serpentinized peridotites is related to steatitization. The evolution of SiO2, H2 and H2S activities is coupled. Dihydrogen ! @# W# @B /2 \# !@@B V $"/@@@"&#!/$$! FQ#~+Y2,aq activity drops, high-sulfur fugacity phases such as pyrite and polydymite precipitate. The sequence of events leads to early perva- V#[/$@BC#$#"+ !"#$#C$![#["V$"&#/ initial stage of serpentinization. In contrast, steatitization indicates increased silica activi- [/#$#$#/B#[[#!BB"!BV#- tion, form as the reducing capacity of the peridotite is exhausted and H2 activities drop. [#[#$#C@#!![/- "$[&$":LV#$f[/fS2,g in the system $!$Y2S,aq, indicating that H2V\#@#$$@# ""U&/FEFF ""#FFF L! @V V ! B"B"LV!"$!#"/"- @/"!B/#@B$B/[ !#@FFFF L""#$# an H2-rich vapor phase may develop in abyssal serpentinization systems, if the pressures $&±F<L!$#/!#/B facilitate the abiotic synthesis of organic compounds. The phase petrological constraints W#@/$/"!#"BV@$- ruite, pentlandite, and violarite.

2.7. *+

}&<Y$!/#!"B"@ <B&/$"#/#}&|- @<+!!$"!@B \Y/#&!V"!" \&$/$"Y:+!#!"- @//$#V@B|_V["|^B##& !$#""@B"#!!L# samples supplied by the Ocean Drilling Program (ODP). ODP is sponsored by the US # `j !!/ # # "/" $ /!#`j[L&#!!$#$"

68 References

!B/"EE$\"#`|+FU|+ FUj@B^\U_W#QL_ B"R

References

+@[ L+ ¬ [ ^ `':'j ~"@ ![ !![ #[ !/B$C$+<$@#</B /BF[E** +@[L+[#[[|&[X[{B[\{[[¬V[<`'::j <B//![~"@![!!! /N/$<_+""N_V[!! +@[[[\|¬X#[<`'*:j##$ "#*[EEE +[^_¬B$[}_[`FFj"!V\#$" #"[ B" B" " /N+ W!" #BEFF [FF@\"""+*[E +[ ^ _ ¬ B$[} _[ `FFEj !C /N $"{B@B"B"\" ""+:[E*E +[ ¬ &[} `'':j #$# !C !N !C!"@#$##$\!B F[''*''' +[¬&[}`FFj!C$@B!$"<+X [<+/N#$#/"B"/\" ""+*[E +[[&[}[[|[}[#&[Y[\[¬|#[\`FF*j Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro W!@B"$#/<+/[ FR`^{/ F'jN+#$#WB/!#B\"B[\!B[\B" :[F:FF[NF:FFFF'UFFF*\FF* +B[ }[ |#W[ +[ |[ X} ¬ [ < `''Fj Y@& $ </B L#[ +~N < ^ #@/ `@B !" $ Mineralogical Society of America). |[}[ \[ [ YVB[ [ #&[ Y ¬ [ <[ `FFEj @ ! N / $" ^ {/ F'[ <+ \"B[ \!B[ \B" [ F'[ NFF'U FFE\FFF*EE |[}[ #&[ Y[ \[ [ $[ |[ <#[} ¬ Y#"![ _

69 2. FeNiCoOS phase relations

`FFj V/ # $ !C N !/!B[ ""B[!!B$!$"<+ `^{/ F'[ *Ej \!B { [ {FNFF'U FF\{F: |[[{"[¬Y""@/[\`'*j\"V$ !B!C[:F: |[[R[¬L[`'*:jB!C [ " §#/V \" "" + E[ EEE Bayliss, P. (1990). Revised unit-cell dimensions, space group, and chemical formula of """</:[** |[ ¬ Y!&[ { `FFFj+ $[ !C B" V[^//"{/*[F:`@+@BjN" ! $ " \# "B # $ \!B F[ *' |CV&[\+[^@#&[+¬XV&[L+`FFj{"!# !B$!+"</:[ |[ < _[ +[ ^ _ ¬ B$[ } _ `''j # $ 2 during !C$VFF FF@\/BE[E |&[<`FF*jL\"R}&@*F@[{NVB of Illinois. |#/#[Y[^"V[{[/[\[@V[+[[{¬"[Y ^`'::j</B$""N<+!# E _B{::[* [ [ |&"[ ^ X[ "[ ^ X[ <+[ _[ [ |[ <[ [ +V/[_[[+¬_[`''*j#/!#$$" +' Q/$"#:[' [ <[ {/@[§[ ##[ [ |#/#[ Y[ B[ [ ^"V[ { ¬ ##[§`''*j"[/@@W!#<+/N //"!!/ /L!B*'[' B[[|#[<\¬X"[X`'':j</"#/"+ /@E N#${/[+<L_U}Y<^_' : Survey. EOS Transactions, American Geophysical Union 79, F920. "6[{¬X!![`':*jY/"!#"B$#!B" 2, Standard enthalpies of formation of pentlandite and violarite. Physics and "B$<E[* "@[+[<{[[L[¬{[\`'jV" <#&W##$_[:: #[{[##[§[|#/#[Y[^V[[_#@#[[|![[

70 References

^!/B[+[+!!#[¬[+`'':jY4 plumes generated by !C$#"[&$ FR$#C <+/\"""+[ #[{[^V[[##[§[|![¬Y"[`FFj\"B of high H2 Y4 V\# #/ $" #"[& @ B"[` ER[<+j"\/B'[E' [<`'':jL""L@NL++[E#$B "$^[</!' &[ {+ ¬ X##[ \ `'j L #$# ! $ B" Economic Geology and the Bulletin of the Society of Economic Geologists 58, ::: VB[<`'*jYB/!C$/ <[ +/B[ $ _" \/B |#$B$_"\/[ VB[<[[[\@[_^[~[_[^$$[\+<¬+/[_ E. (1987). Serpentinization and the origin of hydrogen gas in Kansas. AAPG |#*['E: /[`'*j@B+"</[F /[¬[+`'*j2LV B"\/+$</+$[ +#</[<B['*[+@$! ^[+L`''j\L_$!#"![*E ^[{¬|#/#[Y`':j+!E "+/N! `[j"_L[+"\!B 67, 410. ^[{[|#/#[Y¬[{`''j\""!/B$ <+/[FE NL"!"!"B_ B{F[EEE ^#V[_[#[{[&[_Y[|V#[[V[[^V[[ ##[§[#[^¬+!!#[`FFjL@V\#` ER[ <+jN \# $ #"[ & ! ! " <+ / B" \# " \/B :E[ *E: ^#""[_[`':j|/<W/$YB"#N"_$$ <!VB&[+NBVVB ^#V[{_[{[+§[^#@[\+¬"V[`FFj<@! {B[<+/<@/B*E['*F _&[`'*jL^#"!N+"$$&$# !#""@/@B#"[&_" \/B*F[:F 71 2. FeNiCoOS phase relations

_[ ¬ [ < `'''j "[ W!# /VB /# $ ! $LB # ~ `<+ /[ E j_B{*[EEE _[[Y[\¬_V[|`''*j_$$$!C! / B $ " $#/ !/ / _ B{[::' _[ [ <V[ [ <{[ ¬ </[ + < `FFj $"/$"NL<+ / "!W ER \"B[ \!B[ \B" E[ NFF'UFF\FFFE* [ < _ `':*j ## $ \V&[ 9S8 + B/![ E[ * [|`':j@B$#[[WV"! #$/B[ [ | ¬ |[ `FF*j VB !C# $ /BE:[: #[L[{[[<#"[L[X"[|[L#&[|_¬B[`FFj # V# $ <+ / $LB # ~<\"B[\!B[\B"E[NFF'U FF\FFFE Y[^¬&[_Y`'*j"!" "!</`Ej[::*: Y/[Y[|[LY[/[+¬[L+`'*Fj#$" transfer in geochemical processes involving aqueous solutions. Geochimica et ""+E['' Y[¬|[<_`'''j+@/"$"!$ #B":[FF* Y[ + `FFj L "B" $ #N + V $ ! /" <#/ < L + E[ *'*' &B[^¬B$[}_[`':jYB"!C$! #N_W!"V/$"/B" ""B\"""+F[**: [}[&[_Y¬Y/[Y`''jL'N+$!&/ for calculating the standard molal thermodynamic properties of minerals, gases, ##![$"FFF@FFFF "!# \:[:'''E* X[Y[L&#[¬[L`':j@B$! B"<#"^![':F

72 References

X"[[X&[_[<[^¬B[{`FFEj^{/F' "!/<+/$"E ^_ #F[EF X"[|[X&[_[<[^[`FFE@j_W!BN/ $^//"![F'/[L©N Drilling Program, 75 pp. X"[|[X&[_[<[^[`FFEj{/F'#""BN/ $^//"![F'/[L©N Drilling Program, 139 pp. X"[|[X&[_[<[^¬B[`FF*j{/F'#""BN! F&"& #V @#B B @ <+ /[ E NX"[|[X&[_¬<[^`j/$ ^//"[[#[F'/[L©N ^//"[!! XB[^`''*j#V#!/V"NX[+[ [<[<[^¬_[^`j/$^/ /"[[#[/N^//"[!! ''E XB[ ^ ¬ \[ \ { `FFj $ #@" !#V"N/$"@!\##B \"""+[E XB[ ^ [ X[ +[ |&"[ ^ X[ \[ \ {[ |#[[ ^+[ {B[<^[[_[&[<[[XX[{@[\L[VCC/[ ¬B[+`FFj+$$WB"V[<+ /F #E[*: XB[ ^ [ X[ +[ \[ \ {[ `FFj + ! B"NL{BYB"F*[E:EE XB[ X X `'E'j @# "#/B[ ©[ / temperatures heat-content, heat capacity, and entropy data for inorganic compounds. US Bureau of Mines Bulletin 476. X"[`':'j+"B"#B!B!B"/ B"FFFF V$#/BU#$ aqueous H2\"""+[E X"[¬&[Y`':Ej#/B!$#B/ V"!#!#_B{ *[*': X&C[+¬#/&[+`FFEjL!@4.54.5S89S8 B" "!# $" EFF FF </E[*E X#V[[Y#"[<¬#[§`''j+#@/#$!

73 2. FeNiCoOS phase relations

+"</EE[:'*'FF {[`':jL@V#$#!!"#["!#/ !$Q!RB!!#@B||# `<j`#!j#"[@L"& </#/!<#/E[:F' {[ `':'j </B "B $ # #[ / type spinel peridotite bodies from Ariege (northeastern Pyrenees, France). @#</B/BF[E <"[L<¬|[}`FF:jL"B"B// #/!C$#"[&\"""+ NFFU/FF:FF <"[L<¬[`FFj+"$!$#$ V2 to hydrocarbons during serpentinization of olivine. Geochimica et ""+[*'**: <"[ L < ¬ [ `FFj @ ! "! $ / compounds produced by abiotic synthesis under hydrothermal conditions. Earth B{E[*E:E <[ ^ `FF*j #[ "C :[ <+ /[ ^//"{/F'NX"[|[X&[_¬<[^`j / $ ^/ /" [ #[ F' / [L©N^//"[:!! <[ X `'*Ej L"B" ^ $ / #![ L#{N|# #"V[ \ |[ BC&[ | ¬ X&V&B[ { `'*Ej Y@& $ L"B"^`<N+"C['*j!/[[+N Technical Information Service. [¬/[\`':jYB//$""#&" _B{[F &[_Y`''jL#$V&!&$ _L!$#@V</[F*' R|[ ^[ [ <[ _B[ ^ ¬ /[ } `'':j # [ !"B "# B $ "@ @" $" @ B"!#" R<+/_B {*[ [{¬[<Y`FFEj\""$"""#"[ B"N !C[ /C[ \ @ "B !!\"""+:[ [ | `':Fj ## $ C `3S2j+ B/! |[ *':F #&[Y[|[}[\[<[^Y/<[#[\¬YVB[`FFj

74 References

\"B $ @B ! `<+ /[ FR[ ^ {/ F'j "! $ \#& !/ V" "\/BE[*'F &#&[\[{B[<^[XB[^¬[_`FFj{"!# V!#{BB"[[V$"B/ @!/"""\/B'[E "[¬[L`'*j["$##$9S8 </[**: @[+¬Y"/B[|`''jL"B"!!$" related substances at 298.15 K and 1 bar (105 Pascals) pressure and at higher temperatures. US Geological Survey Bulletin 2131. [ +[ }$&[ { ¬ |"[ X `':*j !C #"[ B" VB <+ / # $ \!B'[E*E* V[L`'Ej+"B"#B$[@&#[# $#*[** [<¬&[+`FFEj"!$!"\"B[ \!B[\B"[FFFE[NFF'U'FFF\FFF'* "[ X[ X&B[ +[ \@@/[ ^[ V[ { < ¬ $[ `FF*j \"B $ B" \# $" #"[ {/VB"[[ <+/"! !V/"\/BE[ B$[}_¬^@@[}_`':Fj!FF FF @N "! $ / $ ! \" ""+EE[F' B$[}_¬^/[X`''j#@#@\B"B"N V$$W["!#[!YV"B $ !/ \# " / N Y#"![ _[ ~@/[ +[<##W[{¬L"[_`j\YB"B"N B[ " |/ \/ \!B </![+"\!B'[E:* B$[}_[[##&[^¬+[^_`FFEj"[B" B""/N"!B!Y[W @#N\"[[{[¬[{<`j< /NYB"|{! E:[+"\!BN\!B</!*:E B$[}_[[##&[^¬[`FF*jWV#"$ #/ !CN+ W!" #B FF [ FF @"!$#"[B"B"" /\"""+*[:*::

75 2. FeNiCoOS phase relations

B[<[{[[^&[Y|¬^#[<`FF*jVV"! reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge FR ^ Y *E+ @# </B /B [ F' /[§`':*j|V$[&[@#$##/!C[ $ YB #"[ & $ X" "/ [ !</[E ![Y[<@"[+[&[L["[\¬|[^X`FFEjY2-rich \#$"!CN/"@"!/$ +"B$$+F[::: "B[¬<"&[L`''jB/!$"N< B B/!B Y@& $ B }/[ ^N+"\!B[!!* [_¬^&[Y|`''jVV"/#"@B"/$ !\"""+'[E'E Stølen, S., Fjellvåg, H., Grønvold, F., Seim, H. & Westrum, E. F. (1994). Phase stability ##!!$7S69S8NY!B"B" !!$7S6"!#$"X'*FX$9S8 from 5 K to *X#$"L"B"[':*FFF #[[+@[L+[[[<#&[|¬<#@[X`':'j !C$+<$[~"@![!!B/ WB/ ! /"B N ! # \ !!+""N_V[!!FF* Thayer,T. P. (1966). Serpentinization considered as a constant-volume metasomatic !+"</[:*F Thompson, G. & Melson, W. G. (1970). Boron contents of serpentinites and metabasalts #N"!$@B_ B{:[ L#&[|_[{[¬X&[<`'':j</"#"### [/""!"!W<+/ #$\!BF[':*': }C[ { ¬ &[ _ { `FFFj ^/#/ #"[ $" @ #@"B"B"@B"!/#V\#"! #$\!BF[:':EF }B[L`''j_U[+$!&/$/""/$## B"N&/VV/#`V*Fj{V"[+N {{V"{@B }B[ L ¬ V[ `FFEj #[ $ "B" $ /""/$"#B"N

76 References

^!"$_/B`j_/B|+"!B[{{ §#&V[<["&B[<[^BB&[|+¬^[+`':jYB/ "/B$$!B!$"&XB& <#^&B+&"#&[EFE

77 Iron Partitioning and Hydrogen Generation During ! */ %$ the Mid-Atlantic Ridge

Abstract Serpentinization of peridotites generates large amounts of aqueous dihydrogen (H2[j[@B!$MB#[[/[# !^B/!#!WC ferrous iron in olivine to ferric iron in secondary magnetite and serpentine. Poorly under- stood is the partitioning of iron and its oxidation state in serpentine although they impose an important control in dihydrogen production. We present results of detailed petrographic, mineral chemical, magnetic, and Mößbauer analyses of partially to fully serpentinized !$"^//"`^j{/F'[<+/`<+j L##$$#/!C "!!#@!! models. In samples from Hole 1274A, mesh-rims reveal a distinct zoning from brucite $!"BV[$@BC$!@#/"/- [B!"/#"""L"!$ W/!`lithology (harzburgite vs. dunite). Model calculations suggest that both partitioning and oxidation $VBV"!#&#/!C- tion. Serpentine and brucite from Hole 1274A may have formed at temperatures ranging $"±FF @#&&`Uj$"±F+#"# V$"#/"/!C"!#@FFMF U±[B/$#/"W"+"!#@VF dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen depends on the availability of an external silica source. At these temperatures the extent of V!C#$[!#"#B/[ #/#/$"#+L±F [B//$ by the formation of brucite, as dissolution of olivine to form serpentine, magnetite and brucite requires no addition of silica. The establishment of the common brucite rims is #V@#!@#[\ metastable olivine-brucite equilibria developing in the strong gradient in silica activity @!BW`!jV`!@#j

3.1. Introduction

!C$!"!![!- #B/U$"$!//[/[- sequences for rheology, chemistry, and microbial habitability of the oceanic lithosphere `/[[''[''_[''*\[FFE Iyer et al., 2008). Besides its geophysical and biological peculiarities, distinct chemi-

78 3.1. Introduction

$#"&B"B"_W/B$ ##[!Y//$"`/"!#j`"!#j[ B/@V@B"$"&B" $"W"V"_`#[FF^#V[ FF_V[FF:|[FF*XB[FFX|[FF' <"|[FF'"[FF*![FFEjL/B#- ing conditions found in continental and abyssal serpentinization settings, indicated by the !$V"UB`/["@['_&['*[ ':X|[FF'&[''j[V!#W$$# in primary minerals (e.g., olivine) of the protolith to ferric iron in secondary phases, ["/!"[@B!CB incorporation of iron in serpentine and magnetite, but also formation of iron-bearing bru- `|[FF^R+X[FFE_VL""$$['* <B['*B$[FF*j+"$$[/"#$$#- corporated into serpentine and brucite lead to less hydrogen production. The conjunction of high concentrations of H2[$##`2,aq) has @#"@BB$V\#"/$"/"!#\ B"B"[##@B@#B/#"[&[ /[@`^#V[FFj{/V`"[FF*jB-

"[<+/`<+j+#/2[$"{ BB"[`<+[&"$$WjV@!#[ #/@W"B[@#$$@B!#@B"

MgO–FeO–SiO2–H2O (e.g., Frost and Beard, 2007). In addition, experimental and theo-

#["#@2,aq and high H2,aq concentra- #/!C`+B$[FF|[''j

Beard (2007) discussed the effect of silica activity (aSiO2) on magnetite formation and underscored that the presence and distribution of brucite is critical for the interpretation $!C!+#/@#@!["/ and brucite is of importance for quantifying hydrogen production during serpentinization, B@!""B$@#W/ serpentine and magnetite in abyssal serpentinites. The paucity of published brucite analy- $"@B!"!"!$!B- "W!"!"#"!_VL""$$ (1972) retrieved from a data compilation of brucite-serpentine assemblages in alpine ser-

79 3. Iron partitioning and hydrogen generation during serpentinization

$"$"@&$@#YV[B"B$@# compositions from linear extrapolation of serpentine-brucite mixed analyses in abyssal !`^//"[^[{/F'j#@#- #!'"!@B^R+X`FFEj$"! V$"#""#<$`^{/ 195). Obviously, there is a need for more detailed and systematic analyses of co-existing serpentine-brucite pairs in abyssal serpentinites to further our understanding of hydrogen production during serpentinization. L""#!V/$"$!- rived from detailed petrographic and mineral chemical investigations of partly to fully !C!$"^//"{/F'`<+ j} & / ! ! " ! !/ considerations of the system SiO2

3.2. Analytical methods

3.2.1. Microscopy and electron microprobe analysis

<!B"!@B`_<+jML!- BV/"\/#/{^<©Y"- ""!<"!BCQ_{#!!@©+ :'FFR"!@VB$X`\"Bj[#!![V V/!V!"<BC/V- /$F&$@"#$F+$#B$#3"@"" |B#"##/ L~+"$+"/`''j<""!!/@&- `|_j"/$!"#"!"!/! observations.

