Petrology of Serpentinites and Rodingites in the Oceanic - E-LIB

Transcrição

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 konzipiert, dass sie unabhängig voneinander publiziert werden können bzw. publiziert sind. Durch
diesen Umstand ist es zu erklären, dass jedes Kapitel nochmals eine eigene Einleitung, Diskussion und ein Literaturverzeichnis enthält. Auch beim Schreibstil, dem Umfang, der Verwendung 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
Abstract
Prologue
Outline
1
3
5
8
1. Introduction
1.1. Serpentinized peridotites at mid-ocean ridges
1.2. Serpentinized peridotites at active oceanic margins
1.3. Hydrothermal systems and serpentinized peridotites
1.4. Mineralogical and petrological aspects of serpentinization
1.4.1. Serpentinite textures
11
11
14
14
16
16
1.4.2. Serpentinization - an isovolumetrical process?
1.4.3. Some crystallographic basics concerning serpentine
1.4.4. A note on the mineral chemistry of serpentine and its value as
a geothermometer
1.4.5. The MgO–SiO2–H2O (MSH) system
1.4.6. Redox conditions during serpentinization
1.5. Rodingitization
17
18
References
25
Abstract
2.1. Introduction
2.2. Geological setting
19
19
20
24
37
37
40
2.3. Analytical methods
2.3.1. Microscopy and electron microprobe analysis
2.3.2. Thermodynamic calculations
2.4. Results
2.4.1. Petrography
2.4.2. Mineral chemistry
2.4.3. Phase diagrams
2.5. Discussion
!"!#"$ %#& 2.5.2. Redox conditions during serpentinization
2.5.3. Redox conditions during steatitization
2.5.4. Implications for a potential H2S,aq buffer in
serpentinite-hosted hydrothermal systems
41
41
42
46
46
51
54
59
'
60
61
62
2.5.5. Sulfur metasomatism
2.5.6. Possible existence of a free H2-rich vapor phase
#
*+&/"
References
3. Iron Partitioning and Hydrogen Generation During Serpentinization of Abyssal
$";<+/
63
66
*
:
69
*:
Abstract
3.1. Introduction
3.2.2. Mößbauer spectroscopy and magnetization measurements
78
78
81
81
82
3.2.3. Geochemical modeling
3.3. Results
3.3.1. Petrography
3.3.2. Mineral compositions
<=>@#!!B@#&"/C
3.3.4. Geochemical reaction path modeling
3.4. Discussion
3.4.1. Serpentinization at Hole 1274A and geochemical reaction path
models
82
85
85
87
'
92
103
FE
104
E
//!"@/#/!C
3.4.3. Fe+2+3 exchange equilibria in serpentinites
3.4.4. Geochemical reaction path modeling and serpentinization
experiments
3.4.5. The formation of brucite and serpentine in mesh-rims
#
+&/"
References
Appendix
EL!/B$%/N/$"/"!
modeling
Abstract
4.1. Introduction
4.2. Method
4.3. Results
4.3.1. Reaction path models
103
106
108
113
119
120
120
120
122
126
126
137
4.4. Discussion
4.4.1. Modeling of rodingitization
4.4.2. The critical role of aqueous silica
4.4.3. Mass transfer by diffusion or advection
EEE"QU#"Q
EE+!V$/"!#%#
circulation?
E
#
E+&/"
References
138
139
141
142
EE
145
E
E*
148
!CW!XYB"[
/N//$XB"%#
supporting a unique microbial ecosystem
Abstract
5.1. Introduction
\/@&/# 155
155
155
:
5.4. Petrography
5.5. Discussion
5.5.1. Origin of the high H2XY%# L]@V" +B@"$XYV%#"B 158
159
161
*
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References
^&/#/ _!!$!"!&`
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:
'
'
171
*
Zusammenfassung
Die Serpentinisierung von Peridotiten erzeugt große Mengen von Wasserstoff.
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von Sauerstoff in Magnetit und Serpentin die Freisetzung von Wasserstoff. Wir haben
"B"^V]]
]]C#""//#|ziehung in fO2,g–fS2,g und aH2,aq–aH2S,aq Diagrammen für Temperaturen von 150 bis
EFF;
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Beobachtungen offenbaren eine systematische Abfolge von Mineralvergesellschaftungen
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Pentlandit + Magnetit bildet sich in partiell serpentinisierten Gesteinen. Die Paragenese
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VB#[#$V/#&&!Cw\C#&C#$ | |/#/ #Q $[ #/$w[CC#"+/$/"\$^
_&#/VfO2,g und fS2[/w!#/$/Y2S,aq Iso!#"!/"##/[$$#/CwYB"%##"QB"/!#$$
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und harzburgitische Gesteinszusammensetzungen untersucht und die Modellergebnisse
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enthalten, sind die Fe+3U}!"/#=`F@
F:j_U<#_/\#+3-Serpentin
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C#"=/@L"!##F]F;
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#|#C&WXw##$^+##/</@nisse und deren Vergleich mit MgO–FeO–Fe2O3–SiO2–H2O Phasenbeziehungen in Maw#$|#/"!#!#|#C|#/
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des KHF vorhanden sind, dar. Petrographische Untersuchungen offenbaren, dass Olivin
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2
Abstract
Serpentinization of peridotite generates large amounts of dihydrogen (H2,aq), in@B!$]B#$#$#/B#Q[/#
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relations in fO2,g–fS2,g and aH2,aq–aH2[/"$"!#@F
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trace changes in oxygen and sulfur fugacities during progressive serpentinization and
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pentlandite assemblages forming in the early stages of serpentinization to millerite +
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of serpentinization. In contrast, steatitization indicates increased silica activities and that
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form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under
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H2S,aq, indicating that H2V%#@#$$
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solution model that includes greenalite and Fe+3!#V//
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the dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen
requires an external source of silica. At these temperatures, hydrogen fugacities are too
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;
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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
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the assemblage serpentine + brucite. Our study indicates that unprecedented details about
the reaction sequences during serpentinization may be obtained from merging careful
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modeling.
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serpentinization into a gabbroic body. Phase assemblages typical of rodingite (grossular
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replaces clinopyroxene. Our model results support the hypothesis that rodingites form
#/!CB%#@B!Cactions are present. Our calculations further indicate that the formation of mineral as"@/ ! "/ V@B / $" %
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activities.
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calculations predict that high H2$B"%#@
attributed to serpentinization of the troctolites and subsequent hydrothermal reactions
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4
Prologue
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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[""#$"[[
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reducing conditions, caused by the generation of copious amounts of hydrogen during
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2 to methane, and both gases are extremely
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2003)).
5
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communities arose in serpentinization because the high H2
Y4 concentrations can
support microbial communities in surface and subsurface environments of such ultramafB"B"`/[+&['':+[FF*XB[
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chemical energy) that thrive independent of photosynthesis and have served as analogue
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Serpentinization also affects global geochemical and geodynamic processes, as it
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Serpentinization has a large impact on petrophysical characteristics of the oceanic
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authors propose that the presence of serpentinite can reduce the integrated strength of
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and suggested that serpentinization is rather a sequence of mineral reactions. The actual
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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
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general.
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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 dur6
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
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#/pentinization (e.g., Honnorez and Kirst, 1975). An equally common – but apparently less
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particular drives rodingitization.
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7
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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
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retrospective character, focusing on the research about abyssal serpentinites and leaving
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during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic
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Wolfgang Bach. He introduced me to the construction and interpretation of activity–acVB$#/B]$#/B/"$/"!#!#[
!"!$!"@[V#"!"&/
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and did all the petrography in our microscopy laboratory at the University of Bremen
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!!! $ "!# #! EFF ;
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Mader and Dr. Peter Appel. My supervisor, Prof. Dr. Wolfgang Bach introduced me into
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the quality of the manuscript.
Chapter two is already published as:
8
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relations in the system MgO–SiO2–FeO–Fe2O3–H2O and uses reaction path models to
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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.
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(Department of Geology and Geophysics, University of Minnesota, USA).
Chapter three will be submitted to Geochimica et Cosmochimica Acta:
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submitted) Iron partitioning and hydrogen generation during serpentinization of
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"["Q&%#@#$$B@B!Ctions.
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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$%/N/$"
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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
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<B@#!!#$]]]!tions and editing the manuscript.
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and Kumagai, H. (in press) Serpentinized troctolites near the Kairei Hydrother"_B{
10
1. Introduction
! V " $" \& W! !#[ /
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1845).
All the early and the majority of the more recent serpentinite research has been
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independent from their provenance all these serpentinite recoveries contributed to the
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the seismic properties of partially serpentinized peridotite did not match those of layer 3
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the oceanic crust. Beneath a sediment cover (layer 1), the oceanic crust is composed of
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LB[#&<Y[Q"B"!$[
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@#B[!/#$"#B/"`{BEj/!tites, residues of partial melting. These features have been a cornerstone of plate-tectonic
11
1. Introduction
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$#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+![
$[V!//["#$##
@"!"
@#!//B
"#$"/[@#!@&/#B
"!`/[
[''<V[FFj<VB#$!