80 3.2. Analytical methods

3.2.2. Mößbauer spectroscopy and magnetization measurements

To quantify the amount of magnetite present and the distribution, coordination and oxidation state of iron in mesh-rims of partially to fully serpentinized peridotites, <%@# !! "/ V/ # $ &</"`

3.2.3. Geochemical modeling

#$#@#"$#$"[$ ##!W"L'$ @`[''jLL'@#"B" $"Y/`'*:j}BV`FFEj$"[& Y/`'::j[&`':'''*j}BV`FFEj$ V/##!#"[#$"- B"$E-[

Tagirov and Schott (2001), greenalite [Fe3Si2O5(OH)4], minnesotaite [Fe3Si4O10(OH)2] and ferroan brucite [Fe(OH)2°$"<"|`FF'j#"$$- @#` ¤E"3 mol-1j&$"}BV`FFEj $"<"|`FF'j$"&`F"3 mol-1j[ the pure Fe-endmember of the brucite solid solution. As experimentally derived ther- +3 modynamic data of Fe -serpentine [Fe2Si2O5(OH)4°V@#

81 3. Iron partitioning and hydrogen generation during serpentinization

\@@$/B$$"`D\°f j$/!B#"!!$ "&"`':'j"B[ #$"!B/V in Holland (1989). In these computations, hydroxide and oxide bonding of metals in the !B"#$Y!B$#$ "!#"/#"X!!R#$

(1) Fe2Si2O5(OH)4 + 3Mg(OH)2¤

![ +3 ¤![ ![ M![ Fe -serpentine chrysotile Fe(OH)3 brucite

Table 1. Calculated thermodynamic data of Fe+3-serpentine Mineral Cp° a b x 103 c x 10-5 V° S° MgO* 9.03 10.18 1.74 -1.48 11.25 6.44 * Fe2O3 25.04 23.49 18.60 -3.55 30.27 20.94 * Mg3Si2O5(OH)4 52.90 75.82 31.60 -17.58 108.50 52.90 ‡ Fe2Si2O5(OH)4 50.85 68.77 44.98 -16.69 105.03 54.52

Polyhedral unit g † § ° Fe2O3 -185.49 Fe2Si2O5(OH)4 Gf -708.36 † SiO2 -204.10 † H2O -57.34 * † ‡ § Helgeson et al. (1978); Chermak and Rimstidt (1989); Fe2Si2O5(OH)4 = Mg3Si2O5(OH)4 + Fe2O3 - 3MgO; Fe2Si2O5(OH)4 = Fe2O3 -1 -1 3 -1 -1 -1 + 2SiO2 + 2H2O; Cp° = heat capacity (cal mol K '%<% mol ); S° (cal mol K ); g -1 ° -1 (kcal mol ); Gf (kcal mol )

Y!B$@#B&$"Y/`'*:j those of Fe(OH)3&$"L`['':jL"- tropy (S°) of Fe+3!"#/O/#"/"P`Y- /['*:"Y/[''Ej$`jL"B" data of Fe+3-serpentine are summarized in Table 1. L"B"!"/##/"!#_U[ V:F`}B[''''@j#"C"B"@"- @#/L'`[''jL_U@/XV# $!#$F<"!#$"FEFF " #$VB$[$V/!# |#"[|^@BY&!"$" }BV`FFEj+VB$[#"@#B$#

![W!$!/#![$VB$[$2 `^#""[':j/_!BB$"#"W- /##!"#!`/ ""@NB[/[&[+3-serpentine), brucite (Mg-brucite, Fe-brucite), talc (talc, minnesotaite), orthopyroxene (enstatite, ferrosilite), clinopyroxene (diopside, hedenbergite), chlorite (clinochlore, daphnite) and tremolite (tremolite, Fe-actinolite). Serpentine in abyssal serpentinites is in most cases chrysotile or lizardite.

82 2.3. Analytical methods

Antigorite is common in alpine-type serpentinites but rarely found in abyssal serpen- +@B_V`FFEj|`FF*j$$@ "B"!!$BCV$ chrysotile to represent the Mg-endmember of serpentine (cf. Wilson et al., 2006). To ac-

#$+![&¯+2Si2O5(OH)4] instead of amesite

(Mg2Al2SiO5(OH)4j#"[@#_!B!@ /"W/$#|V["!$ [W

83 3. Iron partitioning and hydrogen generation during serpentinization

"!#$B"EFF [#/#@#"- @#$"\#!"!#$$[" #@#""!$"\#$@#&B"$#$"- !##"#/!"BB#" $"BB#"[$$VUV$"#B#@#" "@/[$"F*"!#F/"!# $%()"&*# – These models emulate the entrainment of heated $![W"!#!#"B! ##`V 3 molal. For this reason, all reaction paths "UCF L!#$"$N}!&/$ #/_}O!&#!P!#/_ ""#$!#///"!#`F[FF[ F[FFF jL$""#$! "#/U/C@$B !"!L\#[W!""#- [U$!!W"BFL!##W" #@#"@#@#$"\#!$#$ &

84 3.3. Results

3.3. Results

3.3.1. Petrography

Details about the geological setting and comprehensive description of drill-cores $"^{/F'VB@!#@@B#`|[FFEX- "[FFEFFE@j[$&$@VB[! LV/$&B$ of primary olivine (~ 0 – 35 vol. %), orthopyroxene (~ 0 – 30 vol. %), clinopyroxene (~ FMVj!`FMVjL!"B""W- plicitly but are similar to those reported in Seyler et al. (2007). Harzburgites and dunites are partially to fully serpentinized (65 – 100 vol. % secondary minerals). Peridotite from Y*E+VB"W$!- C`X"[FFE@jYV[V[W$- !C/BV@$#B!#!"B"W to each other. The micro-textures of serpentinized peridotites range from pseudomorphic mesh and hourglass textures after olivine and bastite textures after pyroxene to transi- @@W#[!#"!&/W#<"! are extensively veined by paragranular and transgranular serpentine veins. Paragranular V$""C/&!#!!B[- /#V#!!B`RYB[''jM#[B !B"#!$MMMM! "$X|`FF'j $/$#!/!B$BB#- cates, oxides and hydroxides in pseudomorphic textures and veins. Due to the intimate /$!@#[""/#@ ! "!L V" $[#B # "!@ B`_<+j$"[ +M!B!C&["W# V""!@#"" typically found. In samples from Hole 1274A mesh-rims commonly reveal a distinct C/$"@@#$V[$@BC$ !@#/"/[B"!"/ outermost mesh-rims (see Fig. 1). While the brucite abundance decreases, the amount of magnetite and serpentine increases from center to rim of each individual mesh. Magnetite is dispersed throughout the matrix and it becomes more coarse-grained from center to rim. In many cases mesh-rims are bordered by trains of anhedral to subhedral magnetite

85 3. Iron partitioning and hydrogen generation during serpentinization

a) b) Mgt Mgt

PAM Brc Fe-Ni-poor Srp Ol Ol

Brc vein Srp

c)Si d) Fe e)Ni f) S Mgt

Ol Ol Ol Ol

Brc 20μm 20μm 20μm 20μm increasing intensity g) Ol h) Brc Mgt Ol Srp Mgt Brc Brc picrolite vein Fe-poor Srp 0.2 mm 0.3 mm

Fig. 1. Back-scattered electron images, element distribution maps and photomicrographs of partly serpentinized peridotites from Hole 1274A. (a) Transgranular serpentine vein crosscutting pseudomorphic mesh * }< contents. White box indicates the area mapped in Figs. 1c–f (sample 1274A-10R1, 3-10 cm). (b) Pseudo- morphic serpentine (Mg# 95) and Fe-rich brucite (Mg# 80) growing after olivine. Magnetite is present in ]?'_<* Note the rugged interface of olivine and brucite indicating disequilibrium. (sample 1274A-22R1, 24-32 cm). (c–f) Detail from in Fig. 1c (white box). Element maps depict Si, Fe, Ni, and S in mesh-rim. Pure Fe-rich brucite (Mg# 80) is present at the interface with olivine. The proportion of serpentine relative to brucite increases from center to rim. The abundance of magnetite, Ni – Fe alloys, and sulfur-poor Ni–Fe- *'<* Note brownish brucite along veins (plane polarized light; sample 1274A-10R1, 3-10 cm). (h) Picrolite vein crosscutting mesh texture. Magnetite forms a network tracing former olivine grain boundaries and intra-grain cracks (sample 1274A-6R2, 128-135 cm). (Ol = olivine, Srp = serpentine, Brc = brucite, Mgt = magnetite, PAM = pentlandite + awaruite + magnetite)

86 3.3. Results

/C//$"3""F3"&$#!# "$@@#V[@#@# /C$""#////!/VB$ V`$X|[FF'j"&$#!#"`j ![#C}&B!CV!! /B!+`±F)"j@#C##V`- $@B_<+j[B/C@B!"!B}/ W$[/@#$V@"!/VB[ &$@#CV!@ + – In samples from Hole 1274A relics of orthopyroxene are commonly preserved. It has exsolution lamellae of clinopyroxene parallel to the (100) plane. The pseudomorphic replacement of orthopyroxene, i.e., bastite, is by serpentine B@#"/V!!BW&$" Hole 1274A (cf. Seyler et al., 2007) and commonly preserved. VeinsM/VV$![B! "/[@#@##@[#/"/"!_<+

V/B2 contents at the olivine interface and may point to the presence $@#[@#V@&"/W$@## ##VB@V[/#!V##B@#- "/&$#!#"LVB/ `0F3"j"![B!!#"`03"jVV "!/#V["/"/[ "!V/""!V##B V $ "/ &$# ! /# V #- /"!/B@#@B@@#[/" V&@#L/$/#!V`!j composite, laminated isotropic and anisotropic serpentine oriented parallel to both vein @#LB$"/##BV"/@ @B@V/@#@#}!V#@[B B&"/@#

3.3.2. Mineral compositions

Table A1 reports representative EMPA data for olivine, orthopyroxene, serpentine and brucite from ODP Site 1274 (the complete set of analyses is available from the ##j/##

100 a) b)

80 Mg# 70

50 100 c) d)

80 Mg# 70

50 100 e) f)

80 Mg# 70

50 0 10 20 30 40 50 0 10 20 30 40 50

SiO2 [wt. %] SiO2 [wt. %]

Fig. 2. Regression analyses of brucite and serpentine in mesh textures of rocks from Hole 1274A. (a) Sample 1274A-4R1, 104-105cm contains both Fe-rich (Mg# 81) and extremely Fe-rich (Mg# ~ 55) brucite. (b) Sample 1274A-6R2, 128-135 cm contains brucite with a mean Mg# of 79 although some brucite has slightly elevated Mg# of ~ 85. (c) Sample 1274A-10R1, 3-10 cm exhibits a rather uniform brucite composition with Mg# 80. (d) In sample 1274A-17R1, 121-129 cm no pure brucite was detected, however, a rather low Mg# of 72 is indicated. Note the bimodal distribution. (e) Sample 1274A-22R1, 24-32 cm contains brucite with a mean Mg# of 81, however, the whole range is ~ 75 - 85. (f) Sample 1274A-27R2, 5-11 cm contains brucite a mean Mg# of 82 but Mg# is in places 85. size of secondary phases, virtually all microprobe analyses represent the composition of mineral mixtures on a submicron scale. MV

88 3.3. Results

+ H2O + O2

Fe Mg Si

Mg3Si4O10(OH)2

Fe 2Si Mg Fe Si O (OH) 2O5(OH) 2.85 0.15 2 5 4 4 Fe3Si2O5(OH)4 Mg3Si2O5(OH)4

4 (OH) 5 FeSiFeO Mg 2

Mg Fe (OH) Fe 0.8 0.2 2 Mg

Fig. 3. Fe–Mg–Si ternary plot (molar proportions) projected from H2O similar to that by Wicks and Plant (1979). Tie lines extend to hypothetical serpentine Fe+2 and Fe+3-end-members. Serpentine in mesh-rims +2 +3 +3 contains both Fe and Fe as some analyses plot along lines to Fe -serpentine, Fe2Si2O5(OH)4, and greenalite, Fe3Si2O5(OH)4. The presence of Mg-cronstedtite, Mg2FeSiFeO5(OH)4, is not indicated. Most serpentine-brucite mixed analyses plot between serpentine (Mg# 95) and brucite (Mg# 80), however, samples 1274A-4R1, 104-105 cm and 1274A-17R1, 121-129 cm trend towards much higher Fe contents.

$:M'$ME@#+2O3 and 1.0

2O3!BW!`

Al2O3. The composition of clinopyroxene exsolution lamellae could not be determined. Brucite – Regression analyses of brucite-serpentine mixtures in mesh-rims reveal

/!B$@#`2±j#[ #/V#B#/V#$- V#B["[/BV<`j[`Fj and Al2O3`F'j_V#$#`/V3 in Table A1) are most &BM#[!@#"W[#

89 3. Iron partitioning and hydrogen generation during serpentinization

45 100 40 a) 35 90 30 Mg# Mg# 25 80

[wt. %] [wt. 20 2 Ol 15 Srp 70

SiO 10 5 SiO2 Brc Brc + Srp 0 60 0 10 20 30 40 45 b) 96 40 94 35 92 90

30 Mg# Mg# SiO2 88 25 Ol 86 [wt. %] [wt. 20 2 Srp 84 15 82

SiO 10 80 5 Brc Brc + Srp 78 0 76 0 10 20 30 40 50 60 70 45 c) 94 40 92 35 SiO2 Mg# 90 30 Mg# 25 Ol 88

[wt. %] [wt. 20

2 86 15 84

SiO 10 5 Brc + Srp Brc Brc Brc + Srp 82 0 80 0 20 40 60 80 100 120 45 Ol 94 40 d) 92 35 90 30 88 Mg# 25 Srp

[wt. %] [wt. 86

2 20 84 15 Mg#

SiO 10 82 5 Brc 80 SiO2 Brc + Srp 0 78 0 5 10 15 20 25 30 45 94 40 e) 35 92

30 SiO2 90 Mg# 25 Mg# 88

[wt. %] [wt. 20 Ol

2 86 15 Brc + Srp 84 10 SiO 5 Brc 82 0 80 0 5 10 15 20 25 30 35 40 distance [μm]

Fig. 4."J[>_ <*'!=>="?=?# (b) 1274A-6R2, 128-135 cm, (c) 1274A-10R1, 3-10 cm, (d) 1274A-22R1, 24-32 cm, (e) 1274A-27R2, 5-11 cm.

90 3.3. Results

$@#&B@"`FFMFj in Table A1. – The composition of serpentine varies depending on its precursor mineral and textural context. Regression analyses reveal that serpentine in mesh-rims is ""/V[//$"

/B@FL+2O3"B@F[@#"B@ #!F[!@BV$"/! !VB#$#[#///!$M#[ serpentine matrix. #"!!$!BW`@j

'/$`±*j[+2O3`±j[2O3`± j[<`±Fj$`±Fj"!"- !`/[L@+j!"/!/#- V

"/[

3.3.3. Mößbauer spectroscopy and bulk-magnetization

<"$!B$#B!C!!@B" drilling to investigate Fe+3U5V#$!@#!$ "!W$!C|#&"/CB# to relate Fe+3U5V#"/$""|#&"/C- yses and thin-section petrography revealed a positive correlation of magnetite content W$!C}V!["/ /[B"!$#B!C&&B!C!-

Table 4. Mößbauer spectroscopy and bulk magnetization results of micro-drilled mesh-rims Mößbauer Titration* Magnetization +2 +3 +3 +3 % Fe304 Fe Fe Fe 9} Fe 9} % Fe304 Sample 1274A-6R2, 128-135 cm 0 52 48 0.48 0.45 0.33 1274A-10R1, 3-10 cm 14 27 59 0.68 0.62 0.79 1274A-15R1, 106-114 cm 23 26 51 0.66 0.68 1.38 1274A-17R1, 121-129 cm 21 43 36 0.50 0.56 1.20 1274A-20R1, 121-126 cm 52 17 31 0.66 0.68 2.80 1274A-22R1, 24-32 cm 0 70 30 0.30 0.57 0.16 1274A-27R2, 5-11 cm 11 43 46 0.53 0.51 0.79 1274A-22R1, 24-32 cm‡ 0 66 34 0.34 0.57 0.43 * data from Paulick et al. (2006); ‡ micro-drilled bastite

91 3. Iron partitioning and hydrogen generation during serpentinization

["//B![<%@#!V+3U5V# @FFFE:$B#B!`L@Ej/B$#B!- tinized peridotites hosting abundant magnetite, Fe+3U5V#B/ /$"FF:"!`*E+[E"jBC @#&"/C+3U5$!@</CB[" that bastite hosts almost no magnetite. The Mößbauer spectra indicate that about one third $V}![#$"B`#- &[FFj$@#&&!//"<%@##$ the mesh-rims.

3.3.4. Geochemical reaction path modeling

Thermodynamic calculations have been used for several decades to examine the $/""!B$!`/[|"[': _V['*\[FFE['j#[V!- "$!C`$W"![!#"!#j"$"

#V!#@B"

/["!B//#/!- !##&$&//+3#!&@B brucite during serpentinization. Although it appears possible that brucite contains Fe+3, #B#$#"$@#

,"-./01#%(211*3 ()C'"-'6#

This reaction path model investigates the isobaric retrograde hydration of monomineralic dunite in a closed system. Figure 5 depicts a summary of the model results $#@#"""@/"!$"\#$#- $"!#+EFF V"#@#"""@/[ accompanied by trace amounts of serpentine and magnetite (Fig. 5a). With decreasing temperature the amounts of serpentine and magnetite increase at the expense of olivine //CN

(2) olivine + SiO2,aq + H2!"/Y2,aq.

As the formation of serpentine according to (R2) consumes dissolved silica, the SiO2,aq VB!`/[$[/F[j"! the dearth of aqueous silica hampers the serpentinization of olivine in a dunite and thus the effective formation of magnetite and H2[`/[[$j!

//"!#[/2,aq further until the system reaches #V!$V`

(3) olivine + H2!@#"/Y2,aq.

"/$@#$#B@&$V and the coincident formation of large amounts of magnetite and serpentine (Fig. 5a). In

$V[Y2[!B!& #:*""`"<[/j"B[VB$ !!@#`/j+`j! [&$W#$/@C/V reaction is solely driven by the decrease in temperature. After olivine has completely

#`[L±F j[Y2,aq and SiO2[@B#@@ serpentine, brucite and magnetite, the typical phase assemblage found in completely ser- !C#L"#$!"B- ing temperature (because the amount is constrained by the amount of SiO2@#& 93 3. Iron partitioning and hydrogen generation during serpentinization

10 10 a) g) serpentine olivine olivine 1 1 serpen- brucite clinopyroxene trem. tine 0.1 0.1 magnetite magnetite chlorite brucite 0.01 0.01 minerals (moles) minerals (moles)

0.001 0.001

100 1.0 100 tremolite 1.0 b) h) clinopyroxene 0.9

serpentine Fe 0.8 0.8 Fe +3 90 serpentine +3 / /

0.7 90 Fe in serpentine 0.6 chlorite 0.6 Fe in serpentine olivine 80 0.5 0.4 brucite Mg# mineral 0.4

Mg# mineral brucite 80 0.3 70 0.2 0.2 +3 Fe / Fe in serpentine Fe+3/Fe in serpentine 0.1 70 0.0 60 0.0

400 400 c) i)

300 300 brucite in

200 200 ,aq (mmolal) 2 ,aq (mmolal) H 2 magnetite out H magnetite out 100 brucite in 100

0 0 100 100 d) j) 80 80

60 60 serpentine magnetite serpentine magnetite 40 40 ,aq related to mineral ,aq related to mineral 2 2 20 20

0 mol% H 0 mol% H

10 0.990 e) 12 0.988 k) 0.988 11 9 0.986 aH2O aH2O pH 0.986 10 pH

8 aH

aH 0.984 9

0.984 2

O pH

7 O

pH 8 0.982 0.982 6 7 0.980 0.980 6 5 0.978 0.978 5 4 0.976 4 0.976 0 Cl 0 Cl f) l) -1 Mg Na Na -2 Ca -2 K K -3 Ca -4 Al Si -4 Fe -6 Mg -5 -6 -8 Si -7 Fe Al -10

Log concentration (molal) -8 Log concentration (molal) -9 -12 25 100 175 250 325 400 25 100 175 250 325 400 Temperature (°C) Temperature (°C)

Fig. 5.[<T> 1 kg of seawater was equilibrated with 1 kg of dunite (a-f) and harzburgite (g-l), respectively, at temperatures between 25–400 °C. See text for discussion.

94 3.3. Results

B"j["#$@#B#"[! particular brucite become increasingly Fe-rich as the temperature decreases. This is due to the temperature-dependent changes in sub-reactions of the brucite-serpentine-magnetite equilibrium that increase the stability of the Fe-end members of serpentine and brucite relative to magnetite. That is, the equilibrium constants for (R4-6) favor increasingly the !#/"!#N

(4) Fe3O4 + H2O + H2,aq + 2SiO2[¤3Si2O5(OH)4

(5) Fe3O4 + 2H2O + H2[¤`Yj2

(6) 2Fe3O4 + 7H2O + 6SiO2[¤2Si2O5(OH)4 + H2,aq

The shifting equilibrium of (R4) and (R5) reduces the amount of H2 present at equilibrium. Overall H2[!/"!#["BV@B`j

The activity of SiO2[@B\#&#@`@#$$@B![ ""@/[/[!@#jV"&`#"jB- gen is implicitly acounted for in the reaction path model. As (R4–6) proceed to the right /"!#["/"!BW#@ the system is entirely controlled by exchange equilibria among serpentine and brucite +3 (Fig. 5a). The H2[VB!@B$ -serpentine (Fig. 5b, [jN

(7) 2Fe3Si2O5(OH)4 + 2SiO2,aq + 5H2¤2Si2O5(OH)4 + 3H2,aq

(8) 2Fe(OH)2 + 2SiO2,aq + H2¤2Si2O5(OH)4 + H2,aq.

+/`M:j"!#@B$+3-serpentine, and thus the activity of H2[["@B2[VB[\B@#$$@B- N

(9) Mg3Si2O5(OH)4 + H2¤2,aq + 3Mg(OH)2.

With the exhaustion of magnetite, serpentine becomes increasingly magnesian as tem- !#![@#@"/B`/@:M'j L!Y$\##/$"E:EFF ' +!!@B<"|`FF'j!Y/B"@B@B of brucite, but is ultimately controlled by the entire mineral assemblage. Except for high "!#[V

95 3. Iron partitioning and hydrogen generation during serpentinization

,%.%(211*3 ()C'"-'7#

L&"!#/"##! "!![[C@#/`N!WN!W¤:FNNVj+#/ !!![/##[@# remains largely unaltered in most samples from Hole 1274A (cf. Klein and Bach, 2009). Figures 5g–l summarize the model results in terms of equilibrium mineral assemblages "!$"\#$#$"!#+EFF #- @#"""@/$V[!["" amounts magnetite (Fig. 5g). The predicted concentration of H2,aq is higher compared to model 1A, because orthopyroxene provides the SiO2 needed to produce serpentine and "/`/j//CN

(10) olivine + orthopyroxene + H2!"/Y2,aq.

At temperatures above the quasi-invariant point of olivine-serpentine–brucite equilibrium, concentrations of SiO2["!"+`/j[@#"# more serpentine is produced according to (R10). Diopsidic clinopyroxene forms at the W!$"W#"!#@'F "&@B[ VB"@#@#$"/B/"!# `E VF jL"/$@#$$"$"/ serpentine at the expense of olivine according to (R3), resulting in H2,aq concentrations

$'*"<EF L!"W"#"$Y2,aq is similar to that in

!"!#V[@#Y2[!!&@B higher temperature. As the temperature decreases, the concentration of H2[! "/@&&#!@B@#[![#- tions. The amounts of serpentine, clinopyroxene and chlorite remain virtually constant /"!#["#$@#![- rite and brucite become enriched in Fe as temperatures decrease, consuming magnetite `EMj#W#L±E +B"!# of H2,aq and SiO2[@#$$V#@B!@##@#"`/ - j/!/$"2,aq to HSiO3 at pH ~ 8.5 the concentration of SiO2["L± `/&jL/!Y"!" +@&$!"B!BW!BW` L"&R`Lj"!jL@/["#$ \#+"!#@E [@#!BW!#@#" !Y[B"!#!$V "/#"V"!#/@#$$@B![@#- cite, and clinopyroxene. Serpentine is more Fe-rich than serpentine of model 1A and more serpentine is produced due to the higher amount of SiO2B"`//[j-

96 3.3. Results

10 100 1.0 Fe Srp

(a) Brc (f) +3 Srp

0.8 / 1 90 Cpx Fe serpentine 0.6 0.1 80 Brc Srp Chl Brc 0.4 Fe Srp Mgt +3 / 0.01 Mg# mineral 70 Fe 0.2 moles minerals Mgt 150 °C (k) 0.001 150 °C 60 150 °C 0.0

10 100 1.0 Fe Srp (g)

(b) +3 Brc Srp 0.8 /

1 90 Cpx Fe serpentine 0.6 0.1 80 Srp Brc Mgt Brc Chl 0.4 +3 /Fe Srp Fe

0.01 Mg# mineral 70 0.2 moles minerals Mgt 200 °C 200 °C (l) 200 °C 0.001 60 0.0

10 100 1.0 Fe Srp (c) Brc (h) Srp +3

0.8 / 1 90 Cpx Fe serpentine 0.6 Brc 0.1 80 Srp Chl Mgt Brc 0.4 +3 /Fe Srp

0.01 Mg# mineral 70 Fe Mgt 0.2 moles minerals 250 °C 250 °C (m) 250 °C 0.001 60 0.0

10 100 1.0 Fe Srp (d) (i) Srp Brc +3

0.8 / 1 90 Mgt Brc Fe serpentine 0.6 0.1 Srp Cpx 80 Brc Chl 0.4

0.01 +3 Fe Srp Mg# mineral 70 / Fe moles minerals Mgt 0.2 300 °C 300 °C (n) 300 °C 0.001 60 0.0 Ol 10 100 1.0 Fe (e) Srp (j) Srp Ol +3

0.8 / 1 90 Fe serpentine Cpx +3 Fe /Fe Srp 0.6 0.1 Chl 80 Srp 0.4 Mgt

0.01 Mg# mineral 70 0.2 moles minerals Mgt 350 °C 350 °C (o) 350 °C 0.001 60 0.0 0 1 10 100 0 1 10 100 0.1 1 10 100 water to rock ratio water to rock ratio water to rock ratio

Fig. 6. Predicted alteration mineralogy of reaction path models 2A and 2B at constant temperature as a fuction of water-to-rock ratio. (a-e) predicted equilibrium mineral asemblage for model 2A; (f-j) equilibrium mineral assemblage predicted for model 2B; (k-o) predicted mineral composition of serpentine and brucite; black lines denote mineral compositions for model 2A, grey lines denote mineral compositions for model 2B. See text for discussion.

#B[!$V"&B"+

,,(88"-*#

We next treat serpentinization as an isothermal process and compute the effect $//&`Uj\#&#@L#[ #"#$&&/$[#V#/ "/B\#"![V"!#`F[FF[F[FFF j+/#V`

97 3. Iron partitioning and hydrogen generation during serpentinization

0 0 Cl Cl -1 p) Na -1 u) Na Mg Ca Mg -2 K -2 K K Ca Ca -3 -3 -4 Fe -4 Al H+ -5 -5 Si -6 -6 Mg Si Si Fe H+ Fe -7 -7 Al

log concentration (molal) -8 -8 H+ 150 °C 150 °C -9 -9 Fe

0 0 Cl v) Cl -1 q) Na -1 Na Mg Ca Mg -2 K -2 K K Ca -3 Ca -3 Fe -4 -4 Al Fe -5 -5 Si Si Si + -6 H+ -6 Mg H Al -7 -7 H+ log concentration (molal) -8 -8 Fe 200 °C 200 °C -9 -9

0 0 Cl w) Cl -1 r) Na -1 Na Mg Ca Mg -2 K -2 K K

-3 Ca -3 Ca Fe -4 -4 Al Fe -5 Si -5 Si Si H+ Mg H+ -6 -6 Al -7 -7 H+ -8 -8 Fe 250 °C 250 °C -9 -9

0 0 Cl Cl -1 s) Na -1 x) Na Mg Ca Mg -2 K -2 K K -3 -3 Fe Ca -4 Ca -4 Al Fe Si Si Si -5 -5 + H+ Mg H -6 -6 Al + -7 -7 H Fe

log concentration (molal)-8 log concentration (molal) -8 300 °C 300 °C -9 -9

0 0 Cl Cl -1 t) Na -1 y) Na Mg Ca Mg -2 K -2 K K

-3 Fe -3 H+ Ca Ca -4 -4 Si Si Si Al -5 H+ -5 Mg Fe -6 -6 H+ Al Fe -7 -7

log concentration (molal) -8 -8 350 °C 350 °C -9 -9 0.1 1 10 100 0.1 1 10 100 water to rock ratio water to rock ratio

Fig. 6. (continued)[T> *'T>%<'T composition for model 2B. See text for discussion.

"#$!B#!#B[ $VV"B/U\# compositions, mineral assemblages and solid solution compositions of serpentine and @##/$#$U

98 3.3. Results

,"-./01#"-*6#

+F !#@#""@/"B$#" amounts of serpentine and brucite (Fig. 6a). Trace amounts of magnetite are present only @UCEF|#"!/BV//U[

V/Y2[VU`/!/*$*j}

!!$"/U[Y2,aq concentration are no longer buffered by equilibria (R4-6), causing a change in slope in H2V#U`/*j_#@`*j +3 `:j#/VY2 activities, and although Fe !`/&j[

H2[@#B"@""&"L!Y/B- trolled by the solubility of serpentine and brucite, and Mg2+ is the dominant cation apart $"+VU/$V"`/!j[W! $+["/B/U`@Vj+#"#" \##!!`""!j +FF [F FF !@#"#@#" ""@/VU/`/@Mj[""F L"$$"/@&U±+VB "!#!"[!@#@"/B magnesian at higher temperatures, promoting magnetite formation. The molar amount $"/B/U[@"#"# $!@#L\@BV!!$" #@#"B"!/"!#!"

1A (Fig. 5), predicted concentrations of H2[@B$"FFF

`/*j<V[U[$VY2 increase almost expo- B/U/$V"VB "$F "[#/@#$$`/!Mj !#[$V[+/B/ $V"`W!$"#V"!- #!"+j!/U +F #@#"!"@/!"B$- V""#$!"/`/j[\/

@B$VLF #B[Y2[$- V"V"!$V" temperatures.

99 3. Iron partitioning and hydrogen generation during serpentinization

800 300 °C 700 dunite 600 harzburgite

500 250 °C 400 ,aq (mmolal)

2 300 H 200 °C 200 150 °C 100 350 °C 0 0.1 1 10 100 water to rock ratio

Fig. 7. Predicted dihydrogen generation for serpentinization of dunite (grey lines) and harzburgite (black lines) at constant temperature as a fuction of water-to-rock ratio. See text for discussion.

,%."*7#

+F UE!#@#""@/!#@B B$C@#/"B$!@#""/ `U±EF[/$j"#$+U0EB!BW @"!$#@#""@/</@&UCF[[ "U"!"+LV#$# "!$!@#"$"+`/&j !BWB!#!!!"@/!- "B$+[@#"&#!B"$$!#"! VV$"!#/U"$`F"!j UYB/B/"!"+[@# again, H2[!B@&$"/[\/

H2[@#$$/@B!`/*jULB/W- dition of Si provided by dissolution of orthopyroxene according to (R10) supporting (R6). L$!BW"@#&"!/!!$[ //$$!#@#""@/! ##@#[#/"#$"/[ and thus higher H2,aq at equilibrium. +/U!Y"@B\#$

100 3.3. Results

0 300 °C H2O H2 50 MPa -1 pyrite pyrrhotite vaesite -2 pentlandite

polydymite

S(aq)

2 -3

magnetite

Log a H -4 millerite 0.12 wt.% S

-5 heazlewoodite 0.04 wt.% S hematite awaruite -6 -6 -5 -4 -3-2 -1 0 1 0 200 °C 50 MPa -1 pyrite H2O H2 -2 vaesite pyrrhotite -3

S(aq)

2 pentlandite -4 millerite

-5

Log a H 0.12 wt.% S -6 magnetite

-7 heazlewoodite hematite 0.04 wt.% S awaruite -8 -8 -7 -6 -5 -4 -3-2 -1 0 1 Log a H2(aq)

Fig. 8. Fe–Ni–O–S phase relations in H2-H2<*T arrows pointing the direction of increasing water-to-rock ratios. Note the different paths taken for rock with low (0.04 wt. %) and moderate (0.12 wt. %) sulfur contents.

101 3. Iron partitioning and hydrogen generation during serpentinization

V##"@@B#$!"B!BWL"!- nent of orthopyroxene is only partly incorporated into secondary clinopyroxene, leading 2+"`!$"+j&!Y$:U +VB/U`j[$"&"!BV\# +FF [F FF #@#"""@/B ""F [V["$$/

$"#$"/U#B/Y2,aq concentrations at higher temperatures. In addition, concentration patterns of dissolved elements VUVB"$"F #/$/ concentrations for most elements. "+[F #@#"!"@/! "B$!V""#$!BW[-

"/`/j"!"+[!Y2,aq is slightly elevated as more Fe+3-serpentine is generated.

9%;%<%

L"@#"@/@B$!B!C&

$"Y*E+!¯`[[j9S8°#`3Fe) + magnetite, fol-

@B!C`3S2) + magnetite in partly to completely serpen- C&`X|[FF'j+/B#/[! #$#C#UC[#[!`j!/ $#/B /"!/@V$X|`FF'j[#- !"!#$#C$!#$#$#/B!- #UC#@B`':j[ !#@V/!B5$B"+5- !@#$#CB"!C #"/["/5! $""@//C"/

Minimum H2[#@C!# "/"@/$"F"<F "E<EFF `X and Bach, 2009). The modeling results reveal that serpentinization of dunite and harzbur-

/B#$[Y2[@VF @C"@/`$/ *j|"!#$F [!U"@/ stable or not, since serpentinization produces more H2[U`/:j

102 3.4. Discussion

3.4. Discussion

3.4.1. Serpentinization at Hole 1274A and geochemical reaction path models

#"/#"$"!#U!- $M

103 3. Iron partitioning and hydrogen generation during serpentinization

8#9#"#/!

}W"!#@//"!#U- /:$B"!V#$#"$//

H2 and H2V#/!/V!C`_&['*[': Klein and Bach, 2009). Klein and Bach (2009) interpret the occurrence of pentlandite + # "/[ "/ "@/ !B !C &$"Y*E+[@V$B#/!V/ throughout serpentinization. The occurrence of this mineral assemblage indicates that

B"Y2#V"B@@B#@#" separate H2V!!<V[#$#B"!/ @#@##!"@[/[#" #$"@"@/@#B@F +@V"- !##@@#$[B#/- ["/##V$5B""@ #@@!/!#!`/:j+!$"

W"@B#@

3.4.3. Fe+2+3 exchange equilibria in serpentinites

_V`FF:jV!#$#/W

104 3.4. Discussion

+2 of Fe and Mg Fe-1 exchange equilibria in serpentinites and emphasized the importance of Fe+3 in serpentine as it contributes to hydrogen formation during serpentinization. YV[&$+3$@B!RYB^B`''j !<%@##`j/[+3 in serpentinite (about 50 % of j`j/[+3[/@#F`$|# ['*'\CC<[FF/[':C['*'}& }&['*Fj#_<+V'*$"##`@$* oxygens) at the tetrahedral site of serpentine in mesh-rims is occupied by Si and alumi- num accounts for approximately 0.01 formula units at the tetrahedral site (see Table A1). Hence, at the tetrahedral site of serpentine less than ~ 0.02 formula units can be occupied by Fe+3, although serpentine comprises ~ 0.15 formula units Fe (calculated as Fe+2). Mößbauer spectroscopy applied to separated mesh-rims of partly serpentinized dunites C@#/$"Y*E+V!$/["#$+3 in hydrous secondary minerals. ""$&B!C!+3U5@#FF FE:YV[#!&$+3 by brucite cannot be excluded so that the actual Fe+3U5 $!"B@"""$/B$#B!C peridotites the Fe+3U5$B#B!@FF:[ @B/&B!C!L/+3U5"B

@&Y2[[@##/!B$&

#/$$#VUVV$Y2,aq (see R7) or both. Increasing aSiO2,aq is un- +3 &B!"/ U52[@#$$W/BV# "!#@B`'j | F F ` W! "!# / $ !C Y*E`+[FF*|[FFX|[FF'j! Fe+3U5$!!!/BV!#&"/B@#V- /[B/U"!#`/j+F U[@#!V!"! ##V$"Y*E+`

105 3. Iron partitioning and hydrogen generation during serpentinization

3.4.4. Geochemical reaction path modeling and serpentinization experiments

Seyfried et al. (2007) conducted serpentinization experiments of a spinel-lher- CFF F<LB!

/\#"#$Y2,aq generated during serpentinization. The huge

$$@!@VY2,aq concentration is obviously related to &`$""@j$!@##! "$B$`FF*j`[@!! @#j<"|`FF'j#B!/$ !@#"W!"B`<B['*B$[ FF*j@""#VB!" #$"L#!$/!V!$B- drogen yields during serpentinization.