@ $# /" `^&[ ':'jL & ! 12
development of deep rooted faults (e.g., Schroeder et al., 2007). Serpentinized peridotites
$"$#!B!#/WVB
VWVB%
} $"&[ ! /V @B !B @ "/"[ B"[ !L W! @# $
!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$#!#!!!B!C
!!/![""!!B
#&B
':["!"/$Q"#/^{/F'[*F$<+"VBE&"#$
X#C`^&['::j+!!W"B"$!CC@#/#V$"'"!''#$X
#~'"$!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 } Q 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$#"Q{/V[@{
BB"QV$QVB
'F!$#"!V#$#/
#"Q&[/""/V#$!#/
!C@/"!V/`+B$[
FF+ B$[ FFE+" [ FF: | [ FF # [
FF^V[''*^#V[FF\[FF\@[
FFFXB[FFXB[FF<
"|[FF'&#&
[FF&#&[FF:#W[FFE"[FF*&
[FFEB$[FF*j
L{/VB"QV'']''E@#FFF"
!$"#$<+#$;F#~
E;E `|#V [ ''Ej L W! C@#/[ !BW[ C[
[/@@VB//$!|&"&14
1.3. Hydrothermal Systems and serpentinized peridotites
V/"!#`F;
[
#[FF^#V[FF"
[FF*jB"%#[V["$V/&!
"$"!["&/`|/V[''*j
diameters up to 10 m.
^#/{_#''*@B"QV
;E#$+"/"/$@/
! $ @# FF " `## [ ''*jL @ / $#@#
/W!!C#"Q&#V$#B"Q`|/[''*jL#"#@&"&"W"#"
$%#"!#$;
`^#V[FFj
+@%#$#"Q#@"@B!Q!
@#"B$"/V%#
"!"QB"
B"[ V %# $# VB / $ V B/ "$##B/#Q`
#[
FF^#V[FFjL!Y`;
j{/V:
@YB"QYV[B"$%#"
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"!$@"Q#"Q/!!
&B[W!@/"!#%#"B
//{
BB"Q[V''*"
&"B$"!/W$<++"$[
/!$*F'FF"`XB[FFj[!C!!"$"/V%#`&#&[FFj{
BC@B###B`#!F"j@##["/VB`F]';
j[@`!Y']j"!
%#{&/"!#B"B"[$V
B/"V"!"QB"!/!C$#"Q&@"`&#&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
1.4.1. Serpentinite textures
Textural characteristics of serpentinites are fundamental for the interpretation of
!#@`#@jL@W!!@B}&
}&`'**j"!VW@&@BYB`''j
!C #"Q & W@ @ VB $ ## W#
$#!L"!"!&
are only slightly serpentinized, completely transformed massive forms of serpentinite,
B"BC![@$""$!UB"["B!`"#
volcanism).
LW#$!@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$""#"Q&L$!#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#"@Q" L""
!$"!"!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
!`!Q@/$j¡!`!Q@/
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 am16
!@@}&}&`'**jV$#!C$
chlorite also produces bastites indistinguishable from those after chain silicates. Therefore, 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/BLQVB!$VW#N"VV`"/#"!BQj[/V[Q@
V`Q@!!#Vj[!Q@V`Q@"!Vj[V#//BV`"!BQVj
1.4.2. Serpentinization - an isovolumetrical process?
+ # "B ! W# !CV#"!"$"!B
V#"LV#"/#/`¢j$V!
@#`//jN
2Mg2SiO4 + 3H2£</3Si2O5(OH)4 + Mg(OH)2
forsterite
chrysotile
brucite
108.5 cm3
24.63 cm3 2x43.79 cm3
¢¤
¢¤
L$!C$!BWN
3MgSiO3 + 2H2£</3Si2O5(OH)4 + SiO2(aq)
enstatite
chrysotile
108.5 cm3
3x31.28 cm3
17
1. Introduction
!C$C@#/N"$V!BW`:
VVNEV!BWj#V#"$E
Mg2SiO4 + MgSiO3 + 2H2£</3Si2O5(OH)4
forsterite enstatite
chrysotile
F:" 43.79 cm3 31.28 cm3 ¢¤E
V#"!C"!"V$</#$%#&
N
6Mg2SiO4 + 2MgSiO3 + H2O + 10H+£</3Si2O5(OH)4 + 5Mg2+ + 3SiO2(aq)
forsterite
enstatite
chrysotile
3x108.5 cm3
¢¤F
6x43.79 cm3 2x31.28 cm3
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
##!!@BQV/Q"#$""V
#/!C{#`/[YB[''j#"
!C&!@B@BB$B#![
/V#"/$%@B@#$#
shear zones in serpentinites. The expected volume increase during serpentinization has
"`/[{['*Ej$!C#!"@B
$/""{`'*Ej#$"$#/B"#"#@Q/##!$!
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+""Q/#$BV
18
#/@#V#$B`YB[
''j{C"!$!B[B"!$
B[$"B/NB!BV[#/QB
1.4.4. A note on the mineral chemistry of serpentine and its value as a
geothermometer
_V`'**j!VQ!/"$!!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"#/!"##$QVC$
!!B"!@BQ"!$/B$$$
!#</""@`/["[FFj+#/"B$!
approximates Mg3Si2O5(OH)4, common substitutions include aluminum and ferric iron
for silicon in tetrahedral coordination and aluminum and ferrous or ferric iron for mag#"`YB^B[''}&['*'j
&["#"["/#@#$"/#"[@##@#
are relatively minor.
1.4.5. The MgO–SiO2–H2O (MSH) system
|"!#
@/![!"B"!
principally composed of MgO and SiO2"#@#$$</`##ally < 15 mol. %). Most secondary phases of a serpentinite are Mg-rich, silica-poor and
+#/!"[V@
them in the MSH system. The major primary Mg-phases in peridotite are forsterite and
^!/$UB$!
$"$$$B</!"@/`j
serpentine, (2) serpentine + brucite, or (3) serpentine + talc (see Fig. 1). The hydration of
$$"!@##""#[B
$B!B&N"$$
V@#L&`j#
19
1. Introduction
of SiO2"V$</$"[!/$!
@#@#$$VBV[!/$!tine and talc buffer the silica activity to higher levels (e.g., Frost and Beard, 2007).
L$!<YB"/QB/
/W"![VB$@/V
"&!@##"!![@#"/$"ditional phase to satisfy the phase rule. The actual content (and oxidation state) of iron in
!@#`!BW!j!/B
"!#VB$#/!CL""trol the partitioning of Fe during serpentinization into serpentine, brucite and magnetite
are poorly constrained. Being able to better constrain Fe-partitioning during serpentinization is a paramount importance to reliably determine the amount of dihydrogen produced
during serpentinization.
In chapter three, the results of geochemical modeling, phase petrological, petrographic, mineral chemical and Mößbauer spectroscopic (for the determination of iron va!jB$!C!$"^{/F'Y*E+[
@!V#/@VB
and Fe-distribution during serpentinization and their implications for dihydrogen generation.
!C&/W"B#/`_&[
'*[':<
"|[FF'&[''![FFEjLV$VB/"#%#$"#"Q
B"QVB<+`
#[FF^#V[
FF[&#&[FF"[FF*jB"
1.3. The conjunction of serpentinites and extremely reducing conditions is even more evident from active continental serpentinization settings, that emanate hydrogen- and meth/`+@['::|['*:
VB['*
VB[
':*#[':'§#&V[':j<&#W
$$#V!BW@B$"/!@
$/$B/#/!C[##BW!N
3Fe2SiO4 + 2H2£3O4 + 3SiO2(aq) + 2H2(aq)
fayalite
magnetite
regarding hydration of olivine and
20
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 + H2£3O4 + 3SiO2(aq) + H2(aq),
ferrrosilite
magnetite
//B$!BW[WB/B$$"$"/W$""!$@B21
1. Introduction
/LVB$B//#%#WB/$#/B
of the system via the Knallgas equilibrium H2`j ¤ F2(g) + H2(aq). Partially ser!C ! #/ B ""B &] B
+#`3j"""B!B[@##`
j[
`j!#V`
"@['~#['':jV
been reported. Frost and Beard (2007) noted that although iron alloys are reported from
!["/"/@#$$N
Fe3O4¤;2(aq)
magnetite iron
"#WB/$#/B!+V""B"&
#!/QV#"$!["@##"
redox buffer – they merely react to redox conditions superimposed by other mineral –
%##@|`FF*j$#WB/VN
4Fe3Si2O5(OH)4£;3O4 + 4SiO2(aq) + 8H2O
in serpentine
iron
magnetite
#!#/Q"#$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`j£3O4 + 6H2O
in brucite
magnetite
2Fe(OH)2£;Y2O + O2(aq).
in brucite
iron
LVV[@#$"$"/V
iron.
+#/ " B #$#! #Q !!B @#$$ W
equilibria during serpentinization, they can be used as a redox monitor. Frost (1985) noted
!$]B!/!WB/$#/B
$#QV/#@<`$B"/#Cj@#$$[/!#@WB/$#/WV/#
@<""["@"B"$#[
! `|CV& [ FF Y[ FFj V@[ 22
fugacity–fugacity diagrams for O2 and S2 can be recalculated to better constrain redox
#/!C!QV$!VB]VB$#/B]$#/B/"$]]]]
]]!