106 3.4. Discussion

@V#BB$#!@##" the predicted amount of H2,aq generated during serpentinization relative to a model that $!/$!@##"[ predicted H2[@/# !C"$V `"+j!$:"<FF U$!- ing serpentinization model of harzburgite (model 2B) the predicted H2,aq concentration is 'E"<{&#@B<"|`FF'j[ similar to those measured by Seyfried et al. (2007) (77 mM). The match in H2,aq concen- B[@#"!!#W!""- #"$$!!/$!"![ !C"!#$FF [!#$F<U yield more than about 100 mM H2,aq. An obvious explanation for this phenomenon is that all compositions used have serpentine-brucite-magnetite phase relations that govern the

H2V$&"!@""@/ FF [[!MM"/[B/$#/W!@"#

[/VB#@C!¯3Si2O5(OH)4¤3O4

+ 2SiO2,aq + H2O + H2 (cf. Frost and Beard 2007) Allen and Seyfried (2003) conducted serpentinization experiments of olivine `

0.3 mM) and H2[`"<"

$`!3<"#3

107 3. Iron partitioning and hydrogen generation during serpentinization

$QW 4W] (Q 7OF )R

7OF (Q 2S[ U[Q 7OF 6US )R 6US

/RJD6L2 PHWDVWDEOH2O%UF GHYHORSLQJ

ZLWKLQ wJUDGLHQWLQ6L2

6US )R 2O U[Q %UF

7HPSHUDWXUH r&

Fig. 9. Temperature-SiO2 activity plot depicted the phase relation in the system MgO–SiO2–H2O. Thick lines are stable phase boundaries, while thin and dashed lines are metastable ones. See text for discussion.

FF [F<UF[$V"/" #!/"V"[/[X[@VV- tive during serpentinization. The concentration of H2"#/ W!"["!"!!@ To conclude, our modeling results are, at least semi-quantitatively, in a very good /" W!" #[ #/ V $ # /"!"$!$"\##@#- /!CYV[W!"BV"B"$ serpentine and brucite solid solutions exist, our results should be regarded provisional.

3.4.5. The formation of brucite and serpentine in mesh-rims

Independent of the extent of serpentinization, mesh-rims exhibit a distinct zon- /$"@#$V$@BC$!@#-

108 3.4. Discussion

/"/[B"!"/#"`/[ 2 and 4). The reaction path models that provide phase equilibria changes in the system

MgO–SiO2–FeO–Fe2O3–H2/$&! @V"C/L"!#V#/V! @#W@BW"B[!$"# change the invariant nature of the iron-free system.

_#@ @ V[ ! @#

(11) 3 Mg2SiO4 + SiO2,aq +4H2¤

(12) 2 Mg2SiO4 + 3H2¤

(13) Mg2SiO4 + 2H2¤2,aq + 2Mg(OH)2

(14) Mg(OH)2 + 2SiO2[¤

+@VF [V#@V!$/`j$ W#$`/[$"!BW@&j[

-3 Tlc

-4 Berndt et al. 1996 Ol ,aq Brc Fo 2 aH 2O = 1.0 -5 Srp aH2O = 0.5

rp S rc B l + log a SiO O -6 Brc

aH2O = 0.1 -7 100 200 300 400 Temperature (°C)

Fig. 10. Temperature-SiO2 activity plot showing in the phase relations during olivine breakdown in greater detail than Fig. 9. Solid lines show stable phase boundaries. Black dashed lines show metastable phase boundaries. White dashed line denotes the aSiO2,aq path of model 1A. Polythermal olivine-serpentine- brucite equilibrium is possible if aH2O < 1. Also shown is the range of silica observed in experiments from Berndt et al. (1996). See text for discussion.

109 3. Iron partitioning and hydrogen generation during serpentinization

W!B"#!+[V@#M!M $[##`j$"V/!B/!C$

&$"Y*E+`j!V!Y2O ~ 1 and T ±F V##B$"!@# There is a number of possibilities that can explain the coexistence of olivine, serpentine, @#[\##VB$`$[':j[@#B" gains a degree of freedom if p < p [/$#V$#!#@ H2O total the MgO–SiO2–H2B"`/Fj[VB"B@# !C$[#"_"$ $#@W!`/[$[':j@#@V&$"Y *E++[@#C@V!V- pentine-brucite equilibrium, as serpentine and olivine are physically separated by brucite. "$"&B"@#@@V[! brucite or arrested reactions in olivine serpentinization need to be considered to explain @#"`j@@&#@`j`Ej[ @ & W! " C/ "" $ `j &![#[\##"V!@B[[ and serpentine may ultimately form once the Gibbs energy required for its nucleation is available. In this sense, the brucite rims may represent an arrested reaction. While bru- !V$C$[#""&@#BL @W!@B/VV!/B peridotite undergoing serpentinization (Fig. 9). When metastable orthopyroxene reacts L±EFF [W!$"!L/# \#VVB/VB[!@#$"/ $"V@&"!VBV@V# /VB`/'j$$#V!@- thopyroxene and olivine. Interestingly, the metastable olivine-brucite phase boundary is #@!@#!@#$$`/'j! B""@V[@#B$"[ $"!BW@&!V#/W"B" V"&!$" Experimental data may help shed some light on this issue. The only experiment $V@#/##"$W!"$ |`''jL##!CW!"!#- V`

110 3.5. Conclusions

Fig. 10 illustrates the isothermal reaction path of SiO2[`E:$- !/j@@#V!@#B$`Ej"-

@@$`j$2 increase further until (at 362h) a maximum is [""@@$`j#@#B of SiO2 drops until it reaches the univariant phase boundary of (R14). This experimental !"B@&V$`j&/!@$`Ej[ ultimately control the SiO2VB$B"VW#<

!C W!" #$[B # 2,aq analyses are needed to constrain this further.

3.5. Conclusions

Our study indicates that unprecedented details about the reaction sequences during serpentinization may be obtained from merging careful petrographic, mineral chemi- ["/[<%@#!!B"!V"B" "/}V#B//!"!#[& "![\#U&#/!C|FMF - B@B!M"/M@##@}V![B !/$V@#VV- pentine. Model calculations reveal that both partitioning and oxidation state of iron is very V"!#&#/!C+"!- $ W/ ! `

111 3. Iron partitioning and hydrogen generation during serpentinization

+/["#$V!@#V- hydrogen generation. Textural evidence indicates that olivine is replaced by brucite (and not serpentine) along the grain boundaries so that the formation of brucite appears to be the initial step of a serpentinization reaction sequence. We propose that brucite is meta- @@B[$!##/"B"V$! nucleation is available. The formation of Fe+3!"&B@#B//[ !#"!#&<%@#!! #@#FF$!U@#""V- lent, irrespective of subbasement depth and the orthopyroxene content of the precursor &#/"!""#VB/" !!#$!C"!#FF If the Fe+3-component of serpentine is neglected in geochemical reaction path models, magnetite is predicted to be part of the equilibrium assemblage over the entire temperature range. In contrast, if Fe+3!!["/ !$"L±FFF [W!- "#BB$`FF*j!&$"/$"#/ !C$!FF L##"!$- ering the Fe+3"!!#"YV[+3 #@#$!#B[# "/##@/!VL"!V!V!$ serpentinization models, experimentally derived thermodynamic data of Fe+3-serpentine and thermodynamic parameters for the serpentine solid-solution are necessary.

8#;#*+

L##&&<Y$!/#! "B"@<B&/$!/"#/- #}&|@<+!!$ "!@B\Y/#&!V"!" thin sections. This research used samples supplied by the Ocean Drilling Program (ODP). ^!@B#`j!!/#- #"/"$/!#`j[L& #!!$#$"!B/"EE$\" #`|+FU|+FUj@B^\U_W- #QL_B"R

112 References

References

>*J*<*J**??Y'*T < < > study at 400ºC, 500 bars. Geochimica et Cosmochimica Acta 67, 1531-1542. >*****[^*+**+* (2007). Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the Mid-Atlantic Ridge, 15°20’N (ODP Leg 209): A sulfur and oxygen isotope study. Geochemistry, Geophysics, Geosystems 8, Q08002, doi:08010.01029/02007GC001617. >**XX#'*>}>| << particles. Microbeam Analysis 4, 177-200. * + * * ^< * [ ^* " * ??='* seawater-peridotite interactions – First insights from ODP Leg 209, MAR 15ºN. Geochemistry, Geophysics, Geosystems 5, Q09F26, doi: 10.1029/2004GC000744. * [ ^* + * * * * [* ^ S. E. (2006). Unraveling the sequence of serpentinitzation reactions: petrography, mineral chemistry, and petrophyscis of serpentinites from MAR 15ºN (ODP Leg 209, Site 1274). Geophysical Research Letters 33, L13306, doi:13310.11029/12006GL025681. "* +* J * + ^* * * ^* X]@'* internally-consistent thermodynamic data by the technique of mathematical

programming: a review with application to the system MgO-SiO2-H2{* of Petrology 27, 1331-1364.

Berndt, M. E., Allen, D. E. and Seyfried, W. E. (1996). Reduction of CO2 during serpentinization of olivine at 300°C and 500 bars. Geology 24, 351-354. Blaauw, C., Stroink, G., Leiper, W. and Zentilli, M. (1979). Mössbauer analysis of some Canadian chrysotiles. Canadian Mineralogist 17, 713-717. Brand, R. A. (1987). NORMOS Programs. Internal Report, Angwandte Physik, Universität Duisburg. *XXY'*JT* +<"X]=@Y=!* Cannat, M., Bideau, B. and Bougault, H. (1992). Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15°37’N and 16°52’N. Earth and Planetary Science Letters 109, 87-106. * >* * "* "* * +* "* X@#'* *J

113 3. Iron partitioning and hydrogen generation during serpentinization

Canadien des Sciences de la Terre 2, 188-215. * * *[* }| * [* ^ * ??'*

Geochemistry of high H2 and CH4T "<Y@=#>"'*+<X* ***XX]'*>>}> webbook.nist.gov/. *>*"**X]X'*J<7+ 7^'X]<* American Mineralogist 74, 1023-1031. #>***??='* from the South Chamorro Seamount (Ocean Drilling Program Leg 195, Site 1200): inferences for the serpentinization of the Mariana forearc mantle. Mineralogical Magazine 68, 887-904. J** *{J*^*[**}* *[*}| *[*>[*??'*"T Y@=#>"'T > " < T* +< 184, 37-48. * J* * X]'* ^< } Effects on Mineral Precipitation. University Park: The Pennsylvania State University. Eckstrand, O. R. (1975). The Dumont Serpentinite: A model for control of nickeliferous |<*J Geology 70, 183-201. J¤*^+*J*XX!'*J_ strength and the style of normal faulting at slow-spreading ridges. Earth and Planetary Science Letters 151, 181-189. Evans, B. W. (2004). The serpentine multisystem revisited: chrysotile is metastable. International Geology Review 46, 479-506. 2+ Evans, B. W. (2008). Control of the products of serpentinization by the Fe Mg-1 exchange <*[<=X]!Y]]!* J * * * { ^* * X!@'* < of chrysotile and antigorite in the serpentinite multisystem. Schweizerische Mineralogische und Petrographische Mitteilungen 56, 79-93. J***X!'*JTJ^< on Duniten. Schweizerische Mineralogische und Petrographische Mitteilungen 52, 251-256. }*"*X]#'*{<* [<@Y@Y* }*"***??!'*{<_*

114 References

Petrology 48, 1351-1368. }++* *<*>**[>*<**+<*??='* Serpentinization of oceanic peridotites: implications for geochemical cycles and biological activity. In: Wilcock, W. S. D., DeLong, E. F., Kelley, D. S., Baross, * >* < * * *' * Washington, DC: American Geophyscial Union, 119-136. González-Mancera, G., Ortega-Gutiérrez, F., Nava, N. and Arriola, H. (2003). Mössbauer < < _ *^<=]=X@!* ^^**<**^****X!]'*< critique of the thermodynamic properties of rock-forming minerals. American !]>X* ^***X]X'*< A review and a predictive model. American Mineralogist 74, 5-13. Hostetler, P. B., Coleman, R. G., Mumpton, F. A. and Evans, B. W. (1966). Brucite in alpine serpentinites. American Mineralogist 51, 75-98. <*>^***??]'*_ lithosphere and some geochemical consequences: Constraints from the Leka Ophiolite Complex, Norway. Chemical Geology 249, 66-90. <*"*<*J**X]@'*^<_ within the oceanic crust: Experimental investigations of mineralogy and major element chemistry. Geochimica et Cosmochimica Acta 50, 1357-1378. * * { J* ^* ^ ^* * XX'* ¦["X > package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1-5000 bars and 0-1000°C. Computers & Geosciences 18, 899-947. Kaneda, H., Takenouchi, S. and Shoji, T. (1986). Stability of pentlandite in the Fe-Ni- Co-S system. Mineralium Deposita 21, 169-180. [*J***[< ***??='*{[ ?X >"=@*{J Y?=?* [**J***>**"* *<*}* ***>*^***}¦*+* +*+**+**+**+**^< **+****[*[^*"* Schroeder, T., Seyler, M. and Takazawa, E. (2004b). Site 1274. Proceedings of the Ocean Drilling Program; initial reports; drilling mantle peridotite along the Mid-Atlantic Ridge from 14 degrees to 16 degrees N; covering Leg 209 of the {J"%"_ +%@]!#@<@<??Y*[@>©

115 3. Iron partitioning and hydrogen generation during serpentinization

University Ocean Drilling Program College Station TX United States. <***>***}++* **>* <**{J***{*"** +**"__ [*[<>** ??'*>< Atlantic Ridge at 30 degrees N. Nature 412, 127-128. Klein, F. and Bach, W. (2009). Fe-Ni-Co-O-S phase relations in peridotite seawater *[<#?Y!#X* McCollom, T. M. (2000). Geochemical constraints on primary productivity in submarine hydrothermal vent plumes. Deep-Sea Research Ietters 47, 85-101. McCollom, T. M. and Bach, W. (2009). Thermodynamic constraints on hydrogen generation _*+>* < * * X!@'* > < _ olivines. American Mineralogist 14, 462-478. Nickel, E. H. (1959). The occurrence of native nickel-iron in the serpentine rock of the Eastern Townships of Quebec Province. Canadian Mineralogist 6, 307-319. O’Hanley, D. S. (1996). Serpentinites: records of tectonic and petrological history. New {¦<[* O’Hanley, D. S. and Dyar, M. D. (1993). The composition of lizardite 1T and the formation of magnetite in serpentinites. American Mineralogist 78. Olsen, E. (1963). Equilibrium calculations in the system Mg, Fe, Si, O, H, and Ni. >@X=YX#@* [ * * X@]'* %<&* > Mineralogist 53, 201-215. [ * * X@!'* _ * Mineralogy and Petrology 14, 321-342. [ * * " * ^* ??='* + < _ _ T chimney precipitation. Geochimica et Cosmochimica Acta 68, 1115-1133. [ >* < * * } * "* * XX'* ^< reducing conditions during Alpine metamorphism of the Malenco Peridotite

(Sondrio, northern Italy) indicated by mineral paragenesis and H2 T inclusions. Contributions to Mineralogy and Petrology 112, 329-340. Ransom, B. and Helgeson, H. C. (1994). Estimation of the standard molal heat capacities, entropies and volumes of 2:1 clay minerals. Geochimica et Cosmochimica Acta 58, 4537-4547. Rozenson, I., Bauminger, E. R. and Heller-Kallai, L. (1979). Mössbauer spectra of iron in 1:1 phyllosilicates. American Mineralogist 64, 893-901. Schmidt, K., Koschinsky, A., Garbe, S. D., de Carvalho, L. M. and Seifert, R. (2007). +< < T < # > "%

116 References

investigation. Chemical Geology 242, 1-21. < * J* * } * * } * ??!'* " transfer during serpentinization; an experimental and theoretical study at 200 °C, #?? < < ocean ridges. Geochimica et Cosmochimica Acta 71, 3872-3886. <* *[*^****??!'*[ reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge 15°20’N; ODP Hole 1274A. Contributions to Mineralogy and Petrology 153, 303-319. Shock, E. L. and Helgeson, H. C. (1988). Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Correlation algorithms for ionic species and equation of state predictions to 5 kb and 1000°C. Geochimica et Cosmochimica Acta 52, 2009-2036. Shock, E. L., Helgeson, H. C. and Sverjensky, D. A. (1989). Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of inorganic neutral species. Geochimica et Consmochimica Acta 53, 2157-2183. Shock, E. L., Sassani, D. C., Willis, M. and Sverjensky, D. A. (1997). Inorganic species T< of aqueous ions and hydroxide complexes. Geochimica et Cosmochimica Acta 61, 907-950.

Sleep, N. H., Meibom, A., Fridiksson, T., Coleman, R. G. and Bird, D. K. (2004). H2-rich T_*[ the National Academy of Sciences 104, 12818-12823. * * ??'* > T * Geochimica et Cosmochimica Acta 65, 3965-3992. _ * "* J* * ???'* <<<T* +<"?#]YX]Y=?* J**J*}*X!?'* polymorphs: a discussion. American Mineralogist 55, 1025–1047. }**[>*+*X!X'*J< of serpentine textures. Canadian Mineralogist 17, 785-830. * < +* < * * " * * * and Shibata, M. (2006). The effect of iron on montmorillonite stability. (II) Experimental investigation. Geochimica et Cosmochimica Acta 70, 323-336. <**XX'*JY@>| systems: Package overview and Installation guide (version 7.0). Livermore, Ca: Lawrence Livermore National Laboratory. < * * XX'* JY" > [ + >|

117 3. Iron partitioning and hydrogen generation during serpentinization

Speciation-Solubility Calculations: Theoretical Manual, User’s Guide, and Related Documentation (Version 7.0): Lawrence Livermore National Laboratory. <***>*XX'*J@>["[ Modelling of Aqueous Geochemical Systems: Theoretical Manual, User’s Guide, and Related Documents: Lawrence Livermore National Lab. < * * "* * ??Y'* ¦# JY@ ]*?'* Albuquerque, New Mexico: Sandia National Laboratories. <***}*??='*< Geochemical Modeling of Mineral-Water Interactions in Dilute Systems.

118 Appendix

Appendix

Table A1. Selected electron microprobe analyses

Hole 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A Core 4 6 10 17 22 27 4 6 10 17 22 27 Section 121112121112 Depth (cm) 104-105 128-135 3-10 121-129 24-32 5-11 104-105 128-135 3-10 121-129 24-32 5-11 Depth (mbsf) 22.79 32.78 49.33 89.51 122.34 147.65 22.79 32.78 49.33 89.51 122.34 147.65 Rock type Hz Hz Du Hz Hz Hz Hz Hz Du Hz Hz Hz Lab code none Ap-86 AP-88 AP-95 AP-99 AP-103 none AP-86 AP-88 AP-95 AP-99 AP-103 Mineral Ol Ol Ol Ol Ol Ol Srp Srp Srp Srp Srp Srp Texture mesh mesh mesh mesh mesh mesh mesh mesh mesh mesh mesh mesh

Wt. %

SiO2 40.84 40.73 40.85 41.10 40.87 40.31 39.72 40.76 40.91 40.70 40.58 39.95

TiO2 0.02 0.02 0.00 0.00 0.03 0.03 0.01 0.03 0.01 0.01 0.05 0.03

Al2O3 0.04 0.03 0.03 0.04 0.02 0.02 0.28 0.11 0.08 0.17 0.09 0.26

Cr2O3 0.01 0.01 0.01 0.02 0.02 0.02 0.00 0.04 0.03 0.02 0.00 0.02 FeO 8.14 8.27 8.26 8.18 7.67 8.20 4.00 4.28 4.20 3.08 4.23 3.45 MnO 0.11 0.12 0.11 0.13 0.09 0.10 0.06 0.08 0.10 0.08 0.09 0.09 MgO 50.22 49.89 50.26 49.86 50.70 50.73 37.70 38.69 39.44 39.58 38.45 39.05 NiO 0.38 0.37 0.38 0.38 0.41 0.40 0.30 0.32 0.42 0.46 0.27 0.40 CoO 0.03 0.04 0.02 0.01 0.03 0.02 0.01 0.02 0.02 0.01 0.02 0.01

SO3 0.00 0.00 0.02 0.00 0.01 0.01 0.13 0.06 0.05 0.13 0.04 0.10 CaO 0.07 0.09 0.25 0.07 0.03 0.06 0.06 0.08 0.10 0.05 0.04 0.06

Na2O 0.02 0.02 0.00 0.00 0.00 0.00 0.03 0.03 0.00 0.00 0.01 0.02

K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 Total 99.88 99.59 100.19 99.79 99.88 99.90 82.31 84.50 85.36 84.29 83.87 83.45

Formula Si 1.00 1.00 0.99 1.00 0.99 0.99 1.99 1.99 1.98 1.98 1.99 1.97 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe 0.17 0.17 0.17 0.17 0.16 0.17 0.17 0.17 0.17 0.13 0.17 0.14 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 1.83 1.82 1.82 1.81 1.84 1.85 2.81 2.81 2.84 2.87 2.81 2.87 Ni 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.02 Co 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 S 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ca 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total 3.01 3.00 3.00 2.99 3.01 3.02 4.99 4.98 5.02 5.00 4.98 5.01 Oxygens 444444777777 'T{^__

119 <\= Insights from geochemical reaction path modeling

Abstract L"B" ! " !! !V / $"$/"[U#"[@#L"#! V/$\#M&#@\#"V$"!#// !C/@@@BV/ $/W$$!C\#/@@FF [FF [EFF "@/B!$/`/#!/j !$"FF FF [@#B\#- B#$$@B/@@[[#"[& [#[!C\#+\#@""$$ @B/@@[!!!/"! !BW\#"B"!B@B/@@[ !"@/B!$/$N@!/[[ [!L!"VB" is observed in natural rodingites from different settings. Our model results hence support B!/$"#/!CB\# controlled by serpentinization reactions are present. Our calculations further indicate that $"$""@/!"/V@B/- $"\"BV@B!VB/!#- #!\#<$$&B@B$$#$Y+ species, !VB!/"[M#"[ @#B/!@#$/@B$!/ 2 activities. Our model calculations further predict the formation of diopsidite $"/@@!BW!#&FF FF [#/// VB/"!#$:FF #$"$!- sidite veins.

4.1. Introduction

/@V$""[$&\# V@$$@B#"[&`LB['"['* ['*_V['**[':RYB[''j!C # / \# @" &[ / `B$ ^@@[':F&BB$[':B$[FF*j#\#& $""//`XB[FFXB[FFj[!`

120 4.1. Intoduction

[ ' | ['* | R[ '' /[ ':+@['::j[$ regions of subduction zones (Mottl et al., 2003, 2004). It is hence not surprising that rodingites have been found in a range $ /[ #/ \ !/`+#"{#@[ '*YCX['*|# [''Y&[''j[$ continental margins (Beard et al., 2002), !`|/B$[':+#- " V&[ FF:j[ / @ ` [ ':' RYB [''+[FF" }"[ FF*j[ ! / `['* _V['** ^#@& }[ ''' { [ FFE <#C and Shanina, 2007), and suprasubduction C `{ [ FF*j /C is a metasomatic process, and the dominant mass transfers involved are removal $ $ #" `- "['*j #B[ B mineral assemblages in rodingites com- !/$M+[#/ C[ ![ /#UB/#- lar, and vesuvianite. Other phases are usually diopside and chlorite. In many cases, rodingitization is multistage (e.g., Schandl Figure 1. Photomicrographs of rodingitized gabbro [':'RYB[''{ from ODP Leg 209 at the MAR 15°N (Kelemen et [ FFE " }"[ al., 2007). (A) Patchy replacement of plagioclase by prehnite (prh) (sample 1274A-11R-1, 46–49 cm). 2007) or rodingites are overprinted by lat- (B) Clinozoisite (czo) replacing plagioclase (sample er metamorphic events (e.g., Frost, 1975). 1274A-21R-1, 12–17 cm). (C) Euhedral grossular (grs) crystal (sample 1274A-21R-1, 12–17 cm). All During later stages of rodingitization, the *!# direction of mass transfer may be reversed mm wide. `["B@$"&jL

121

Table 1. Comparison of major element compositions of rodingites from the MAR with average compositions of peridotite and basalt

Depleted mantle MAR N-MORB MAR rod. 1 MAR rod. 2 Alpine rod. 1 Alpine rod. 2 Munro rod.

SiO2 44.90 50.01 36.45 35.25 37.43 37.68 39.68

TiO2 0.13 1.11 0.32 3.88 0.19 0.06 0.57

Al2O3 4.28 6.31 20.00 8.07 15.34 18.81 11.68 FeO 8.07 9.73 4.67 11.85 2.92 3.19 5.80 MgO 38.22 8.67 16.30 10.38 29.32 13.58 8.15 CaO 3.50 11.75 10.53 17.83 29.32 21.74 28.86

Na2O 0.29 2.52 0.54 0.34 0.08 0.10 0.28 Source Salters and Stracke Klein (2004) Honnorez and Kirst Honnorez and Kirst Li et al. (2004) Li et al. (2004) Schandl et al. (2004) (1975) (1975) (1989)

!"&$[#!!"$!["M\# equilibria. The situation is less complicated in rodingites from mid-ocean ridge settings, @#"!$\#`![/@@[- j$ _W"!$B!""@//V$"\ !/"!$/!VL@"!- $!#!!"!"/@`<|j from the mid-Atlantic ridge (MAR). Oceanic rodingites from the MAR (Honnorez and X['*j[!/$"~"`{[FFEj[/$" +/@`[':'j""/#V- ably, rodingites are depleted in SiO22[$"! @/BV@##B/B[!#/ !B/#B[/C""B/ """[#$B!C@!BW@& #/!C`/["['*jL"B"V/$$/- CB!C"$"/V$!C/\#`- "['*{[FFEj"@$[

4.2. Method

"!#$#@#"$W[$#-

122 4.2. Method

= 1 ous species and dissolution of minerals in B"

ZU ZU Y##/L' = 0 )OXLG IORZ = 1

ZU @`[''j ZU

'LIIXVLYH WUDQVSRUW L $ L' @ the thermodynamic data set from Helge-

,QILQLWHVL]H `'*:j$"[& 3HULGRWLWH

ZU = 0 Y/ `'::j[ & Y/- `''Fj & `''*j $ *DEEUR = 1 dissolved inorganic aqueous species. The ZU = 0 'LIIXVLYH WUDQVSRUW 3HULGRWLWH @##!/$ ,QILQLWHVL]H slop98.dat and spec02.dat databases (see Figure 2. Schematic representation of the rationale behind using titration models to describe metaso- Wolery, 2004 for details). Data for Fe-ser- " ! `"[ $" [ ''*j + is ![@#[& the reaction progress and ranges from zero at the $"<"|`!j#- $!\#"! @B W ! "!$E , "# $ & @ MgSO4o$"`<"[FFFj \#[ #@ & as aqueous Al complex data from Tagirov A titration model is appropriate in assessing the qualitative phase relations developing either in an and Schott (2001). The data base consists VV/"`/\#\!j of standard-state thermodynamic param- $$#V/"[&@BB$" [ <MXB $[[ \#[ [# `+j B $" @#- B/#W\# equation of state parameters for miner- composition (B). Table 2.VV$!"

# Reacting solid /\# Temperature }M& +

1 Peridotite MFF 1 0 to 1 (T) 2 Peridotite MFF 1 0 to 1 (T) 3 Peridotite MEFF 1 0 to 1 (T) 1a Plagioclase Fluid 1 FF .FFF F`}Uj 2a Plagioclase Fluid 2 FF .FFF F`}Uj 3a Plagioclase Fluid 3 EFF .FFF F`}Uj 1b !BW Fluid 1 FF .FFF F`}Uj 2b !BW Fluid 2 FF .FFF F`}Uj 3b !BW Fluid 3 EFF .FFF F`}Uj 1c Gabbro Fluid 1 FF .FFF F`}Uj 2c Gabbro Fluid 2 FF .FFF F`}Uj 3c Gabbro Fluid 3 EFF .FFF F`}Uj 1c‘ Gabbro Fluid 1 FF .F F`}Uj 2c‘ Gabbro Fluid 2 FF .F F`}Uj 3c‘ Gabbro Fluid 3 EFF .F F`}Uj

123

Table 3. Composition of peridotite starting material als and aqueous species that are used to

Minerals wt. % composition "!##@#"`{/Xj

Olivine 77 Fo90 for temperatures and pressures up to 1000 Orthopyroxene 18 En90 FF<`[''j Clinopyroxene 3 Di90 We calculated equilibrium constants for Spinel 2 pure 50 MPa and temperatures from 0 to 400 Oxides wt.% ""@/ SiO2 45.24 X@$#_UL"- Al2O3 0.91 MgO 44.02 !#_U\#! FeO 8.77 "#@B# CaO 1.06 as reaction path geochemical modeling of

Composition of gabbro starting material \#M& `}B &[

Minerals wt. % composition 2003). We used the B-dot equation for #$VB$[$- Plagioclase 50 An80

Clinopyroxene 50 Di85En8.5CaTs6.5 V / ! |

Oxides wt. % W ^@BMY& !" " $"}B SiO2 50.61

Al2O3 18.09 `FFEj ^V # ! MgO 7.79 / #B VB $[[ W- FeO 1.39 CaO 20.83 !!/#![$ Na O 1.18 2 2VB$[$"^#"" `':j# Solid solutions are included in the models for many minerals, assuming an ideal molecular mixing model. The solid solu- "!$/"`""@jN!`B[/[ amesite), talc-ss (talc, minnesotaite), brucite (Mg-brucite, Fe-brucite), olivine (forsterite, fayalite), orthopyroxene (enstatite, ferrosilite), clinopyroxene (diopside, hedenbergite), plagioclase (albite, anorthite), tremolite (tremolite, Fe-actinolite), garnet (grossular, an-

j[!`C[!2FeAl2Si3O12(OH)), and chlorite (clinochlo- [!j!"!@"!V

124 4.2. Method

#"#YV[ hydrogrossular in most rodingites has

BMY2O (Honnorez and Kirst, '* [ ':' RYB al., 1992), so that the uncertainties intro- #@B"/&"! in the calculations are probably minor. {&[ $ VB silica activities (vesuvianite and xonotite) are also not considered in the model, although both are not uncommon in rodin- / # " & # W"BVB$ !L"&"!- `Lj !BW considered explicitly in a solid solution model. Instead, clinopyroxene used in the ! " !- `$}B&[FFj so that the presence of several mol. % of L!BW#@# for. Minerals suppressed during the models of rodingitization comprise an- dalusite, antigorite, boehmite, corundum, diaspore, dolomite, gibbsite, huntite, hy- Figure 3. Results of reaction path model calculations, WB!C[&[&B[[ in which 1 kg of harzburgite (Table 2) was reacted with 1 kg of seawater at temperature between 25 °C magnesite, margarite, monticellite, para- and 400 °C. Shown are: (A) variations in the compo- gonite, pyrophyllite, and sillimanite. Also T' system, and (C) composition of solid solutions. #!! # $ #$ and carbonate by dihydrogen. Suppressing reactions that are not usually observed is a common practice in examining metastable equilibria (e.g., Palandri and Reed, 2004). [@VFF / brucite should form instead of chrysotile, "@!`_V[FFEj

125

YV[$[B$B¤/@#""! $[B$!!!#B#//@ }&/#"!/C!C&! "!#B[$/!!#"#/CN }#!$&/$ #/_[#/ _[##!!\#&/$!- !"#[M&B"`jFF [ `jFF [`jEFF @BF<}#!! \#!/`+:Fj[!BW`^:_:Lj[ "/@@`F!/[F!BWL@j[/! /"#$&[W"#$\#<#W" /!#/!/V@`+j[$#$"!- #[$#$#$&L &/!/B"&" VV#/[W"#$\#`&/jL for using this type of titration model for metasomatic processes is provided in Fig. 2 (cf. [''*j"""[\#"!@#B$B" @BW!`+¤Fj+B$"@#B[/\!"!B /@#B[\#"!@#$$@B &`+¤j

4.3. Results

4.3.1. Reaction path models

+$!"#`L@jL[#- ""#//$\#/!`L@ jFF [FF [EFF L\#/!/[ clinopyroxene, and gabbro (Table 3) to simulate the interaction of gabbro and gabbroic "!\#!#@B!"!#$FF [ FF [EFF `/EMj"[FF/$/@@ &/$\#$"!M"W""W- \#/VB&`/*j

126 4.3. Results Results of Figure 4. plagioclase titration * - tion progress in terms of plagioclase added, 1 g of plagioclase add- T* T- tions were calculated in reaction path models depicted in Fig. 1. Temperatures are 200 °C (panels A–C), 300 °C (panels D–F) and 400 °C (panels G–I). The upper three panels plot the changes in T plagioclase is added, the middle three panels show the equilibrium mineral assemblages, and the lower three panels indicate the solid solution compositions.

127 Results of is the reac- + = 1 represents + g of clinopyroxene add- T* T - tions were calculated in reaction path models depicted in Fig. 2. Temper- atures are 200 °C (panels A–C), 300 °C (panels D–F) and 400 °C (panels G–I). The upper three panels plot the changes T clinopyroxene is added, the middle three panels show the equilibrium mineral assemblages, and the lower three panels indicate the solid solution compositions. Figure 5. clinopyroxene titration models. tion progress in terms of clinopyroxene added, where

128 4.3. Results is + Results of gab- = 1 represents + Figure 6. bro titration models. the reaction progress in terms of gabbro added, where g of gabbro added to 1 kg T*T compositions were calculated in reaction path models depicted in Fig. 2. Temperatures are 200 °C (panels A–C), 300 (panels D–F) and 400 °C (panels G–I). The upper three panels plot the T - sition as gabbro is added, the middle three panels show the equilibrium mineral assemblages, and the lower three panels indicate the solid solution compositions.

129 is + = 1 represents Results of gab- + :' the reaction progress in terms of gabbro added, where 100 g of gabbro added T*- grams start in the left with 1 g of gabbro added (Log + anextension of Fig. 5 to T * T- sitions were calculated in reaction path models depicted in Fig. 2. Tempera- tures are 200 °C (panels A–C), 300 °C (panels D– F) and 400 °C (panels G– I). The upper three panels T composition as gabbro is added, the middle three panels show the equilibrium mineral assemblages, and the lower three panels indicate the solid solution compositions. Figure 7. bro titration models.

130 4.3. Results

L#$#!/"$\#"![ &"!["#"!+EFF [!BW- !@B!"[V@^!//"- position of the solid reactant, talc may also form. With decreasing temperature, a number $/\#"!!&!+: [- B!BW!"[#[!@B/ "!#@* +!/!/"!# @!@B|`FF*j}$" !!#"!V"@B`$<"|[!j /[B$$"!$!C\#L\#!Y!

@##`!Y¤[!XWEFF j"#$V $""U&/`"

FF !XW$F'L\#@""&- /"!#[$"F/#@V#BEFF /#@V #BFF [/#@V#BFF ""!#

/[#@#"`{/Xj$`2(aq) + H2¤ + HSiO3 + H j$"FF:*[!!Y2(aq) VB$\#V"W!@B!! /+#["!$\#!##/!M

/B"!#!/$"&[2(aq) at FF #"#["2`jEFF [V// #/#

Reaction of plagioclase #$!/M\#FF [FF [EFF !B /E}/!/+#$"$"/ !BW/!!!!!/ FF L"\!V# $!VB$\#|/@

131

@#!B//VB!+ and aH+&!!!@B!+/ "+[@#VV" reactant plagioclase is added in the model. Secondary plagioclase is predicted to appear ""!!/L"$# suggests that garnet and epidote-ss are close to the Al-endmembers in composition, and clinopyroxene is diopsidic. +FF [W!#!BW// !!!/+/Y+ co-evolve to /V#/+[@//&/!/- appears and secondary plagioclase appears. Al and Si concentrations are similar to each [VV"!FF #""!"#$FF #[@#- B!/"!/ L!#$B"EFF !/ + chlorite. There are subtle but steady increases in Si concentration and proton activity. +!@#/#+ "!#[+/+[@#V/ FF B!/!!!"EFF "!#++$F[!B""@/$ $"/FF /!FF EFF

=,+ #$!BWM\#!"/+FF FF W!B"/B#$"B!BW/- +@"!#[#$"[+- !V+!/M\#!"[ !V/\#"!/FF +EFF [##$B""&B$$N ""/!BW""// |!Y+\#[ L#B""!"/[#@ /+

Reaction of gabbro #$/@@M\#!"!B/*L &#/"!V#

132 4.3. Results

Figure 8. Temperature–activity diagram showing phase relations in the system SiO2–MgO–H2O (blue dashed lines) and selected univariant reaction lines for the system SiO2–Al2O3–MgO–CaO–H2O. Miner- _X]Y'*>'<< tremolite controls aCa2+, while the red lines are for reactions in which aCa2+ is controlled by reactions involving diopside. Magnesium activities in both cases are controlled by clinochlore. The black horizontal lines with dots mark the evolution of silica activities in the reaction path models with the labels on dots represent the log W/R. The lower panel (B) plots the stable parts of reaction lines for reactions that are apparent from the results of the reaction path models. The thin lines are the reactions predicted by the model to take place at higher +, while the thick lines represent reactions predicted to run at lower + (cf. Fig. 7).

Reactions plotted are: R1: 10 anorthite+tremolite + 6 H2O= 7 SiO2(aq) + 6 clinozoisite+clinochlore; R2: 4 clinozoisite+tremolite + 6 H2O = 2 SiO2(aq) + 5 prehnite+clinochlore; R3: 3 prehnite + 5 diopside = 3 grossular + tremolite + 2 SiO2(aq) + 2 H2O; R4: 6 clinozoisite + 25 diopside + 2 H2O = 9 grossular + 5 tremolite

+ SiO2(aq); R5: 19 anorthite + 5 diopside + 10 H2O = 12 clinozoisite + clinochlore + 9 SiO2(aq); R6: 19 prehnite + 2 clinochlore = 14 clinozoisite + 10 diopside + SiO2(aq) + 20 H2O; R7: 5 prehnite + tremolite = 4 grossular + clinochlore + 8 SiO2(aq) + 2 H2O; R8: 25 diopside + 16 clinozoisite + 12 H2O = 19 grossular +

5 clinochlore + 26 SiO2(aq); R9: 5 diopside + 9 anorthite + 4 H2O = 6 clinozoisite + tremolite + 2 SiO2(aq);

R10: 9 prehnite + 2 tremolite = 10 diopside + 6 clinozoisite + 5 SiO2(aq) + 8 H2O; R11: 6 clinozoisite + 19 tremolite + 14 H2O = 50 diopside + 9 clinochlore + 43 SiO2(aq); R12: 3 prehnite + 7 tremolite + 2 H2O =

20 diopside + 3 clinochlore + 16 SiO2(aq); R13: 5 diopside + 8 prehnite = 7 grossular + clinochlore + 10

SiO2(aq) + 4 H2O.