#!@"]]
]]!
!$#["//W$"B
/!CQB#@#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"!Q #/QB@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@#/#//!CFF];
/
VB&#//"#$"/
thus the greatest amounts of dihydrogen.
B[B$&`FF*j#!CW!"
FF;
F<!#":F""&/`"<jB/
/[#/V#B"/!!"!#
Surprised by this result, they used Mößbauer spectroscopy to analyze the valence of iron
!"$$V@V#B[B
formation of magnetite is important for the generation of extremely reducing conditions
during serpentinization, Fe+3 incorporation into serpentine appears to be an important fac$/$/"#$B/
In the third chapter reaction path models for serpentinization of peridotite are
presented that include the Fe+3-component in the serpentine solid solution. The modeling
#@"!#$""<=>@#!!
B$"!V$"^{/F'[Y*E+$##ing of Fe+3 partitioning into serpentine and its implications for dihydrogen generation.
23
1. Introduction
A:freshrock(plg+cpx±ol)
B:rodingite(grsdi±chl)
SiO2
An
Zoi,Prh
SiO2
An
Zoi,Prh
Tlc
Tlc
Grs
Trm
Parg
Di
Di
Grs
Opx
Trm
Parg
Opx
Serp
Chl
Ol
Serp
Chl
Ol
0.75*CaO
0.75*MgO
An
0.75*CaO
Al2O3
Prh
0.75*MgO
An
Al2O3
Prh
Zoi
Grs
Zoi
Grs
Chl
Chl
Parg
CaO
Di
Trm
Parg
Serp,
Fo, En
MgO
CaO
Di
Trm
Serp,
Fo, En
MgO
Fig. 2. CaO-MgO-SiO2 and CaO-MgO-Al2O3 chemography plots indicating that advanced rodingitization
is associated with a loss of SiO2 and a gain of CaO.
1.5. Rodingitization
/[""B$#/B!C![V#dergone intensive metasomatism as a consequence of the serpentinization of surrounding
! `
"[ ' [ '* YB [ '' [ ':'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!Q
@B!C%#[V@B/#"[`/j[#//"/B`
"['*j B ""@/
/ "! / $ ]+ [ #/ C[ ![ /#UB/#[V#V!##B!+
"!/#[#%#!"!#V
/C[/
VBVBL#"!VBdressed 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[ Y
YV[ Y[ 'F[ _/
[^V#@[§&
+/[[
[\|[<`''j</WB/!
$#$!V$"U@+@BN}"[|`j/$^/
/"Q#[
/[^//"[E
+@[[[\|X#[<`'*:j##$
"#`{j*[EEE
+W[Y![\^`''jL!!N+/#
$!//N[{<[<#[||/[`j
!+L<+/#{N\/B
Special Publication, 3-38.
+[^_B$[}_`FFj
"!V%#$"#"QB"B""/N+W!"#B
EFF;
[FF@\"
""+*[E
+[ ^ _ B$[} _ `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"$#/<+/[;F`^
{/F'jN+#$#WB/!#B\"B[\!B[\B":[F:FF[NF:FFFF'UFFF*\
FF*
+"[<[_#[+[|="[[C&[[|[}[\@=@/[^[[ <[ |&[ |[ {&C[ X Y#$$[ `FF:j #" !
`¨44U40
j$/B"!B[{/VQ`<+/[E;Ej\"
""+*[EF*E
+w#[
`'*jL#$B@##+$N
+V_"\/B*['
25
1. Introduction
+#"[{#@[Y`'*jL<+/E;©!C#"Q#
#$_¤#
dien des Sciences de la Terre 8, 631-663.
|[}[|[[^&[Y||&[_L`FFj^VB$
VB"!/#!/#/
F;_\"B[\!B[\B"[FF'UFF\
FFF*'
|[}[\[
[YVB[[#&[Y[<`FFEj@!]/$"^{/F'[<+ª\"B[\!B[\B"[F'[NFF'UFFE\
FFF*EE
|[}X[`FF:jL!/B$%/N/$"/"!"/{[NFFUFF:FF
|[ }[ #&[ Y[ \[ [ $[ |[ <#[ } Y#"![
_ `FFj V/ # $ !C N !trography, mineral chemistry, and petrophyscis of serpentinites from MAR
ª `^ {/ 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$!ent-day serpentinization. Science 156, 830-832.
|[[[L[`'*:jB!C
["§#/V\"
""+E[EE
145.
|/[+[
[<+[V[<[@[+[##[§[[
Y[[{B[`''*jL@!!#!&&`<+/[+<+/"jN!"B
!${_#_L+"\!B
78, 832-833.
|#V[ | [ XV[ + \[ <&V[ [ &V[ \ +[ XV[ \ {C[§^`''Ej<V#Q!VE;E[<+/|^\_[F
|CV&[\+[^@#&[+XV&[L+`FFj{"!#
heat capacity of pentlandite. American Mineralogist 86, 1312-1313.
|&[
<`''j\"</§&NW$VB
Press.
|#[^[Y@[[Y&[
[<`''j<"!"$!&$"\$L$"`_Q;j
26
References
#$\!B'[F[F*'FF[F''
|"[Y[}/[^[<{[
[L!![^[
$[[[L[[
{[\[X[[L[X"[<[<Q[X[[Y}[<`''j
\/B$L/$N+#@!#@#C!_Ltions American Geophysical Union 77, 325.
|/V[§+[|&V[[&BV[[\#V[_\/V[+
< `''*j+ B! $ " "$"/ B"N |& "& $
B"QE;E#[<+/\/B$
Deposits 39, 58-78.
|[\[#[\[[<\#[`':'j#/@B!
@$"/#E[
|[ _[ _"[ [ [ \[ YC[ B[ Y `'*Ej "Q
carbonate breccias from the equatorial Mid-Atlantic Ridge. Marine Geology 16,
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[[|&"[^X["[^X[<+[_[[|[<[[+V/[_[[+_[`''*j
#/!#$$"
/$"<+/#:['
[ < `''j _"!" $ " & % " /
#$\!B':[EE*
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<+/WVB;*;_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#[#"Q
W!#[ #// $#/ ! <+ / `«E«j
Geology 23, 49-52.
"@[+[<{[
[L[{[\`'jV"<#&W#
#$_¤#
dien des Sciences de la Terre 2, 188-215.
![Y[[X[|B[<[<[|+[
#$[+[X@[{{
{VB[^`FFj+B/@#@#$"@""#B"@B"/#E[
#[{[^V[[##[§[|![Y"[`FFj\chemistry of high H2 Y4 V %# #/ $" #"Q & @B"Q`ªE[<+j
"\/B'
27
1. Introduction
#[{[##[§[|#/#[Y[^V[[_#@#[[|![[
^!/B[+[+!!#[[+`'':j
Y4 plumes generated
@B!C$#"Q&$;F$#
C<+/\"
""+[
2333.
"&[ + "[ ^ `':'j _"/ "B" !!
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[`'*jL@#$!##$
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[@#B[<Y`'*j##"!$
#V\!B!B[*:
"[\`'j![/[#!B!
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~\/#VB[|#*[F
"[\`'*j{"!#C!&$
$[
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$!`'*j
$N!\"*[E
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VB[<`'*jYB/!C$/
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^C[<[|[
|#[^`FFEjLB"&$"
#
$#C+_#!#$
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^&[[W[[#C[[&B[+[X/[{[<B[{B[}
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29
1. Introduction
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/[+
$$[_`FF:jL$"$"
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&#&[\`FFjF[FFFB$B"VB{
BVQF[E'E':
\[\{[[+{#B[
`''j/@!B"!C$_"Y^![:'N<V[
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L"$"#N#${///$^//"Q#[E
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&V[\+
[<`FFFj
!!#Q+$
{/VYB"`<+/[E;Ej\/B$
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\"[
[X&""[\#&[<^`''jL@B"!#"[;[<+\!B{['*'':
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W!#/$B"V_B{
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Y/[}`:Ej#$+@#/}
Y![\^[|"[X#[`'::j+Q["[@!
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X[+{[<`''*jL/$!W!#
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^//"Q#
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^ {/ F' " ! / <+ / $"
E;;^_#F[EF
X"[|[X&[_[<[^[+@[[|[}[
[{[
B[
[
"@[{<[
[<[
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<[\[
[\[[\[<<[\"[^}[\$Q[^}[YVB[[$[|[#[\[$[[<#[}[#&[Y[[
<[[L[B[<L&C[_`FFE@j{/F'#""B/$^//"!/"!