133

Figure 9. Activity–activity diagram showing the phase relations in the CaO–MgO–SiO2–H2O system

(dashed lines) speciated over the phase relations in the CaO–Al2O3–MgO–SiO2–H2O system (solid lines). >Y??#?[ T at high pH and low SiO2. Gabbro (cf. Table 3) is predicted to form tremolite–albite–clinozoisite–quartz 2+ 2 + T /a H ) of about 7.1 and log aSiO2:*X*>_T encountering gabbro will start at the serpentine-brucite-diopside invariant point and develop along the long-dashed line towards equilibration with gabbro. Along much of its path it will make rodingite (diop- '*T not make rodingite along its evolution to equilibration with gabbro (short-dashed line).

"`/EjL#$B""@/FF !BW/!BW/!BW/ !!BW!"- [@#B!B@"@!! $$VB@#$$!!//" ""@/L#/#"#[#"& `!Y*M:j}/+[/@"/#"!- @""$[!BW""/#/# +FF [#"/!BW`/j /`!BW[j!BW!`[/-

134 4.3. Results

Table 4.T| different mineral assemblages at 300 °C and 50 MPa

Srp–Di–Brc Srp–Tr–Tlc Tr–Ab–Czo–Qtz

pH 7.31 5.81 5.30 9 12.5 11.2 27.4 Ca2+ 1.51 1.55 3.82 CaCl+ 8.62 8.86 21.69

CaCl2(aq) 0.74 0.76 1.84 CaOH+ 1.58 0.01 0.04 9 0.0174 1.214 11.4

SiO2(aq) 0.0163 1.212 11.4 HSiO3: 0.0006 0.001 <0.01 NaHSiO3 0.0005 0.001 <0.01

Log aSiO2 :=*!X :*X :*X= Log (aCa2+/a2H+) 10.73 7.74 7.11 2+ *><*?{2 and 0.085 for Ca . j$!VB#@BB["- +$BB#F"<!BW"

135

#"[&[B$"- N[+N$$$ /@@ & " \#"! +FF [#B ""@/$/ V# / +N ! !BW ! !BW" prehnite + tremolite + chlorite + talc + plagioclase. Si concentrations are pre- @B[!B- roxene is stable, and then to increase to values close to quartz saturation (satura- W {/ UX ¤ F }U ¤ 10). Mg concentrations and proton activ- "!$ the course of the model run. In contrast, +"- bined effect that silica concentrations ex- $+@B" $"/#}U±FF#

"!©Mg F*[ other Fe–Mg phases are predicted to be more magnesian. Secondary plagioclase is albitic in composition (An~5 mol%). Figure 10. Summary of the mineralogical changes within a gabbro dike away from the contact with a pe- LW!#FF ridotite undergoing serpentinization at 200 °C (A), 300 !BW! °C (B), and 400 °C (C). At +?T< clinopyroxene + epidote-ss + tremolite controlled by serpentinization reactions (W/R = 105). At + = 1 the W/R is 103, and at + = 2 it is 10. The min- !BW!"- eralogical succession is discussed in detail in the text. !!" The dominant reactions in the order of decreasing +, i.e., with decreasing distance to the gabbro/peridotite ! ! " contact, are (A) = 200 °C: tremolite + prehnite di- prehnite + plagioclase. Mg concentrations opside + chlorite, (B) = 300 °C: prehnite + tremolite and pH remain fairly constant throughout cpx + epidote-ss garnet + chlorite, and (C) = # 400 °C: plagioclase + clinopyroxene epidote-ss + tremolite chlorite + clinopyroxene. steps and plateau close to quartz satura- }U`{/UX¤F}U

136 4.3. Results

¤Fj}M

4.3.2. Phase diagrams

"!VW"$!!/!$/ "/V@#@$`"['*['*_V['** [':RYB[''{[FFE|[FF*j V@B!$!!L!/"/:![@- cause it helps understanding the results of the reaction path models presented in Section

!B"M

MgO–SiO2–H2O (blue dashed lines) as a function of temperature and silica activity. The <+Y!!/:+V$!B! #V[!/#`[j[!/CU !#`UE[*U:jLVB$ /\#}ULV#$VB \#/!#$/"!#!M \#[V$\#$VM!- pentine–brucite boundaries and actually deviate from the boundaries into the serpentine [@#@""$"!#`/jV"!- #/@FFEFF [$$V@\# @B ! @B /@@ "&B"!#L\#$ !"!/:@&@[!//$V"- !###/a`$L@j["&/V/V }U`/#jL\#@B/@@V/ V#@#C#`$EjL"M+ @EFF [!FF !FF L

137

$"!!FF "#`$/j@#$ @B!V!$+"!#[!## silica activity is most pronounced, garnet and clinopyroxene may form in interactions \#@B!C The most relevant reactions indicated by the results of the reaction path models !/:|L'EFF [:FFF [FF <B$V/" !CU!/#<+YB"! talc–serpentine boundary in the MSH system in T-aSiO2(aq) space. +!/"/"@B\#V$2+, H+, and

SiO2`jFF F</'LV! 2+ 2 + "B@#$$ U H and aSiO2`j!C\##/@- @VVBV@#!/\#@#$$ #[@#$$@B![![@#B[$"! VV//#M!@#B//@@- [\#@#$$@B"[![/@@@B"&/ "+"\#@#$$@B"M!M! "B"&!/#[@#/"#/$!\# 2+ 2 + +/!@![ U H and aSiO2(aq) change, but the relative 2+ + $ , H , and SiO2`j//V/$$#& LW"#[!#@B##W" $$!#@B\#@#$$@B$$ 2+ 2 + #"[ "[ "@/ `L@ Ej | U H and aSiO2(aq) differ by #/B$"/#@!M@#M!#"[ @#$$"@/"MCM@M#C"[@#$$"@/ L/2+U2H+[V["W#VB#$$!Y`* V#j|2+V"&@B""/ buffer assemblages considered. These results indicate that rodingitization reactions can- @V@B$$2+ activities. Instead, the differences in the activities of + SiO2(aq) and H generate most of the thermodynamic driving force for rodingitization reactions to proceed.

4.4. Discussion

The reactions during serpentinization have been discussed in numerous recent W!"#`}C&[FFF+B$[FF

138 4.4. Discussion

B$[FFE[FFEB$[FF*|[ FF*<"|[!j#!C"#@ [/$##$!C"!V#- /!C""!BV!#!$!V/#\# are hypothesized to cause rodingitization in intercalated gabbroic material.

4.4.1. Modeling of rodingitization

L"#"@/B!$/$"/@- @\#/#/!CFFFF } \# "! @B /@@ `[ + j[ B! greenschist-facies mineralogy is predicted to develop. In the geochemical models pre- [!/V@+"#$" &@\#+/+$"F[& `/}Uj$"`/EMj$"/*$#[#/ & !BB #@ $ !CM/C B"!$"$$!$R!B#$"B &"$"!"@[B!B@F`+/ [''+/[''\[''@+/[''* +[FF*j/WB/B/!"!$" /#//BVW!B/V#"$\# \#$$$/#`\[''@j"$/" "B"[/}U""$&VB/! $\#@BW!`$[''*j!W"![ W$\#"!@! /@@&/V#"@&$!$@#/B- "[/@@&\#[["- B@#$$@B!B"$" /@@\#@$$@B/@@#!! "B"!BL!/V@+#@ V!WB$$"/@@!/@@`$ Fig. 2). +VV$#"///@@$# $!W"B!/F[@# $"!#"CFF+!M"- MM!/"@/$"//+FF `/F+j!

139

@!@B!M!BWM[BM!BW the distance to the contact diminishes. At all three temperatures the predicted number of !L"!$"EFF [- !BWFF [!BW/FF +FF `/F|j[#}U"@/N!["[ ![!/}/+["!! become replaced by epidote-ss and clinopyroxene. Proximal to the contact, epidote-ss !BW/VB/[!@B!BW at the contact. +EFF /+`[\#"!@B /@@j[!/!BWW"++ "@/!/!"`/Fj+!!- /`+Fj[!"/VBB- !BW[B""\!! $"EFF #""B["!$/+[[ //@@U![!/!BW !"!BWEFF [!" !BW!/FF ["! !BWFF We suggest that our simulations provide valuable information on the sequences $""@/V!/@/@@! /@@[/@@/+|## "#V&!B![+|##- V!"[M#"[@#B@"! !/!@V

,% Gabbroic veins and screens from fracture zones on the equatorial Mid-Atlantic //B/W$/C`YCX['*j- !V#@[!CB$"V !""!L#@@BYCX `'*j$"@B/V&/"#! $"!!+/@`/["['*[ [':'jBB!&!"$!/@B! ![/#L!"$!@B! observed. Honnorez and Kirst (1975) report of a large plagioclase grain terminated by a

140 4.4. Discussion

!V\/B$"!V[#@V$- /#NB/#!B/#/!! @!/@!/+"C!@B #!"$FF FF `/Ej Our model calculations support the petrographic interpretation that grossular-bearing rodingites form from epidote-rich ones as a metasomatic evolution sequence (Schandl [':'+[FFjL&$/#!V$!/ /@@B$"/C$/\# /B@B/@@[#!B!- ity to produce rodingite. The extent of rodingitization does not just vary as a function of distance from the !+BVV\#!$"#"["[- /#/CW/$#/@@@B#$- "\@@"!Y^!WB this type of relation. The least altered samples are characterized by tremolite + chlorite + !!//!"@/[!/C! B/#CB[$" V/V!`Y&[''\[''j"B[/@- @&V$"\#~ a_[ "$@#!V/`|#[''j \~&B/#[V !@B!"/`|#[''j}![/ #$VM![&$@#$$ ##V##//$"

4.4.2. The critical role of aqueous silica

Our model results highlight the critical importance silica activities play in rodingitization. That silica activity gradients play a role in the formation of rodingites and @&#"[&@!!@$|B`'**j noted that diffusion imposed chemical potential gradients mainly of silica and magne- #"/V"$@#$#"[@YCX `'*j!!!"$!@BB/!"@B consumption of silica in the serpentinization reactions. Frost and Beard (2007) realized that the silica activity of brucite–serpentine equilibrium oversteps the reaction transform- /!/#/#V-

141

$!C\#!B/CL! !B#/:@!/$$" garnet, diopside or chlorite, the activities of aqueous silica set by brucite–serpentine equi- @#"!V"B"V$&!L$$ "!$##/"!#@FFMF + higher temperatures, olivine is stable and brucite is absent. The aqueous silica activity $VM!@#$$/B"!#[EFF @V[VB//C&!` F<j+B$`FFj!!\#$"V! B"B"@#$$@B!BWMM""!#$EFF L!/'\#@BM"!Y[[ "&/}##/$"B !"&\#/!YB#\## /@@@"!"B"V/$"&/`/'j

4.4.3. Mass transfer by diffusion or advection

<"!&!@B\#\@B$$#V@B/ VB/`/[XC&[':L"!['*FL"!['*E|& <XC[':*B^!![''j[!!" @#B"""L$$#V\#W$!B"- !/[!/$$"! is the one that diffuses most readily. Our model calculations (Figs. 4–7, Table 4) indicate /C"!//$$V$ ##!"$XC&`':j[O!$B"@P "!"$L"!`'*FjX"!L"! B@#$$@BW!`N!Cj "V$B!\#B"[W

'"['*+#"V&[FF:j$&##2 diffusivi- $FFMFF "!#/$F: to 10'm2 s`}}&[''*j

142 4.4. Discussion

!B$F[$$V@#&@#$[[//#B[ 10 to 10 m2 s}$$#VB[//#B&[ V/$$#V$##"MF&BL" easily matched by estimates for the minimal life span of peridotite-hosted hydrothermal systems (Früh-Green et al., 2003).

8 #!/#$#V/!!&$// 2+V!\#"[M#"[`jL[/ "!$"""V@B\#/- #/!C`/["['*{[FFEj}#//- #$##"##!$L!# L@EY+ species is three orders of magnitude more abundant in /!Y!C\#"!Y\#@B /@@Y["$$/!@@B$$#- $BW"!W$!/ It is popular to assign large mass transfers, such as involved in rodingitization, V$/"#$\#$"""/C#B #@B\#[["$/$"F: `$L@j##""#"&$FF[$"/\# F""U&/##"!/L!@"/- \#W$\#/@@"&/ `FFEjV[FF [!@B$"/- #!Y$":M!B"& $"FF+M! $"FF#@/`2+U2H+j$\#@B /#L!/'B#\## @@!#/L#//[""" B@\#M"@B#"&/$ \#@B!M&/! &L!#@$#B#// ""!"\#!#/#`/[} V[':§B[''j!@"["!# @//#$&"!CB" ""!VV#$/C"&W- "B#&B }!!V$/"#$\#-

143

#/C<&B["$/B$$#" @B//!L!$!- C"!##FFMFF #//V/$$ rodingitization. The thermodynamic constraints discussed here demonstrate that rodingi- C`"""j#"BV@B//! ##"$\##"![ #/!@&"!#@FF B facilitate rodingite formation as suggested by Frost and Beard (2007).

9#9#9#>

L"#W!""!"@V"[M #"[CL"!#!!V/$$!- "@/{/"@B!/"" V/`/[YCX['*[ ':'B[FF*jV$""@#B !//$#!#!!`XC&[':j L!$""V!/@#BEFF F <"BW!$"$$@&`/[['*{ al., 2004). These calculation results also explain the common alteration rim of chlorite #!/\! ^$$#"$"[M#"[@@"" !"//#"[&@"$- //CL!/:#//B ! / \# @B /@@ `[ {/}U¤j##[""! /@@&$#B"@/ $/`/[|[FFEj!/[ [VW@[@W!@#@ ""/[#/B"!$! and garnet (Fig. 8).

144 4.4. Discussion

9#9#$#*@\- tion?

Due to the large entropy-change in dehydration reactions, prograde metamorphism favors the formation of anhydrous mineral assemblages. Moreover, isotopic evidence and theoretical calculations predict that anhydrous mineral assemblages may persist at high "!#V!$###`{#B\##[''<- "&['':j"!/&""!&/ $B#"/$""!#/ good example that this line of thought can be misleading. An intriguing consequence of /C"B$""!@B$$" the solid phase assemblage (e.g., reactions 3, 6, 7, 10, and 13 in Fig. 8). In fact, the diop- M/"@/!$"/&B# `BB#[$/B/j[!V$" "!VB@#///"!#`:FF j $$"`B[FF*j^!#$V\#VB /"!##!#![$##! the accretion of the igneous oceanic crust (e.g., Dunn et al., 2000). There may indeed be petrological and geochemical evidence for deep and high-temperature circulation in the "!`/[|[FFE\/BLB[':j+/$" "!#$!V#!!"""# @ /[ ! @/ ![ /"!# #} high-temperature origin of the diopsidite veins (Python et al. 2007) is possible, our model ##//!&#$""!#FFM FF @B""$/@@!BW\#B externally buffered by serpentinization reactions. The model calculation of clinopyroxene FF FF `/j!L@/"!- $!"B!BW@"!/[B clinopyroxene is virtually pure diopside (Fig. 5). The diopside in the Oman diopsidite !+[[W!$$""!#<V[ small amounts of garnet commonly accompany diopside in the veins (Python et al., 2007), #!!BW! !@!@B"M"/M"!#$EFF +B/"!#[VB$##\#W/V #@/#$#C#`$/|[FF*j[ !@/VB!#/$"## B"/B$"@B\#[##

145

VB@#$$W"BV@B!M@##@#"|# @#W!$"$V@&"!#@VF [ $!/#/@@BB`FF*j- /$""!#$:FF |"! $V!!$/$"!#$ formation for the Oman diopside veins.

4.5. Conclusions

(1) Titration reaction path models can be employed successfully to reproduce the mineral assemblages commonly formed in rodingites and provide crucial insights into the main driving forces of rodingitization reactions. (2) At a pressure of 50 MPa, rodingitization can proceed at temperature around 200 FF [!C\#@B@#M!M- !#@#"+"!#$EFF /[@#@ \#@#$$@BM"M!#\#[&$ ! @& "& \#[ $" / - //@@@$/@@" (some are rodingitized, others are not) can thus be related to the temperature-de- !\#M"#@! `j /C"&B#$$$#"""LVB /!#@#"["[ /!LVB/2+ is virtually ^$$#V"$$/&B$$#$ BW!&@B/\#W$[/@- /#B[/\#\#W&B!#! VB/#V/C# !$$"B" `Ej L$$#V$$"&#//V/C# V@"""#"[&[@W#$ @#[C"B$"![## @B!/#V/@@ `j L$"$!V""B&!"!- #$FFMFF $"/@@!BW!B/ "!#`:FF j#$"$"!B# assemblages.

146 4.6. Acknowledgements

9#;#*+

}&§/#$V@#!#< YL"<"!V#!"@/"B" @B"#V&$#V"" "#/#[Y &+#"["# !!L&#!!@B^##//"$`^\j/ |FU}|@B^\U_W#OL _B"P

147

References

+@[L+[#[[|&[X[{B[\{[[[V[<[ ':: <B/ / ![ ~"@ ![ !!N ! /"\/B*[M +/[[[<[''*WB/!!C! #"[&$"<+/` jNX[+[[ <[<[^[_[^`_j[/$^//"[ [#[V[!!:M:: +/[ [[ \[ |[ <['' </ WB/ ! $- #$!V$"U@- +@BN}"[|`_j[/$^/ /"[[#[VE'[!!EM +/[[Y&[[|#[^[VB[<[''Y@!"!- $##!!"&W!Y^! \!/L!#_B{[:M' +[^_[B$[}_[FF"!V\#$"#"- [B"B""/NW!"#BEFF [FF@\"""+*`:j[ME +[[&[}[|[}[#&[Y[\[[|#[\[FF* Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro W!@B"$#/<+/[ Fa`^{/ F'jN#$#WB/!#B\"B[\!B[\B" `:j[F:FF[NFF'UFF*\FF* +[X[_V[<[|&$[<_[FF\"B$#"[M/ &!C^WV/@[# \#$+$_E`j[ME +#"[[{#@[Y['*L<+/E [©[!C #"[##$_:[M +#"[Y[V&[L[FF:/CB$! V!{&![B{FE[**M': |[}[\[[YVB[[#&[Y[[<[FFE@M!- M[/$"^{/F'[<+ \"- B[\!B[\B"`'j[F'[NFF'UFFE\FFF*EE |[[{"[[Y""@/[\['*\"V$!B serpentinization. Science 156, 830–832. |[[Q[[''L!@\#"$!B!