/<+/$"E//V/{/F'
$#$/V^_#[|C[
\/[|"#:*[<B#BFF/'NLW
+¬<VB^//"
/L©
31
1. Introduction
X"[ |[ X&[ _[ <[ ^ B[ `FF*j {/ F' #""BN
!F&"&#V@#BB@<+
/[E;];NX"[|[X&[_<[^`j
^[#
/[L©N^//"[
XB[^[X[+[|&"[^X[\[\{[|#Q[^+[{B[<^[[_[&[<[[XX[{@[\L[VCC/[
B[+`FFj+$$WB"VQ<+
/F;#E[*:
XB[^[X[+[#\[\{[§/[^[&[L<[|#Q[
^+[YB[<[&[<[[_[&#&[\[&#@[<[
|B[+[{[|[{#/[X[\&[^[|#&"[X[|B[+[
|C[}[[X[_[<{[^#[+[|[<[{B[
< ^[ |[ +[ #""[ _ BV[ `FFj+ !
B"NL{
BYB"F*[E:EE
X&[^`FFj!#@#':[EEE
X[|[}`FF'j
!!#$/BF[*'
{/@[§[|#[^[
[<[X[+<V[
`'':j"Q
"Q!#&#W!/<+/`F«F«j
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<|$"#/;_E;:_
butions to Mineralogy and Petrology 110, 253-268.
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!
@#</B/B:[E:
{[
|`'*Ej!$&\!B#
Astrological Society 39, 465-509.
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<
"[L<`'''j</!#$"/B$
primary biomass production by autotrophic organisms in hydrothermal systems
_#!#$\!BFE[F*'F*E
<
"[L<`FF*j\"
#$<@_/B$
"#!B "QY ^! YB" B"
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33
1. Introduction
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YB[^[[_}&[`''jL/$/$"
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B$[}_[[##&[^#[`FF*jWV#"$#/!CW!"#BFF;
[FF
@"!$#"QB"B""
/\"
""+*[:*::
[`'E'j&$<+/#$\/B*[:''
![Y[<@"[+[&[L[
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and oxygen isotope geochemistry. Ophiolites and crustal genesis in the Philip!+""[N_V[FF*
L&[ X[ \"[ L[ L#/[ [ &B"[ [ YB"[ Y[ [ X Y 35
1. Introduction
Y&[ X `FFEj \" "@/ V $ Bgen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem
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}C[{&[_{`FFFj^/#/#"Q$"@#@"B"B"@B"!/#V%#"!
#$\!BF[:':EF
}&[[+\`'*'j_"!@WB"@"#
$!W#
<*[*::F
}&[}&[_}`'**j!W#!C
</[E'E::
§#&V[<["&B[<[^BB&[|+^[+`':jYB/
"/B$$!B!$"&XB&
<#^&B+&"&#&[EFE
36
Interactions
Abstract
!C$@B!&!#W"Bducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron
in primary phases to ferric iron in secondary minerals by H2O. We have compiled and
V#"B"$
!"!#!
in fO2[/fS2,g and aH2[Y2[/"$"!#@FEFF;
F <} # "! $ ! changes in oxygen and sulfur fugacities during progressive serpentinization and steatitiza$!$"<+/;F®#~`
^//"{/F'j/!@V#//B"/$"
#"/!C"/!"@/
$"/B/$!C"!B!BB""
"@/C&+#@V@#@/!B!C&+!!B[@#$$/$VV#@B!$
@#$$"$/"#$B/[$" $ #+ !" #$#C $ ![ #Q
"V $" & #/ / $ !C [ CV/#$#$#/B#Q[#
!BB" !BV #[ $" #/ !B $ peridotite is exhausted and H2V![#Q
#$#C@#!!#$#$&LV#
of f[/f[/B"$!$Y2S,aq, indicating that H2S in
V%#@#$$[Y2V%#@#$$@B
!["B"V#$Y2V%##$
!/V&L#$!#"/cates H2[V/%#@B"$L!$B//!#BC/!B$##
facilitate the abiotic formation of organic compounds.
2.1. Introduction
< ! ""B W! % $ # spreading mid-ocean ridges by detachment faulting that initiated close to the spreading
W`/
[''*L#&['':j/""!B
$&#$`!Cj!
/!C/B%#/B$!`_[''*j[/"@#/$`L"!¬<'*F¬^&[''j["@![[@V%`+¬&['':|['':XB[FFj+"&@$#
$!C/B#/#$/%#[
@B#$V"B!`&[''
"@
37
2. FeNiCoOS phase relations
['{[':+@¬[':'j##/$"#"Q
"$[ $ / " ![ W@ W!B
high concentrations of dissolved dihydrogen (H2[j`LB['|['*
¬ /[ ':j ! % B" B" high H2[$F""U&/`
#[FF^#V[FF
XB[FF&#&[FFjL/Y2,aq concentrations are due
to the oxidation of Fe+2&@B+3 in magnetite that forms along
!@##/!CB$`FF*jV!
experimental data suggesting that incorporation of Fe+3 in serpentine may also generate
considerable amounts of hydrogen.
L"@$$$%#&$#$&"![&["!#@W"@B!!
"`/}C¬&[FFF¬[FFE<
"¬|[FF:j[
@# &/ $ "%# #@ /V %# "!
#_W!"#`/B$¬^/[''B$
[ FFE[ FF*j !Q "%# @#$$ Y2,aq and also
H2[B"#V/%@B"[$[
!B!B"/`<j@#$$""BY2,aq and H2S,aq in the
/%#`B$¬^/[''jLY2,aq and H2S,aq
!B"&B$`FFEj#//
@!B"/@#$$Y2,aq and H2[%##/$"
submarine peridotite-hosted hydrothermal systems. In other studies it has been suggested,
V[!#B@V!!ence of H2,aq concentrations of the order of hundreds of millimoles in serpentinization
%#`![FFE|[FF¬|[FF*jL/!
concentrations of hydrogen are corroborated by similarly high H2,aq concentrations in
%#$"B"W!"`&B¬B$[':|[''
Y¬|['''<
"¬[FF+¬B$[FFB$
et al., 2007).
L#$"%##@#W"/!tions and associated H2,aq and H2S,aq activities in hydrothermal solutions is currently
"@B&$"B"$"B$!@#
!`/![#[/V&[!BB"[V[VC$"#$!#"!C!/VL@j_&`'*j`':j&#!B"
#V"B¯/#`3j[""B
@V !° W"B WB/ $#/ `/ / $"/#@<FF;
j/#$#$#/¯/F$
"/#@<`[':j°+¬&`'':j/C#
a common phase forming in the early stages of serpentinization of abyssal peridotites.
38
2.1. Introduction
Table 1: Idealized formulae of opaque
minerals in serpentinized peridotites
Mineral
awaruite
tetrataenite
pentlandite
godlevskite
heazlewoodite
millerite
polydymite
violarite
magnetite
pyrrhotite
pyrite
linnaeite
cattierite
jaipurite
wairauite
chalcopyrite
Chemical Formula
Ni3Fe
NiFe
(FeNi)9S8
Ni9S8
Ni3S2
NiS
Ni3S4
FeNi2S4
Fe3O4
FeS
FeS2
Co3S4
CoS2
CoS
CoFe
CuFeS2
+# "B ![ V[ !V# #/W!B%B"B"@#$B
$"B"LV"$Q#["B"
$"#!$
B"
L'`
[''j@_U`}B[''j@`F<j"B"@$"!#$"FEFF;
;
"#$
#`/Xj$!$!B#V@"#/VY$$#!!#"$#B
!!$"^//"
`^j{/F'[<+/`<+j;["!@
Y[Y[!/`':j[#!#V#!$!#/"Y2,aq and
H2[%#"@/@V"!!"#QW!"#
`&B¬B$[':<
"¬[FF
#[FF^#V
[FF+¬B$[FF&#&[FFB$[FF*j
39
2. Fe
1275
1274
1273
15°20'N FZ
Fig. 1. Location of the study area in the vicinity of the 15°20’N Fracture Zone. Investigated samples are from Sites 1268, 1270,
1271, and 1274; redrawn from Kelemen et
al. (2004c).
1272
1271
1269
1268
LHF
1270
2.2. Geological setting
L$;F#~`~[/j!/`±"U
year, full rate) MAR has been explored in detail by numerous surveys (e.g. Rona et al.,
':*|#/#['::
[''*
B['':_¬
[
'''_[FF#[FFj|V"!$
E;/V&"BB!"/@
`_<|j`^¬|#/#[':^[''jL"/"E;
;/V"|#/#/VB"[/
@/VB@#B/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:+$VB#$;F®~W
E*""!BC@#/[#[/B"/"&
"BCL&$$@B!VV!CB#!"!!VV!"$!@B`Cj#Q
@#!#$$
*F #" $ [ %& $
40
<+VB${/VB"QÈ;E®j
"VB$"Y*F
!VVB!CC/@@V+#/$!$"Y*F
*F^"B&!#["!B
/B$"[&/@@#V@"!B
"[Y*F
*F^"@
and oxide veins.
Site 1271 is located on the inside corner high of the MAR spreading segment south of
the 15°20‘ FZ. Drill core 1271A is mainly composed of completely serpentinized dunite.
Drill core 1271B comprises variably serpentinized dunite and harzburgite. Steatitization
"&
*E&"$;F®~$VB
'EF"!*FF"$"$"$#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$X"`FFE[FF*j[
|`FFE[FFj#&`FFj
L!BV/"%/#/
{^<©Y
"""!<"!BC
®_{#!!@©+:'FF"!@VB$X`\"Bj[#!!QVV/!V!"<BC
/V/$F&$@"#$F+$#B$#
´"@""|B#"#L
#/~+"<""!!/@&
"/$!"&$#!#""@/
#"!"!/!@V['/!B$&$#!#"@+#/"B$B
@#$"/C$"$
"[V#
!##//!!/V!C
41
2.3.2. Thermodynamic calculations
L"B"###/
L'`
[''j"!#L@$
L'$`':
K and 105j"B"!"[<XB$Q[#$
state parameters for pure minerals, aqueous species and gases for the calculation of equi@#"`/XV#j$"!#!##!FFF;
FF
MPa. The database used for this study combines all upgrades from the slop98.dat and the
!F@`}B¬V
[FFEj/"##¨
¨
/\"}&@ (GWB jV*F`|&[FF*j+"B"
database for GWB¨"@$!#$F<"!#$F[[
FF[FF[F[FF[F[EFF;
{/XV#@"!#
@B
L'##/VY$$"!#W![//$$$!#`@j{/XV#$#$"
/VL@+VB$Q$Y2[#$/^#""
`':j$
2[[VB$Q$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$$"`¢Yf°j$:E*F&U"$"!`4.54.5S8),
#/!$$"$`j"`j$"@¬
Y"/B`''j
"ì¬X!!`':*j!¢Y°f$:**E'&U"[
/"##+!!\@@/B$$"`¢\°f ) of
:&U"V#/"!$[/V@B
@¬Y"/B`''jL#"@¢\°f¤:&U"$"
42
Table 2: Equilibrium constants for dissolution of selected opaque minerals (P = 50 MPa)
Reaction
log K
Mineral
no.
0 °C
25 °C
100 °C
200 °C
250 °C
300 °C
350 °C
400 °C
72 21
1
awaruite
231 70
196 80
161 91
120 06
104 74
91 77
80 46
2
tetrataenite
11943
103 67
83 38
61 90
54 05
47 41
41 63
37 38
3
pentlandite
57 71
56 73
56 35
59 09
61 67
65 15
70 08
71 77
4
heazlewoodite
3068
26 61
16 94
7 73
3 93
0 32
3 23
5 40
5
godlevskite
87 45
84 42
79 62
78 71
80 01
82 44
86 50
87 29
6
millerite
9 13
8 83
8 42
8 49
8 73
9 09
9 64
9 84
7
polydymite
121 00
113 88
99 44
89 24
86 54
85 08
84 96
83 59
8
violarite
118 14
110 42
94 59
83 35
80 31
78 57
78 21
76 67
9
vaesite
15 48
14 63
13 44
13 45
13 90
14 62
15 72
16 65
10
wairauite
120 48
109 25
84 27
62 91
55 11
48 63
42 79
38 57
11
cobaltpentlandite
82 22
79 11
73 66
71 80
72 65
74 64
78 29
78 83
12
jaipurite
8 25
8 00
7 64
7 72
7 94
8 29
8 83
9 03
13
linnaeite
112 41
105 66
91 84
81 96
79 30
77 80
77 56
75 74
14
cattierite
17 94
16 81
14 98
14 41
14 64
15 17
16 11
16 91
15
H2S,aq
7 28
6 86
6 37
6 53
6 82
7 22
7 78
8 49
16
H2O
50 66
46 30
36 35
27 58
24 31
21 53
19 09
16 84
Reaction no.
1
Ni3Fe + 8 H + + 2 O2,aq = 3 Ni2+ + Fe 2+ + 4 H 2O
2
NiFe + 4 H + + O2,aq = Fe 2+ þ Ni2+ + 2 H 2O
3
Fe4 5Ni4 5S8 + 10 H+ = 4 5 Ni2+ + 4 5 Fe2+ + 8 HS
4
Ni3S 2 + 4 H + + 0 5 O2,aq = 3 Ni2+ + 2 HS
+ H 2,aq
+ H 2O
5
Ni9S 8 + 10 H+ + 9 Ni2+ + 8 HS
6
NiS + H + + Ni2+ + HS
7
Ni3S 4 + 4 H + = Ni2+ + 2 Ni3+ + 4 HS
8
FeNi 2S 4 + 4 H + + Fe 2+ + 2 Ni3+ + 4 HS
9
NiS 2 + H 2,aq + Ni2+ + 2 HS
10
CoFe + 4 H + + O2,aq = Fe 2+ + Co 2+ + 2 H 2O
+ H 2,aq
11
Co9S 8 + 10 H+ + 9 Co 2+ + 8 HS
12
CoS + H + = Co 2+ + HS
13
Co3S 4 + 4 H + = Co 2+ + 2 Co 3+ + 4 HS
14
CoS 2 + H 2,aq = Co 2+ + 2 HS
15
H2S,aq = HS
16
H2O = H2(aq) + 0.5 O2,aq
+ H 2,aq
+ H+
/¬`'*j|#/"!#!B&/[
#VY$$W!
L'$["!#/
K values for dissolution of pentlandite. A standard molar volume (V°) of 153.3 cm3U"
#$#!$"/V@BX#V`''j
Heazlewoodite
¢\°f[ ¢Y°f and S° $ C `3S2j & $" @ ¬ Y"/B
(1995). We used high-temperature heat capacity data from Stølen et al. (1991) to calculate
<XB$QL° (40.655 cm3U"j$C#
using cell constants given by Parise (1980).
Awaruite
Y`FFj!¢\f° [¢Yf° and S°$#`3Fe).We calculated log K
V#$#@B"$VY$$W!
L'$
[@#/"!#"#V@L° (26.96 cm3U
43
"j#$"/V@B+B`''Fj
Tetrataenite
Y`FFj!¢\f°[¢Yf° and S°$`j}#
/XV#$@B"$VY$$W!
L'
$[@#/"!#"&/L° (13.84
cm3U"j#$"/V@B+@`'*:j
L"B"!!$/V&`9S8j@$"!B
"#"@Bµ`''Ej$7S63S2+¢Yf°$:F'&U"
calculated from H298.15Y0 `*E'FE U"j #/ !B $ $"
$/V@B@¬Y"/B`''j¢\f°V$"¢Y°f and standard
"!$/V@B@¬Y"/B`''jL° for a natural
/V&È:*"U"j#$"/V@B`'::j
Millerite
L"B"!!$"`j&$"@
¬Y"/B`''jL$¶"*';
"&
#$#$#@#"EFF;
Vaesite
¢Y°, S°!B$V`2j&$"L`
['':j
f
¢\°fÈ:&U"jV#/¢Yf°"!$
The V°&$""B¬<
"&`''j
Violarite
}#¢\°f and S°$V`2S4j!@B
/`'*j¢Yf°&
$"
"·¬X!!`':*j+!BV@[#/X
V#$#$V#/VY$$W!
L'
$[L°&$""B¬<
"&`''j
¢Y°f , S°!B$!BB"`3S4j&$"L`
[
'':j¢\°f`':&U"jV#/"!$/V
@B@¬Y"/B`''jL°&$""B¬<
"&`''j
Cobaltpentlandite
¢Yf°`:E*'&U"j°È*U"!Xj$
""@`
9S8)
$!#&$"V`'Ej¢\°f (-836.43
44
&U"j"!##/¢Yf°"!$
/V@B
@ ¬ Y"/B `''j Y/"!# !B ! $"
Kelley (1949). The V° (147.102 cm3U"j # #/ !" $"
"¬`'*j
Wairauite
¢\°f¢Yf°$#`
jQ/#!"@B
compounds database (Dinsdale, 1991). We calculated dissolution constants by means of
VY$$W!
L'$
L° (14.09 cm3U"j
#$"/V@B|B`''Fj
¢Y°f , S°/"!#!B$!&$"
<`'*EjL¢\°f$"##/¢Yf° and standard molar
!$
/V@B@¬Y"/B`''jL°$`
3S4)
`
2j&$"@¬Y"/B`''j[$!#
`
j$"#"V`'*Ej
45
2.4. Results
2.4.1. Petrography
} /# B! $ & N !C $ ! atitization of serpentinite. At Site 1274, peridotites are partially to fully serpentinized,
*F[*:[!B$#B!C!V
undergone additional steatitization to variable degrees (see Bach et al., 2004). Microtextures of the serpentinized peridotites range from pseudomorphic mesh and hourglass tex#$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 $/ !/! ! $# !# " "@/ variably serpentinized and steatized peridotites. Because of the small grain size of most
&$# !# "[ Q @B % / "!B $"!@LV"!@"#"@//
!!/V!CBC"
compositions of opaque phases by electron microprobe and used the compositional data
$!Q
$"^{/F'/B"!@B"
""!`#&[FFB[FF*j#"##
46
2.4. Results
#QW®@@$"/"/[@@B_&`'*j$
^#"![#Q#@BV!BW[@@B
{`':'j[/C"!V/"B#Q
""B"!B!@BB#QB$"#/!CB`FF*j!#$"/"#Q`!B
@@ $ ![ @[ !B V / @#j
!@@B##/""!/$!"
!B$#B"!$":
##Q!"