148 References

#"[!@"!C[}\- logical Society of America Bulletin 80, 1948–1960. |/[+[B$[}[':^V!"$/@&!- [_{/#[B@#</B/B:E[EM 151. |[[#/[^[[+X[FF\@@!/"#[@+@B- [^{/*[F*FN"/""#/$"V- $/\!/#$/BE`j[::M'F |&[<[<XC[^[':*L!$"@B\##/"- "!"@#</B/B'[:EM' |#[^[Y@[[Y&[[[<[''<"!"$! &$"\$L$"`_[ aj#- nal of Geophysical Research 96, 10,079–10,099. |[^["[<[|[[[+[^#[<[+/[[FFE^! /"!#B"#"!N!- /!V#$/BE`j[:MF: |B[|['**<"C""!&\""- mica Acta 41, 113–125. "[\['*{"!#C!&$$[ Oregon, and Washington. United States Geological Survey Bulletin 1247, 1–49. ^#""[_[':|/"W/$B"\#N"$$ mineral precipitation. Ph. D. thesis, The Pennsylvania State University. ^#@&[_[}[+['''{B$"/@&$" C" `{ [ jN "/ $ "" !- cesses during serpentinization and serpentinite recrystallization. Mineralogy and Petrology 67, 223–237. ^#[+[L"B[^["[[FFFL""## !B!!$#"@_[ ' Fa#$\!BF`|Fj[[*M[ _V[|}['**<"!"$!!!+#V of Earth and Planetary Sciences 5, 398–447. _V[|}[FFEL!"#B"VNB"@- \/BVE[E*'MF B[<[^!![\<[''#\["["""\- ogy 19, 211–214. [Y[/[<[@[_['+#B[$[!/$## "\"""+[*M:

149

[|['*""!"$![@&- /B\_B[[}/#$/B 16, 272–313. [|[|[[FF*VB!C#$- ogy 48, 1351–1368. \[\{[XB[^|[|[<[X[+[{#/[X+[|#- [[^+[|[[&#&[\[FFF[FFFB$B" VB{BV[F[E'ME': \[\{[[+[^Q+/[{[''</@! $!B!$#!!#/@@$_[ `Y^![:'EjN<V[[\[X<[+[[<B[`_j[ /$^//"[[#[VE*[!!M 254. \[\{[[+[{#B[[''@/@! B" !C $ _ " Y^![:'N<V[[\[X<[+[[<B[`_j[ /$^//"[[#[VE*[!!F'M 163. \/B[L[LB[Y[':+WB/!![$# #["!["N_V$:@#$$/$ @B!`&"jB"#"/# of Geophysical Research 86, 2737–2755. Y&[ [ |#[ ^[ #[ [ "[ {[ +"[ [ {[ [ |#"[[''/B$_[##!!"W- !Y^!`#[j#$\!B 98, 8069–8094. Y/[Y[^B[<[@[Y}[|[^X['*:#""B# $"B"!!$&$"/"+"#$ Science 278A, 1–229. YC[[X[['*/B$/$"#<+ $# C / /[ @# </B and Petrology 49, 233–257. &B[^[B$[}_[':YB"!C$!- #NW!"V/$"/B"" "B\"""+F[*M*: [ }[&[ _Y[ Y/[ Y[ '' L'N $ !&/ for calculating the standard molal thermodynamic properties of minerals, gases,

150 References

##![$"MFFF@FMFFF "!#¬ Geosciences 18, 899–947. X"[|[X&[_[<[^[{/F'!@B[FF*{/ F' #""BN ! F&"& #V @#B B @ <+/[E M NX"[|[X&[_[<[^ `_j[/$^//"[[#[VF'[ pp. 1–33. XB[^[X[+[|&"[^X[\[\{[|#[[^+[{B[ <^[[_[&[<[[XX[{@[\L[VCC/[[+LF !@B[FF+$$WB"V[<+ /F #E[*M: XB[^[X[+[\[\{[§/[^[&[L<[|#[[ ^+[ YB[<[ &[<[ [ _[&#&[ \[ &#@[<[ |B[+[{[|[{#/[X[\&[^[|#&"[X[|B[+[ |C[ }[ [ X[ _[ <[ ^#[+[ |[ <[ {B[ <^[|[+[#""[_[BV[[FF+!- B"N{BYB"F*[E:MEE X[_<[FFE\"B$/##NY[Y^[L#- &[XX`_j[L\"B_V[+""[!!EM EXC&[^[':LB$""C/<#"^- posita 3, 222–231. XC[[':B"@$&$"/"+"</:[**M 279. {#B[[\##[\[''WB/#"!"!$Y^! /@@`Y:'E:'E\jN/"!#$ #BN<V[[\[X<[+[[<B[`_j[ /$^//"[[#[VE*[!!*M 234. {[©[~/[{[}[[+[§[[[FF*/B$/V$" /L[#$<"!/B[M 382. {[©[[<[|#[X[FFE<"!!/$~- "!\/BVE[:M <"[L<[FFF\"!"B!#VB#@" hydrothermal vent plumes. Deep-sea Research. Part 1. Oceanographic Research Papers 47, 85–101. <"[L<[|[}[!L"B"B//

151

#/!C$#"[&\"""+ NFFU/FF:FF <"[L<[&[_{['':#M&#N "B""$B"#$\!B- search 103, 547–575. <[<[X"[[B[[<B[{[FF^!@\#$#W"!- +<$!"#VN^/- /" {/ ' \"B \!B \B" E `j[ NFF'U FF\FFF:: <[<[}[\[B[[\@[[<[|[FFE"B$!/ <$!/VVC$#@#/ !\"""+:`j[E'ME' <#C[[[[FF*#/"/$/@// from the Karabash alpine-type ultrabasic massif, Southern Ural. Geochemistry International 45 (10), 998–1011. [[/[\[':YB//$""#&" _B{[MF "[[}"[+_[FF*B""/$- /$"NV$"/\##<+@- "[+@[#@\"L`:j[NF:UE* 4866-8-11. QYB[^[[_[}&[[''L/$/$"[ |#"@[#"L`Yj#/!C \"""+['*MF: [{[[<Y[FFE\""$"""#"[ B"N!C[/C[\@"B!- !\"""+:`j[M B[<[#[\[[§[|[+[+[[FF*"!N /B / $ VB / "!# B" # "!@!/_B {[:'MF [<Y[''*YB"!\#"! N|[Y{`_j[\"B$YB"^!}B ¬[§&[!!FM [ <[ ': <"!" $ /N ! ! $ B" M

152 References

[<[&[+[FFE"!$!"\"\- !B\B[FFFENFF'UFF\FFF'* [_[QYB[^[}&[[':'/!C#"[ &$+@@\@[</*[*'M' B$[}_[^@@[}_[':FM!FF FF@N"!$/$!\" ""+EE[F'M B$[}_[##&[^[+[^_[FFE"[B" B""/N"!B!Y[W @#N\"[[{[[[{<`_j[< /NYB"|{! +"\!B[}/[^[!!*M:E B$[}_[##&[^[#[[FF*WV#"$ #/!CNW!"#BFF [FF@ "!$#"[B"B""/- \"""+*[:*M:: &[ _{[ Y/[ Y[ ':: # $ "B" ! !!$##!/!#"!#N- /"$!#$!&@FFF \"""+[FF'MF &[ _{[ Y/[ Y[ ''F # $ "B" ! !!$##!/!#"!#N! "!!$/!\"""+E`Ej[ 915–945. &[_{[[^[}[<[V&B[^+[''*/!/- / \#N "/ " "B" !! $ ##BW"!W\"""+[ 907–950. L/V[|[[[FF+#"#"!#\#V\" ""+['M'' Thayer, T.P., 1966. Serpentinization considered as a constant-volume process. American Mineralogist 51, 685–710. L"![+|['*EM$$#C@"@! #$/B[EME L"![|['*F\"!B"\""- chimica Acta 34, 529–551. }[[V[<[':!#!/""-

153

!"@#</B/B*'[M* }[_|[}&[^+[''*^$$#$VY\[ "!$"!##!!"@# Mineralogy and Petrology 130, 66–80. }C[{[&[_{[FFF^/#/#"[$"@#@" B"B"@B"!/#V\#"!# of Geophysical Research 105, 8319–8340. }B[L[FFE#[$"B"$/""/$ "M#B"N^!"$_- /B`_j|+"!B[{{ }B[L[&[{[FF$Q<#_U`V:Fj- {@[+@###[<W §B[|[|[Y[$$[+[''_V$/\#V #/"#""!"#E'[ME

154 Serpentinized troctolites exposed near the Kairei BEJ=J L\ unique microbial ecosystem

Abstract LV\#$XYB"`XYj //#CL!#B!LBVVB/Y2 [VB/["&@BY4UY2 ratio. B[!#@!XY[@#B"\# are suggested to support a hydrogen-based hyperthermophilic subsurface lithoautotrophic "@B"`YB!{<_j[@&B"/# for the early Earth ecosystems prior to photosynthesis. Despite the increasing interest in \#"B@[/$###"B$B- "\## Y#//$BV$" small hills near the KHF, provide a possible explanation for the composition of the KHF \#^V"#@"@&FFV!/#[ [V/@@[/B##<- !@VVVB"!!B"!B replaced by serpentine and magnetite, indicating the generation of H2 by serpentinization @VB"\#L"#! that the high H2/$B"\#@@# !C$#@#B"@- &#XYL##///$XB" B"[!#&"!!$#[ @!@$###"B$XB"\# V/#$YB!{<_

5.1. Introduction

VB$@&"&V/"B"$# communities (Spiess et al., 1980), submarine hydrothermal systems and associated biota have attracted the interest not only of geoscientists, but also of chemists and biologists `/[Y#"![''^V[FFF}&[FFEj!$- cades, it has been revealed that the diverse populations of the hydrothermal vent-endemic animal communities are generally dependent on the primary production of symbiotic and free-living, chemolithoautotrophic microorganisms. These obtain energy from inorganic substances, such as H2[2, H2[Y4[V$"B"V \#`/[<[':j<B[!#@! to archaeal methanogens supported by H2B"\#[@#B!

155 5. Serpentinized troctolites near the Kairei Hydrothermal Field of microbial ecosystem is considered to be an important modern analogue to the early B"$_[W$!" `/[XB[FF[FF[FFL&[FFj LXYB"`XYj/`j /#CL!#`LjV+#/#FFF[B @VB"V`\"[FFY" [FFjL""!$V\#[#/"V!-

`/[[[

H2 (8 mM) despite the similarity of the other mineral and gas element compositions to B!@"/B"\#`^V[FFL& [FFE\^""[FFX#"/[FF:jFFE[#// that a hydrogen-based hyperthermophilic subsurface lithoautotrophic microbial ecosys- "`YB!{<_jW#@\V"$XY`L&[ 2004). This microbial ecosystem is sustained by the primary production of hydrogeno- trophic, hyperthermophilic methanogens, utilizing H22 as the primary energy and carbon sources. The H22 are completely photosynthesis-independent substances, !VB@B//`B"j!["!B/YB!{<_ &B"/#$B_B"!!B`L& al., 2006). YB//"!#B"V\#"X- \#V@!$"VB"V[$@[ {/V[+C[@#//<+/

`<+j`#[FF[FF*<[FF:jL<+Y2-rich hydro- "V\#V/B@@#!C$@B!- @B#!$"!B"V[ `#[FF^#V[FFj[@//V- dence indicating the involvement of peridotite in the generation of H2-rich hydrothermal V\#XY`^V[FF\^""[FFj-

[#/!B"\#/BW@/Y4 and "!@`/[#[FFj[

XB"\#VY4 and Si concentrations similar to typical mid-ocean /B"\#`\^""[FFjL#//@ &#W!"!\#"![$ the unusually high H2$XB"\## FF[V!C&`B!"!- jXY`X#"/[FF:j[#@###"- !$XB"\#Y[

156 5.1. Introduction

70o00'E 70o30'E 30 N A

15 N

Central 0 Indian Ridge CIR-S2 (CIR) AFRICA PLATE AUSTRALIA PLATE 15 S 25o00'S Rodriguez Triple Junction (RTJ) 30 S Southwestern Indian Ridge Southeastern (SWIR) Indian Ridge (SEIR) B Kairei Hydrothermal Field ANTARCTICA PLATE CIR-S1 45 S 45 E60E75E90E95E

25o30'S 25o30'S RTJ

SEIR

SWIR

69o00'E 69o30'E 70o00'E 70o30'E

o o o o o o 3900 69 50'E 70 00'E 70 10'E 70 08'E 70 10'E 70 12'E 3400

0 3300 250 0 3 3800 50 B 3800 3700 3100 C 0 3700 3 0 00 000 3600 33 250 3000 3500 0 3600 3400 3200 320 250 0 3400 3500 o 0 4000 2900 25 10'S 3 50

0 0 3300 0 3300 5 3200 3 3100 0 0 0 3 3000 500 Uraniwa-Hills 3000 0 300 3200 0 0 0 0 7 3000 2 4 4 00 2900 0 2800 50 3000 0 3 2900 2800 1 250 0 0 350 0 3100 2800 0 0 2 0 250 8 5 0 3 0 Hakuho Knoll 2800 0 2900

C 3000 North Hill 3100 00 3200 40 3 00 2700 300 0 0 2900 0 00 4 3 00 3000 00 3 3500 28 30 000 2900 0 0 CIR-S1 00 4 0 0 3000 25o20'S 3000 4 2500 South Hill 0 00 350 3000 2700 0 0 2 350 5 0 4 3000 0 000 2900 Kairei Hydrothermal250 Field 28 3000 00 3500 3000 2900 0 2800 3500 3 000 3100 3

0 0 0

0 0

350 2900 310 3200 2900 00 2800 300 4 00 30 0 0 2900 3000 3000 400

0 0 3100

35 2500

o o 0 o o o o 69 50'E 70 00'E 0 70 10'E 70 08'E 70 10'E 70 12'E

-5000 m -4500 m -4000 m -3500 m -3000 m -2500 m -2000 m

Figure 1. (A) Bathymetric map, based on SeaBeam data, of the Central Indian Ridge (CIR), Southwest ""'"J"'"_"'* The location of the Kairei Hydrothermal Field (KHF) is indicated by the star symbol. Note that the abyssal <<""'* (B) Bathymetric map showing the Hakuho Knoll and Uraniwa-Hills. At the Uraniwa-Hills, olivine-rich rocks of plagioclase dunite, troctolites, and olivine gabbros were discovered. The location of the KHF at the western slope of the Hakuho Knoll is also shown. (C) Bathymetric map of the Uraniwa-Hills, showing sampling localities of plagioclase dunite, troctolite, and olivine gabbro. Note that both the North and South Hills are elongated perpendicular to the trend of the surrounding abyssal hills.

157 5. Serpentinized troctolites near the Kairei Hydrothermal Field

&$@B/#/@#"B$ B"\#XY

5.2. Geological background

LXY[/"$""B$L ` 'R[*F FER_j!$EEF"`\"[FFjLB- "V#!$Y&#X[$$ W&$[&"$/W`/[@j#BFF[ !$"\@V&"!/#/"#@"@ &FFV!""!V#/$//@&- ground of the hydrothermal activity at the KHF (Kumagai et al., 2008). ^V@V!$Y&#XV& B"#!$!@VW!#$!& @V`X#"/[FF:jL#$!V#- V/!$"#XY`^V[FF\^""[ FFj_W"$\"!/B/[@|"@- B"B["!/!/`$$Yj &"$XY` 'R[*F FER_X#"/[FF:j`/@jL Y"[Y#Y[V@BFF "!_}/VB`/jL!B"!/B$" ##/@BW//!`}_j`/j L#/##!!#/WV!# Y`/jLY/"#Y[- #/#/##@V#`/j^#/#V Y[V&!B!W#" #+#/#!$"!"!$!& #[!/!BW!#$!&#// Y!"!W`j`/[|&"['':j

5.3. Analytical methods

158 5.4. Petrography described in Morishita et al. (2003a, b). <\# # #/ L' @ `- ''j[#!#<"&`'':j}B[ `FFEj!##$"!#!/" _U`}B[''j#["B"$"## !$"L'@[/XV##@B L'!/"$"!#$"FEFF !#$FF@+V- B$[$##!##/|#`Y/[ 1969).

5.4. Petrography

}#EV/@@&"@B$"- Y!/#['[$#V/@@$ &B!#![#@W!$"!"/ #!!"[@#B&B#$!!$ #U#"@#B`/[^&[FFFj"/V B@!$"+`/ FR# ~L&C[FF*+<$[FF:j[#/// &$!&"#$ Both the plagioclase dunite and troctolite are mostly composed of subhedral to euhedral V`'F*EF"[!VBj!/" amounts of clinopyroxene (< 2 modal %) and spinel (< 1 modal %). All the samples are intensively altered to serpentinite. Thin-section observations reveal that these samples BW@!#"!""W#`/[@j[ "W#$!C#`/[}&}- &['**jVWVB!@B!"/[#/- V!V"!`/[@jL

159 5. Serpentinized troctolites near the Kairei Hydrothermal Field

(a) Mt

Ol Serp Ol Serp

Serp

(b) Serp Vn

Mt Ol

Serp Mt Serp

Figure 2. Photomicrographs of the olivine- rich gabbroic rocks from the Uraniwa-Hills. (c) (a) Mesh texture composed of serpentine and Cpx magnetite in a plagioclase dunite. Relict olivine crystals are partly recognizable. (b) Mesh Mt texture composed of serpentine and magnetite in a troctolite. Relict olivine crystals, as well Ol as serpentine + magnetite vein, are also iden- *'{<<< serpentine and magnetite in an olivine gabbro. Serp Clinopyroxene and plagioclase are relatively Serp Cpx unaltered. Mineral abbreviations: Serp = serpentine, Mt = magnetite, Ol = olivine, Cpx = clinopyroxene, Pl = plagioclase, Vn = serpen- Pl tine + magnetite vein. Scale bar represents 0.5 mm. alteration is less than in the plagioclase dunite and troctolites. Most of the olivine (85 - 82 $

160 5.5. Discussion

5.5. Discussion

5.5.1. Origin of the high H2LB\

The exceptionally high H2XY\#"&@ @V\#"B/V$"/#

Y2C/"""B$XY\# #@[\^""`FFj!!" basaltB!@$\#"B[#- usually high concentration of H2/@B!!$"#[ "[#N

2+ + (1) Fe + 2 H22 + 2 H +H2, + 2+ + `j # + Fe + 2 H2#2 + 0.5 H2 + 3 H .