#"@$"!V/$"*F[*[*E#Q/#
"/#"Q&$$!"!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/`FF¹""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""#/[&$#!#
"!!QB"/!"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 <
¹""#"/B#@"@B%/
immersion microscopy or conventional quantitative electron microprobe analysis. Semiquantitative 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
(a)
(b)
(f )
(e)
(d)
(c)
(h)
(i)
(g)
Polydymite-ss
YC
\V&
Magnetite
Magnetite
(k)
(j) Pentlandite
Polydymite-ss
Millerite
e
Magnetite
Millerite
e
Pyrite
\V&
Magnetite
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
\
!=>]"]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
Core
Section
Depth (cm)
Depth (mbsf)
Rock type
Lab code
Pentlandite
Co-Pentlandite
Awaruite
Heazlewoodite
Godlevskite
Millerite
Polydymite-ss
Magnetite
Pyrite
Chalcopyrite
Serpentine
Brucite
Talc
18
O
1274 A
10
1
3-10
49.33
Du
AP-88
++
+++
+++
1274 A
15
1
106-114
75.06
Hz
AP-92
1274 A
15
2
39-46
75.86
Hz
AP-93
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+
+
+++
+
+++
+
+ vein
6
+++
+
+++
++
4.8
5.4
Hole
Core
Section
Depth (cm)
Depth (mbsf)
Rock type
Lab code
Pentlandite
Co-Pentlandite
Awaruite
Heazlewoodite
Godlevskite
Millerite
Polydymite-ss
Magnetite
Pyrite
Chalcopyrite
Serpentine
Brucite
Talc
18
O
1268 A
2
1
10-16
14.10
1268A
2
2
108-115
16.48
1268A
4
3
26-35
28.04
1268 A
13
1
46-55
68.74
Hz
13R1
1268 A
20
1271 A
4
1
105-110
29.55
Du
AP-55
Hz
AP-02
Hz
AP-03
Hz
AP-08
+
+
+
7.4
1274 A
16
1
44-52
84.14
Hz
AP-94
+
+
+
1274 A
17
1
121-129
89.51
HZ
AP-95
+
++
++
1
8-12
103.65
Hz
none
1274 A
18
1
83-93
94.13
HZ
AP-96
1274 A
20
1
121-126
104.11
Du
AP-98
1274 A
22
1
24-32
122.34
Hz
AP-99
1274 A
27
2
5-11
147.65
Hz
AP-103
+++
++
+
+++
+
+
+
+
+
+++
++
+
+++
+++
+++
+
+++
+
5.7
5.4
1271 B
10
1
30-35
50.8
Du
AP-63
+
++
1271 B
17
1
98-102
85.49
Hz
AP-67
+++
++
+
+++
+++
+++
1271 B
7
1
15-22
36.35
Du
AP-61
++
+++
+++
++
+
+++
+
+++
+
++
+
+++
+
+++
++
++
+++
+
++
+++
5.9
+
3.7
+++
4.8
++
4.1
++
+++
++
++
+++
++
+++
+++
+++
+++
++
+ vein
+
+
+
+++
++
+++
+++
+++
18
* O 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
/@$V/#V[&$#!#$$!"!#BQ[@#/CB!B/
(up to 50 μm in diameter) than in meshes or paragranular veins. In transgranular veins of
!B!C!!$/#"
@B"/[#///!#!!/#@#"
#@#B"@B"/`/j[!B
!@B#"/`/@jL@BV
$ "/ ` @j[ "/ ! "&@B[ #
#!@!$#B
CU"/`/$/j"!!B!#C
+ magnetite assemblages in transgranular veins of almost fully serpentinized peridotites
micro-scale element mapping revealed the presence of relic cobaltian pentlandite. It oc##B!B"¹"VB/C @ C "/[ #/// C "//W!$@!YC#/
/V&"/"B"@!#[
&/ /V&L /V& B / W!$@!|#/V&W#VB$#$#B!C&["&B/V&!C`/j
Q/$!C""!B!C!$"Y
1268A magnetite in veins is partially replaced by pyrite.
Bastite
Serpentine veins crosscutting bastite (serpentine pseudomorphic after pyroxene) are
V$"/!#/#U"/tite, the pentlandite occurring as a solitary phase in bastite exhibits a distinct octahedral
cleavage (Fig. 2a). Where serpentinization is advanced, pentlandite in veins crosscutting
@""@BU/#`/j[#///#
!!+#@W#VB$#!landite.
!V"//#B$"!B//$C`/&j
"!BC&!B[
B!@B"U/`+[FF*j+#[![
C[/V&$!C!BC & `/ / j } ![ B " @B "/[ !/ from reaction to millerite or other higher sulfur-fugacity phases. Relics of the assemblage
!#"/$#!B!C!V
#/C[$"@/!C
50
2.4. Results
"/!/V&"/$#$#B!C
peridotites that have undergone steatitization. With increasing degree of steatitization,
#$#!#Q!/VB!@B#$##Q`/&j
<"@##Q!BC"![!
C /V& } C V[ " $ V!BB" # `!BB"j / W! $ ! ""/`/&j"!BC!"/
"!B!@B!B
!B@"!$"*F[*
*E#B$C"!$"Y:+/
!B<`FF*j$#!B!B"$"
"#Q#"!$"$$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#$##QV!W#VB
C&"[V`!B@j["$!BB"B!/"/V$#"$#B!C"!
$"Y:+L!B@/!Q/$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
#/!!@"$
"!@`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$"$@!##//@BQ$
#"#/$#
!B "! / "B #
@
9S8`j!!!W"B#!!$@"`#!!"B^L@+/jL@$!V$"
V#B
$`±F"j
È"jL""U#$#
$"!/'U:`jL$#/[V[@
1.06 and 1.65. Rather than real variations in pentlandite composition, the elevated ratios
QB / # C !L /B
""U#$#$!$"U#$#/$
# ! ! @B Y ¬ & `'*j B ! !@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#$#!VB
/[!@B/"/#
52
2.4. Results
Co9S8
1268
400°C
1270
300°C
200°C
1271
1274
Fe9S8
Atomic percent
Ni9S8
Fig. 3. Ternary pentlandite diagram redrawn from Kaneda et al. (1986). Pentlandite forms a continuous
Co9S8$}
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
!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/[$#Q#
"!BB"L"B$!B/V/!!$
V"!È:"jLB!
±F"[@#@/
"`#V:E"j$!
!!!B
@"$FF!!"
Chalcopyrite
!B&$"Y:+W@""!`#!!"B^L@+'j""#$
`±FFE"j
detected.
54
2.4. Results
}#/"#/!/fO2 vs.
log fS2 and log aH2,aq vs. log aH2[!@##$
!#$F<["!#@FEFF;
Y2¤`/Ej<@V#B&/#"!`@#[V&[V[
j"$"/"""$"@#@#"
$&B$V"
B$`FFEj#VBVB/"[!!
Y2Y2Y
B"EFF;
F<@BQ$"@BB$`FFEj[@BQ
$V[!BB"[/V&[@/![#
!"@!"[/V&[!BB"
V[$!!/"L@B/
$"#//@BB$`FFEj/QB#C
!V\V&W!@"@V
/Y2,aq and H2[V#W#"@!
<""[V["/V&V#/
!#/}$!@#$/V&
$ W" $ !! "@ ! L @B Q $
#/$V&@BB$`FFEj$
!!$C/Y2[V"&@B[@BQ
$#//##$""@B$"!|#
&/"!V/[!@#$
tetrataenite as grey continuous lines to account for the manifested coexistence of pent#"/{/VBVB/"$
Btem in the H2[Y2[!$"!#@FEFF;
F
<`/j|#$"!B$@!"!$
![C$!@BQ/B!
L/
$!"QW!
H2,aq and H2[V+##!@![
#@W!@BQ$#/$#
L@V!!#@!`/jYV[/@#$V
B"!"@B
$&$#!V@!@![
W/!BB
!#"!
!#"@V@![$
project.
55
0
0
150 °C
–1
200 °C
–1
Pyrite
–2
Pyrite
–2
Vaesite
Vaesite
ill
–4
M
Pentlandite
–5
Godlevskite
Tetrataenite
ill
er
ite
–4
Pentlandite
er
Pyrrhotite
ite
–3
M
log a H2S,aq
Pyrrhotite
–3
–5
Tetrataenite
Magnetite
–6
ite
od
–7
Magnetite
Hematite
–8
–8
–7
0
250 °C
–6
–5
–1
Pyrite
–2
Vaesite
o
lew
z
a
He
–4
–3
–2
–7
Hematite
Heazlewoodite
–1
0
–8
1 –8
–7
0
300 °C
–5
–4
–3
–2
–1
0
1
Pyrite
Po
ly
dy
m
ite
Pyrrhotite
Pyrrhotite
Vaesite
–2
Pentlandite
Pentlandite
–3
Tetrataenite
Tetrataenite
–3
Magnetite
Hematite
Heazlewoodite
–7
–7
–6
0
350 °C
–5
–4
–3
–2
–1
–5
Hematite
Heazlewoodite
0
–6
1 –6
–5
0
400 °C
–4
–3
–2
Pyrite
Pyrite
ite
Po
l
–1
0
Pyrrhotite
Pentlandite
Tetrataenite
Pentlandite
–2
–2
P
M olyd
ill
er ym
ite ite
M
ill
er
ite
Tetrataenite
Magnetite
–3
LHF
RHF
–3
Magnetite
Godlevskite
–4
–3
–2
log a H2,aq
–1
0
Awaruite
Heazlewoodite
Awaruite
–5
–5
Hematite
Godlevskite
–4
1
Vaesite
Pyrrhotite
yd
Vaesite
–1
ym
–1
Awaruite
M
M
Magnetite
Awaruite
–6
Godlevskite
ill
–4
er
ite
Godlevskite
er
ite
–4
–5
log a H2S,aq
–6
–1
ill
log a H2S,aq
Awaruite
Godlevskite
Awaruite
–6
–4
Hematite
Heazlewoodite
–5
1 –5
–4
–3
–2
–1
0
1
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 H2
\

^}'""^}'<

*??%
*
2002).