Table 1. Selected chemical and thermodynamic parameter

Kairei 6 Edmond 12 Comment

Temperature (°C) 365 370 a Fe2+ 6.0 13.1 a, d log aFe2+ -6.19 -6.17 b

log aH2S -2.41 -2.33 a pH (@25 °C) 3.44 3.13 a in situ pH 4.42 4.17 b H2 1.214 1.301 c observed H2,aq 7.9 0.25 a, d Log K (rxn1) -2.17 -2.20 e

predicted H2,aq 0.055 0.027 b, d Log K (PPM) -3.68 -3.64 f

predicted H2,aq 0.86 0.99 b, d Log K (methan.) 8.59 8.35 g

7+methanogenesis -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/kg 2+ e: = * 10^[log K + 2*pH + 2*log aH2S+log aFe ] f: = * 10^[(log K + 2*log aPo)/(4/3)]

g: CO2,aq + 4 H2,aq = CH4,aq + 2 H2O

161 5. Serpentinized troctolites near the Kairei Hydrothermal Field

We calculated the equilibrium concentration of H2 predicted for reaction (1) to as-

$"#[!!V@W!$@VY2 concentra-

XV\#LY2 concentrations controlled by the PPM (pyrrhotite-

!B"/j@#$$`!BEUY2¤U"/!BEUY2j # # # $ V \# $" X

_"[V""!#`L@jY2 concentrations for

`j±F"<@\#[<@#$$!/VY2 $""<L"W!"[/ !$@EFFE `B$

[FFj$W!/V$Y2X\#`: mM), and reaction (1) falls short of supplying enough H2 by a factor of 140. These simple ##//[$["#[!!VB#&B!- sible for the unusually high concentration of H2XB"\# YV/##!@BB/!#/@

B"!#:"<Y2[WW"$- &W!VB$XY!VV@W!$ high H2"#XB"\#& !C$"!["@BV[/W- tremely H2B"\#`/[|[''}C&[FFF +B$[FFj+#/&$"Y typical mantle peridotites, this process could also provide the H2XV\#[

$&#/V!#Y2 during their hydrothermal alteration. LV/B//!B$[#/- "!"#$EFF FF@[#/@#&"B$B!#@ typical primary modes of 75 % olivine, < 25 % plagioclase, and < 5 % clinopyroxene `L@j|#!//"![- $/!/#"@+:F`!/!B "!$!"V@&j@#$"!`![-

Table 2. Composition of rocks used in reaction path models wt. % Troctolite Basalt

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

Na2O 0.4 3.0

162 5.5. Discussion

!BWj/$"#@#$@#L "#!C$& of unity can produce more than 16 mM of H2["Y2 concentrations in V\##/$"@""@B!`#[FF" et al., 2007).

$#$#"#<M/@

L!C$V!#/B/@N

(3) (Mg0.9Fe0.1)2SiO4EUFY2

V!@#"/B/

#"![V[VB&!B"!B !@B!"/[@#B@|##- stable at high-silica activities, such as imposed by an external source of silica, according N

(4) 3 Mg(OH)2 + 2 SiO2`j

@###!

L@$@##"!"B$"!B#@B high-SiO2VB\#!##/!/L&/ @$@#""!$V![ !C$&##B@@N

(5) 2.85 (Mg0.88Fe0.12)2SiO4 + 0.67 SiO2(aq) + 3.66 H2

1.76 (Mg0.95Fe0.05)3Si2O5(OH)4 + 0.14 Fe3O4 + 0.14 H2

V##!"/B/

L!$""#$!/V#[ 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 re- \##/$&[$"$@V chlorite-rich coronas at the olivine-plagioclase contacts. The replacement of plagioclase 163 5. Serpentinized troctolites near the Kairei Hydrothermal Field

-1 A Kairei soln.

Qtz saturation -2

Tlc 2 troctolite calc. Tlc Ol -3 Srp

Srp Ol log aSiO -4 peridotite calc.

Ol Srp Brc -5 Brc

250 300 350 400 450 Temperature(oC)

-1 B Qtz saturation -2

-3

-4 SiO2 H+ -5 log moles dissolved species -6 0246810 grams of basalt encountered

Figure 3.'T{{2-H2O system as a function of temperature and activity of aqueous SiO2 at 500 bars. Quartz saturation curve is shown as 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 the dark gray star. (b) Change in concentrations of H2 and SiO2(aq), as well as pH, in the hydrothermal solution as a function of weight of basalt encountered and reacted with the solution. Calculations were performed at the P-T condition of 400 °C and 500 bars.

@B!&!V!@`| X[FF:[FF:jL@V@&$!/V to prehnite and chlorite

`j +2Si2O8 + 1.25 Mg2SiO4 + 2.5 H2

F2Al2Si3O10(OH)2 + 0.5 Mg5Al2Si3O10(OH)8 + 0.25 SiO2(aq)

164 5.5. Discussion

$!!##

W! ! V[ # $ L equilibrium silica activities of this reaction are much higher than that of forsterite-talc #@#"N

(7) Mg3Si4O10(OH)2¤

¤$!##

!#@#"[[#!"$ V@B#@VYL!$/#[ V[/BV!B#/!- C\#$"BVBV`"#$ #@#"j[#!/"/!C- \#V@#$$VBV#@B@#! #@#"`|[FF*|X[FF:jL a range of SiO2\#$$Y L!"$!/@B/#@W!$"!# @FEF [@#@#@/"!## ##VB$VM!@#$$/B"!- # `<" |[ FF:j |# !@@B # ` V @B j#@#/#$"L"!#/ C$XY\#[@B"/B$ V\#!!}#$&"!- #"#W"#@#"! B"EFF "\#"!"!

$V\#L"##//"<##2 /\#[/B/!-

#`F"<V/}C&[FFFj2 concentra- @VXB"\#`*"

SiO2L"!$[2 must V@"/\#\!$XB"B"

165 5. Serpentinized troctolites near the Kairei Hydrothermal Field

0.5

0.4 mantle olivine array

0.3

0.2 NiO, wt% NiO,

0.1 Pl-dunite troctolite Ol-gabbro 0.0 0.92 0.90 0.86 0.84 0.82 Fo mol % of olivine

Figure 4. NiO vs. forsterite content of olivine in plagioclase dunite, troctolite, and olivine gabbro samples from the Uraniwa Hills. Mantle olivine array (Takahashi, 1986) and compositional range of mid-ocean ridge basalts from FAMOUS segment on the MAR (le Roex et al., 1981) are shown for comparison. Note << mantle peridotite, but similar to those of mid-ocean ridge basalts.

1 H2(g) H2O Aw 0 Pn –1 Po (aq) 2 –2 Hz Rainbow Kairei

log aH –3 Mt Py –4 Mi Hm Bu –5 –5 –4 –3 –2 –1 0 log aH2S (aq) Figure 5. Phase diagram for the Fe-Ni-S-O system at 400 °C and 500 bars (Klein and Bach, 2009). Fe phases hematite (Hem, Fe2O3), magnetite (Mt, Fe3O4), pyrite (Py, FeS2), and pyrrhotite (Po, FeS). Ni- and Ni-Fe phases are awaruite (Aw, Ni3Fe), bunsenite (Bu, NiO), millerite (Mi, NiS), heazlewoodite (Hz, Ni3S2), and pentlandite (Pn, (Fe,Ni)9S8'* <<- "<<*[^2-aH2S conditions for troctolite- seawater and peridotite-seawater reactions in early stage of the serpentinization are shown as light and dark <<*"T*

166 5.5. Discussion

$#$#8#*/LB@\

}V#@W! $"V"!$V\#[

W!"&@/Y2$XY\#LXB- "[@$&BV\# @B\!\#/#/ / #/ @ V \# "! $ @ / L$#W!\#"B[/"! "[#/&""!$@`L@j "!#[/"!# EFF `&"$j[#/\#!/VB @`[&"$jL#$"# `/ @j B " "# $ @ `± / @ ! &/

\#j 2 B" \# !V#B close to quartz saturation (Fig. 3b). These results suggests that even limited interaction

@\#@&$XY#!#/2

$XB"\#Y2 is not predicted to decrease notably during ""#$@B"#!\CL!! hybrid model can hence explain both high SiO2 and high H2 concentrations.

Kairei Hydrothermal Field Uraniwa-Hills

Ridge axis mafic rocks

2 mafic rocks 1 Hydrothermal circulation Heat source Olivine-rich rocks

Figure 6. Schematic representation of the hydrothermal system at the KHF. (1) Hydrothermal reaction of circulated seawater with the troctolitic rocks in the Uraniwa-Hills results in the unusually H2-rich, but CH4- <T^}*'|<

KHF could cause the high concentration of aqueous SiO2<T*

167 5. Serpentinized troctolites near the Kairei Hydrothermal Field

$#$#9#J

$#!#B$XY\#"@W!Y4 LB! ! B" B" & W@

/Y4`#"

''*#[FFj&/[Y4$XY\# #@B@_"Y\#[$$F

H2`^V[FFL&[FFE\^""[FFX#"/ et al., 2008). In H2!B"B"[Y4 is considered to be

!#@B#$2N

`:j 2 + 4 H2Y4 + H2O.

& & $ # $ 2 Y4 are #// # B! B" ` [ FFj L- $[ L! B! `LLj @@ B " B U

W "B !B BC/ $ $" $ Y4 in hydro- " \# `/[ Y |[ ''' ##& B$[ FFEj

B[ # #[ !! @ W B $ 2 conver-

Y4 during serpentinization of peridotite (e.g., Horita and Berndt, 1999). LB!"!&@"#$["B- /VB[$VV /@@ "! /[B `/ Ej |# "# $ B

B$$&$#$2Y4 (Horita and Berndt, '''j[$##B$LLB-

Y&[#/$Y4 relative to H2 XYV\#[B@B

$VB$2V$#$#!`[':X|[ f 2009). Assuming that O2 is mainly controlled by SiO2 activity during serpentinization

(Frost and Beard, 2007), relatively high-SiO2 activity of serpentinization of the troctolitic f &`@Vj##VB/ O2 conditions compared to ser- !C$B!@B!L&W!V higher S content than typical mantle peridotite due to its high incompatibility (Puchelt f et al., 1996). Both high O2/VB/#$# V!$$&BV! # [ #[ `C "j @" @`/jLW!@$B!C !@#[#/#B"!-

[!B!VBC/"$"$"2 reduction, so that "$"/["##/\#[

168 5.7. Acknowledgements

#!!B!+VB["$\# /"!#C$/["/ L!/#!!\#! "W!XB"\#VV`/j- /##Y[#@#B-

"&!#B"\#/Y2@#

Y4LB"\#$#@&#B/

XY//[@"2. These processes in the Kairei B"B"!#"B$B"\#

5.6. Conclusions

!C&V$"Y[&"$ XY[!V&B//$###"B$XB-

"\#Y/$Y2X\#@#!-

C$V&+$!/&2 f \##/!C[##VB/ O2 condition f compared to serpentinization of typical abyssal peridotite. The higher O2 condition, as /#$#VB#/#$#$&[## #$B&WB$LL

@@BL##!!$Y4 formation, resulting

###BY4UY2$XB"\#LB" \#!#@B$&$#@

&#XY[@"//B2. It is concluded that combination $/CY#@- sequent basalt-alteration at discharge zone under the KHF causes the distinct chemistry $B"\##!!##"@B"

$#Q#*+

}VB"#@+{$$#V$"#- !}#&&§"&FF!"< $U§&#&$&$##!![!@ [!B$V#@@#/§XF##- V""@B{##|/B"!V#B$ "#!L#B[B#!!@B\+$[ $"<B$_#[##[![L/B$

169 5. Serpentinized troctolites near the Kairei Hydrothermal Field

170 References

References

+[ ^_[ B$[ }_[ FF "! V \# $" #"[ B" B" " /N+ W!" #BEFF [FF@\"""+*[E +[<[<V[[|#[+<[_[[FF*^B"! BCVNB!! \"\!B\B:[FF[NFF'UFF\FF* |[}[#&[Y[\[[$[|[<#[}[Y#"![_[FF V/ # $ !C !/!B[ " "B[!!B$!$"<+ `^{/F'[ *Ej\!B{[{F[NFF'UFF\{F: |[}[X[[FF:L!/B$\/N/$"/" !"/{[NFFUFF:FF |[ <_[ +[ ^_[ B$[ }_[ [ '' # $ #/ !C$VFF FF@\/BE[E |&"[^X[[[[|["[^X['':/$W "!WN_V$"<+/+#~ Geophys. Res. 103, 21315-21333. #[{[^V[[##[§[|![[Y"[[FF\"B $/YYEV\##/$"#"[&@ B"[` ER[<+j"\'[E' #[{[^V[[X[[|[^[#&V[[|![[##[ §[[B$__L_#[FF:Y/B/@ B@$"#"[B"@ <+/#$__L_#_L+\[ 88, Fall Meet. Suppl., Abstract T51F-04. ^&[Y|[[Y[+[[|[}[|#[^[\[[Y//[[Y/[ \Y[ Y[\[ Y"[ <[$[ |[ #[ \[ [ |_[ XB[ ^[X&[_[X/[+[{#W[[<[[<B[[<[^[ #[Y[#[§{[@[L[[[![+[L"@B[}[ }"[Y[+[§[FFF+/#$#N #$^{/*/#/_ {*'[ ^V[[#[{[^V[_[$XB[[##[§[CV[_[ |![[V[<[\"[[{_#[B[ ''*Y/YYEB"\#$"@B "! ER+<+/"<+/`V/{_ #[#B''*jN"!<+_L+\*:[ Meet. Suppl., Abstract V51E-06. 171 5. Serpentinized troctolites near the Kairei Hydrothermal Field

^#V[[#[{[&[_Y[|V#[[V[[^V[[ ##[§[#[ ^[+!!#[ [FFL @ V \# ` ER[ <+jN \# $ #"[ & ! ! " <+/B"\#"\:E[*E: ##&[^[B$[}_[FFEYB@B"V\#NL role of chromium-bearing catalysts. Science 304, 1002-1005. [|[':@B$#[[W[V"! Petrol. 26, 31-63. [|[|[[FF*VB!CE:[ 1368. [ |[ |[ [ <\/[+[ $$[ _[ FF: L $" $ " / $" ^ Y F'^N XB #/ ! $ !C[E'[*':: \[<[^""[X{[FF\"B"\#$" X_"[ [/\" \!B\B*[FF:[NFF'UFF\FFF* \"[L[@[Y[§"&[L[&#[L[Y"[[L#[[@[ [X&[[L#/[[&"#[X[[§[[[FF" $ B V @& "& \# B" !#" /#C L! #[ / _{'[**' Y" [ [ [ \"[L[ @[ Y[§"/#[L[L#[ [ &#[ L[ }@[ Y[ §"&[ L[ XC[ <[ FF B" V communities from the Indian Ocean discovered. Zool. Sci. 18, 717-721. Y/[Y[''L"B"$B"B"V"!# !#+"[[*':FE Y[[|[<_['''+@/"$"!$ under hydrothermal conditions. Science 285, 1055-1057. Y#"![ _[ ~@/[ +[ <##W[ [ L"[ _[ '' \ YB" B"[ B[ "[ |/[ \/ Interactions. Geophysical Monograph series 91. American Geophysical Union, }/[^[E! [Y}[<[<[':\"@/B$!B"V Science 229, 717-725. [}[&[_Y[Y/[Y[''L'N+$!&/ for calculating the standard molal thermodynamic properties of minerals, gases, ## ![ $ FFF @ F FFF [ "!# Geosci. 18, 899-947. XB[^[X[+[|&"[^X[\[\{[|#[[^+[{B[ <^[[_[&[<[[XX[{@[\L[VCC/[[

172 References

+LF!@B[FF+$$WB"V[< +/F #E[EE' XB[^[X[+[#\[\{[§/[^[&[L<[|#[[ ^+[YB[<[&[<[[_[&#&[\[&#@[<[ |B[+[{[|[{#/[X[\&[^[|#&"[X[|B[+[ |C[ }[[ X[ _[ <[ ^#[+[ |[ <[ {B[ <^[ |[ +[ #""[ _[ BV[ [FF+ !Y _B"NL{BYB"[F*[E:EE X[ [ |[ }[ FF' ! ! [F[*'[NFF'U!/BU/F* X#"/[Y&"#[X[<[L[L&[L[&[X[@#B[L[/#[ L[[["[<[[L[L&[X[FF:\/@&/#$ X_"B"V[//N /"B@V\#\\#[:[' 251. W[ +[ _&[ +[ "[ Y^[ ': \" "/ V$#$"/"B!Q"#R /@<[**[E* <"[L<[&[_{['':#&#N L"B""$B"\!BF[E* 575. <"[L<[|[}[FF:L"B"YB/\ ^#/!C$"[&\"""+[N FFU/FF:FF <[|[^VB[}[\"[[{&C[X[$[[}[<[ <[[§/[^[|&[_L[#&[Y[&"#[X[FF: V$/"!#$$WV/$!#U"NL @#/B"[[#<+/_ {*[' <[L[+[[\[^Y[FF_V#$+!BW Y" [ !N ### $ ""/ \# Petrol. 44, 1237–1246. Morishita, T., Arai, S., Tamura, A., 2003b. Petrology of an apatite-rich layer in the !/!![}+!"!$V#$ ""//{'[*ME' [XY[/&[[L&[X[FFYB/V#@#$#! "@B"`{<_jNBWB#L Microbiol. 13, 405-410. #[Y[[Y<[|[~[<B[[''#["/B[#$# [#$#!"!$"[#"[&$"{/

173 5. Serpentinized troctolites near the Kairei Hydrothermal Field

147. Proc. ODP, Sci. Results 147, 91–101. "[X[X&B[+[\@@/[^[V[{<[$[[FF* \"B $ B" \# $" #"[ {/V B" [[ <+ /N L"! ! investigation. Gem. Geol., 242, 1-21. [[~V[<§[<"[L[FF_W!"V/$/ @"!##B"\"""+ 70, 446-460. B$[ }_[ ^/[ X[ [ |[ FF _W!" @ $ "@ !/!\# #@ V "!# !#N +!!"B$B"\#"/[} &[^!_V"\"BN+@# ^V+[!!:*:[L\"B[ ![[<[X[+[L[|[[C[+[@[^[W[ [^C\[<[#[[\#[[Y&[[YB"[[ Y[[##[L[X[<[{[[{#BB&[|[<#/[^[ <[ [ "&[ }[ #[ [ /[ [ ':F _ [ N Y Springs and Geophysical Experiments. Science 207, 1421-1433. L&[ _[ ': / $ @ "/""! $" ! "/ W!"V$"|#!F[ 17-40. L&[ X[ \"[ L[ L#/[ [ &B"[ [ YB"[ Y[ [ XY[ Y&[X[FFE\""@/V$B/ based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem `YB!{<_j@V!B"[_W"!:[ 269-282. L&[X[&"#[X[#C#&[X[/&[[[XY[X#"/[Y[FF "[YB""YB/YB!{<_`Yj&/N &B/B"@B"+!B" systems. Paleontol. Res. 10, 269-282. L&C[ _[+@[ [ B[ <[ <#[ }[FF* YB@C $ # /@@"Y*|$"<+/ N"! $ " \ #!! " ^[ # F'[ NF'*U!!F'FFFF* ^V[ {[ FFF L _/B $ ^! YB" University Press, Princeton, 352p. ^V[{[Y#"![_[[^[V#/[<[[[\$$[ X[Y"[[{B[<^[B@[+{[&[L<[^""[ X{[ |[ +[ \[ <[ \C[ ^[ \[ ^[ Y[ [ Y"[ L{[ Y#[{+[[[<X[~[<[[[_[[{[

174 References

L#![<[}[§[§#/[[[&[[FF|//!B and Ecological setting of Indian Ocean hydrothermal vents. Science 294, 818- 823. }C[{[&[_{[FFF^/#/#"[$"@#@" B"B"@B"!/#V\#"!\!B Res. 105, 8319-8340. }&[ [ }&[ _}[ '** ! W# !C Mineral. 13, 227-243. }&[}}[^{/[_[XB[^[|[+[B[[FFEL#@\ Biosphere at Mid-Ocean Ridges. Geophysical Monograph series 144. American \!B[}/[^[EF:! }B[L[''_U[+$!&/$/""/$## B"N &/ VV /# ` *Fj[ { {V"{@[{V"[$ }B[L[ FFE #[ $L"B" ^ $ \" </ $<}^#B"N^!"$_/B `j|+"!B[{{

175 Danksagung

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