56
, main
a[
Py
Py
0
, main
a[
2.4. Results
Vs
Pd
Mi
–5
Pyrite
Vaesite
Polydymite
Mi
Hz
Millerite
log f S2,g
Pn
Mt
–10
Pentlandite
log aH2S,aq = -1
Pyrrhotite
–15
Hematite
Heazlewoodite
Pn
Pn
Hz
Magnetite
–20
Mt
Awaruite
Aw
–25
–35
350°C
50 MPa
–30
–25
Mt
–20
log f 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 H2
\<
buffered to values around 1 mM.
57
0
0
200°C
150°C
–1
Pyrite
–2
–2
Pyrite
Cattierite
ae
ite
–3
–4
–4
Pyrrhotite
–5
–5
Cobaltpentlandite
Magnetite
–8
–8
0
He
m
at
ite
–7
–7
–6
–5
–4
Cobaltpentlandite
–6
Wairauite
–6
–3
–2
Magnetite
Cobalt
–7
Hematite
–1
0
1
–8
–8
0
–7
–6
–5
–4
–3
–2
–1
0
1
300°C
250°C
–1
–1
Pyrite
Cattierite
Pyrite
ae
ite
–2
Pyrrhotite
–2
Pyrrhotite
nn
Cattierite
–3
ite
Li
nn
–4
ae
–3
Hematite
Cobaltpentlandite
Li
log a H 2S,aq
Pyrrhotite
Li
nn
ae
ite
–3
Li
nn
log a H 2S,aq
Cattierite
Wairauite
–1
–4
Cobaltpentlandite
–5
Wairauite
–6
Cobalt
Hematite
–6
–5
–4
–3
–2
–1
0
1
–6
–6
0
350°C
–5
–4
–2
–1
0
1
Pyrite
–1
Pyrite
Cattierite
Pyrrhotite
Cattierite
Pyrrhotite
–2
Li
nn
ae
ite
–2
–3
–3
400°C
–1
log a H 2S,aq
Cobalt
Li
nn
ae
ite
–7
–7
0
–5
Wairauite
Magnetite
Magnetite
Cobaltpentlandite
–3
Cobaltpentlandite
Magnetite
–5
–5
Cobalt
–4
–3
–2
log a H 2 ,aq
–1
0
Wairauite
Hematite
–4
Wairauite
Magnetite
–4
Hematite
Cobalt
–5
1 –5
–4
–3
–2
–1
0
1
log a H 2 ,aq
Fig. 6.><$<
|
}
$${$<
#?
=??#?[*
<
<
<
%
continuous lines are boundaries of cobalt, wairauite, cobaltian pentlandite, linnaeite, and cattierite stabil<
*
`
*
58
2.5. Discussion
2.5. Discussion
"#$#%#
&
rock interaction
The phase diagrams displayed in Figs. 4-6 indicate considerable temperature dependences of the positions of invariant points and univariant reaction lines in the H2[
H2S,aq activity plane. Hence, before the H2 and H2$/%#@"!V/"!#$%#&
are required. These can be estimated using phase relationships (Bach et al., 2004) or oxy/!"!`+[FF*j[!QB!"
of olivine by serpentine, brucite and magnetite in the presence of fresh clinopyroxene
$"#!!$$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
$"Y*E+[/"!#`FF;
j
@B¨18&V#ÈE»j:L!
B"V$"@#$""!#`
/['*X[':+¬&[FFX&C¬#/&[FFEj
}V/$"""![$¨18&
V@`+[FF*L@jL/¨18"!@
&$"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#$"!È:*E»[+[FF*j
@"!#+/"!$"Y*|
`FFj$#@![#""!#F;
#@B+`FF*j@¨18$&$"
Y:+"&$"*#Q¨34S.
$" "! *E+FE " ! $" Y
59
:+$##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.
V##V#Q[WBB"
V$W#/!C`/_&['*[
':+¬&['':j#!/!V/V!C
$@B!$"^{/F'"!@B//
! "@/ !B !C ![ ! # "/!C"/""@/
"!$"^{/F'V#"B/Q`±
0.1 vol.%) and hence incapable of buffering H2[
/"@/
mineralogy apparently monitor changes in H2,aq activity superimposed by reactions beV%#!</22Y2O system.
A reaction commonly observed in thin section is the desulfurization of pentlandite to
# "/ V @B / # $ Y2,aq released during
!CN
`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 + heaC"/Y2[V#@B/#$
%#[#///!@&@BN
`j
60
4.5Fe4.5S8 + 6H2O '3S2 + 1.5Fe3O4 + H2,aq + 5H2S,aq
2.5. Discussion
#@&@BN
`j
3Fe + 6H2S,aq + 4H2O ' Fe3O43S2 + 10H2,aq
|/WB/$#/#///
#$#$#/"!#/@&$!#+#/"!V$!@&C
"/`/$j[##@&
!Q"@/[#$`j&!+EFF
;
# @B Q W! !B Q 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 #$#! #Q !/VB ! @B #$# #Q L /
WB/ #$# $#/ ` _&[ '* [ ':j ^#/ C
$!C!"/W!$#$#!#Q
C/V&`/jL!"$!#@B"@V[#/"B&!$fO2!BN
Èj
3S2 + H2S,aq ' H2[
`j
9S8 + H2S,aq ' H2['
L!"$"/@B!B!@BN
(6)
Fe3O4 + 6H2S,aq ' 2H2,aq + 4H2O + 3FeS2
Èj`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
`: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"$#Q"@/@VV`#Vj Y2S,aq concentrations measured in high-temperature vent
%# $" #"Q B" B" # V/ @ {/VB"QV#$"Y2$#""U&/
`"<j`
#['':[FF"[FF*j}##Y2S
$ / %# " #$# WB/ $#/ $"
![$W!#@#"$N
(11)
62
S2,g + 2H2[¤Y2S,aq + O2,g
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
/$V¹"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
Q#!$![V[Y2S,aq is set by pentlandite desulfurC<#Q"!!`!#
##Qj[!!#&BB@#$$"%#!#V
H2[W!!!"C#!%C#@#!!
the levels of dissolved H2{/V@B"%#B
!CEFF;
#!"B#
there is no unique H2[Y2S,aq buffer in peridotite-hosted systems, but H2S,aq should
@@B!#@V##"<"!##FEFF;
YV["V"!$&!C$
/"!#V%#&$"Y*E+V"!#[
&$"Y:+V!
H2[Y2[B"$V%#L!@"$
unique H2[Y2[@#$$@#$$"/@#$#W"
2.5.5. Sulfur metasomatism
L#$#$&$"^{/F'/$"FFF
`#&[FF+[FF*jLBV@B!"!
!#!!"`FF¬&[FFEj+#!graphic observations reveal, main-stage serpentinization results in desulfurization of pri"B#Q`+¬&['':j
#B[#$##@$"&
during serpentinization. Indeed, sulfur concentrations in many serpentinite samples are
63
@FF`/*j!$2V!B!C!
"@#`2±EFjVB"!
"!B!CC&LV#$#$
$}#$#$#B#Q@"
the course of steatitization? Hydrogen produced in copious amounts during serpentiniza&!#$#$#/!##$##$!"B#QV
H2N
(12)
S2,g + 2H2[¤Y2S,aq
"B#Q$!#$#C@"@#/!CN
`j
`!"B#QjY2[¤Y2[`#j
When serpentinization nears completion the conditions become less reducing and reac`j!$[/"C/#$#$#/B"blages such as observed in Hole 1268A to develop. One possible explanation for the
sulfur enrichment in completely serpentinized peridotites is a moving serpentinization
front. Sulfur is leached from the peridotite during active serpentinization, removed by
!C$!!&!C"!
!!$B"#/V!C/
!/%#!&#!Y2[B#@#B#"!#Q
#$#WB/$#/!VLY2[$[V[#
V@$B#@#[V#$#$#/B!#@#$$Y2S,aq
V#$$"<`@VjL#Q#"#@VY:+[#%#W#@"#$!C %# #/ C WB / #$# WB/
$#/#C#!B@!!!$B"#!%
C[#!/#%#"WB#
!B"/$%#"W/@#V#Q!![@##$##W#$#$#[#
@#"/B@BB/V#!/%#[
#$#!"!$B"#QV$"Y:+`¨32¤
»+[FF*j##$/Q#$
#$#@#Q$"@""B[|¬Y!&`FFFj
$#"#"#$VYF:`{/*@</j##Q!!$"/
#/%#V/$"!!!@#$# """"YV["&BC!
64
2.5. Discussion
1268A
1270A
1270B
1270C
1270D
1271A
1271B
1272A
1274A
2.0
S wt.%
1.5
1.0
0.5
0.0
30
35
40
45
50
55
60
65
70
SiO2 wt.%
Fig. 7. Whole-rock concentrations of sulfur vs silica for samples from ODP Leg 209. Partly serpentinized
peridotites (< 40 wt. % SiO2) have sulfur concentration that are slightly enriched or markedly depleted
relative to depleted mantle peridotites (0.012 wt. % S). Sulfur is strongly enriched in silica-metasomatized
(i.e. brucite-free) serpentinites (> 40 wt. % SiO2) and steatites from Hole 1268A. Data plotted are from the
literature (Kelemen et al., 2004b; Paulick et al., 2006; Alt et al., 2007). (See text for details.)
VW/B/#$#`/*#&[FFj/
the sulfur-metasomatism preceded the silica-metasomatism. In those samples, steatitiza!V!!#"!!BW`@j
contrast, serpentine replacing olivine is apparently unaffected by steatitization. Because
bastite and vein serpentine are usually devoid of brucite they can be readily transformed
[@#/!"W#/#"
#&#!@$!@$"+!!B[duction of silica to the system leads to increased oxygen and sulfur fugacities that, in turn,
!"#Q!!+##!B/WB/$#/[
B/!#"/$"@$V
$W"!N
(14)
3Fe2SiO4 + 2H2O ' 2Fe3O4 + 3SiO2,aq + 2H2,aq
or
(15)
Fe3Si2O5(OH)4 ' Fe3O4 + SiO2,aq + H2O + H2,aq
+/@#![&!V[@#!@#$$[E$"/#@#C#`/¬
Beard, 2007). As silica activity goes up, reaction (15) may be reversed and Fe-rich serpen65
$"L#!V&$Y2,aq, required to pyritize magnetite
[see reaction (5)]. Because talc does discriminate against Fe much more than serpentine,
#Q"!/!&#/"""""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$#Y:+
;F#~`X"[FF*j
2.5.6. Possible existence of a free H2-rich vapor phase
+"@V[!#"/#@"!BB/
#@B$B/[!#@
FFF;
`/j|#"!#"$$&$"^
{/ F' /B V! "!# /[ $ Y2-rich vapor phase may
exist in abyssal serpentinization systems. In continental settings active serpentinization
produces H2/"`/LB['
VB['*|['*:
§#&V[':
VB[':*+@['::#[':'j
+#/ B !# ! #@B $ Bdrogen, serpentinization of abyssal peridotites may produce a free H2-rich vapor phase.
L@!!!V#B[@W!"&`<
"¬[
2001, 2006) and theoretical considerations (Sleep et al., 2004). Our calculations provide
additional support for the idea that H2 concentrations close to or exceeding hydrogen
solubilities may develop during serpentinization. Figure 8 compares the H2 concentra!/#!"/C#@#"N
`j
1
1– ¤'1
–H2,aq + 21
–Fe3O4E3S2
4.5Fe4.5S8 + 9–3H2O + 2
2 3
3
3
$ #@#" Y2[/ ¤ Y2,aq. As indicated in the phase diagrams presented
@V[ #@/ "@/ ! W"B / Y2,aq concentrations
#[ !# FF FF ;
"!# /L $$ $
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$$QV+#@/
"/L!B!B$/"!#B66
2.6. Conclusions
1.5
1
Log aH2,aq
0.5
0
0
3D
-0.5
H2,aq solubility
-1
-1.5
100
Aw-Hz-Pn-Mt
control
200
300
400
500
Temperature °C
Fig. 8. Comparison of the amount of H2,aq corresponding to H2{$
$
_
$
$
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 corresponds 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}#//
!$#&/%#"BVWV
a H2/![#"&VB$QB$/B`<
"¬[FFj}B//!$"
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
;
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
$"/@@@"&#!/$$!
;F®#~+Y2,aq activity drops, high-sulfur fugacity phases
such as pyrite and polydymite precipitate. The sequence of events leads to early pervaV#Q/$@BC#$#"+
!"#$#C$![#Q"V$"&#/
initial stage of serpentinization. In contrast, steatitization indicates increased silica activi[/#$#$#/B#Q[#!BB"!BV#tion, form as the reducing capacity of the peridotite is exhausted and H2 activities drop.
[#Q#$#C@#!![/"$[&$":LV#$f[/fS2,g in the system
$!$Y2S,aq, indicating that H2V%#@#$$@#
""U&/FEFF;
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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. *+
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<B&/=$"#/#}&|@<+!!$"!@B
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samples supplied by the Ocean Drilling Program (ODP). ODP is sponsored by the US
# `j !!/ # # "/" $
/!#`j[L&#!!$#$"
68
References
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References
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+[ ^ _ ¬ B$[} _[ `FFEj !C /N
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+[ ¬ &[} `'':j #$# !C !N
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+[
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[<+/N#$#/"B"/\"
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+[
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Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro
W!@B"$#/<+/[;F`^{/
F'jN+#$#WB/!#B\"B[\!B[\B"
:[F:FF[NF:FFFF'UFFF*\
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+B[ }[ |#W[ +[ |[ X} ¬ [ < `''Fj Y@&
$ </B L#[ +~N < ^ #@/ `@B !" $ Mineralogical Society of America).
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69
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""B[!!B$!$"<+;`^{/
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!B!C[:F:
|[[[¬L[`'*:jB!C
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Bayliss, P. (1990). Revised unit-cell dimensions, space group, and chemical formula of
"""
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! $ " %# "B # $ \!B F[
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of Illinois.
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Survey. EOS Transactions, American Geophysical Union 79, F920.
"·[{¬X!![`':*jY/"!#"B$#!B"
2, Standard enthalpies of formation of pentlandite and violarite. Physics and
"B$<E[*
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<#&W#
#$_[::
#[{[##[§[|#/#[Y[^V[[_#@#[[|![[
70
References
^!/B[+[+!!#[¬[+`'':j
Y4 plumes generated by
!C$#"Q&$;F$#C
<+/\"
""+[
#[{[^V[[##[§[|![¬Y"[`FFj\"B
of high H2 Y4 V %# #/ $" #"Q & @
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"\/B'[E'
[ < `'':jL""L@N L++[ 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
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/[¬[+`'*j2LV
B"\/+$
</+$
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+#</[<B['*[+@$!
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^[{¬|#/#[Y`':j+!E;"+/N!
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67, 410.
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71
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transfer in geochemical processes involving aqueous solutions. Geochimica et
""+E[''
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for calculating the standard molal thermodynamic properties of minerals, gases,
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72
References
X"[ [ X&[ _[ <[ ^ ¬ B[ { `FFEj ^ {/ F' "!/<+/$"E;;^_
#F[EF
X"[|[X&[_[<[^[`FFE@j_W!BN/
$^//"![F'
/[L©N
Drilling Program, 75 pp.
X"[|[X&[_[<[^[`FFEj{/F'#""BN/
$^//"![F'
/[L©N
Drilling Program, 139 pp.
X"[|[X&[_[<[^¬B[`FF*j{/F'#""BN!
F&"& #V @#B B @ <+ /[
E;;NX"[|[X&[_¬<[^`j/$
^//"[Q#[F'
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[<[<[^¬_[^`j/$^/
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XB[ ^ [ X[ +[ |&"[ ^ X[ \[ \ {[ |#Q[ ^+[
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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.
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aqueous H2\"
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73
+"</EE[:'*'FF
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`<j`#!j#"Q@L"&
</#/!<#/E[:F'
{[ `':'j </B "B $ # #Q /
type spinel peridotite bodies from Ariege (northeastern Pyrenees, France).
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2 to hydrocarbons during serpentinization of olivine. Geochimica et
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compounds produced by abiotic synthesis under hydrothermal conditions. Earth
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Technical Information Service.
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74
References
\"B $ @B ! `<+ /[ ;F[ ^ {/
F'j "! $ %#& !/ V"
"\/BE[*'F
&#&[\[{B[<^[XB[^¬[_`FFj{"!#
V!#{
BB"Q[V$"B/
@!/""
"\/B'[E
"[¬[
L`'*jQ"$##$
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 #"Q B" VB <+ / ; # $
\!B'[E*E*
V[L`'Ej+"B"#B$[@&#Q#
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B[ " |/ \/ \!B
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75
B[<[{[[^&[Y|¬^#[<`FF*jVV"!
reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge
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Stølen, S., Fjellvåg, H., Grønvold, F., Seim, H. & Westrum, E. F. (1994). Phase stability
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!!$7S6"!#$"X'*FX$9S8 from 5 K to
*X#$
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Thayer,T. P. (1966). Serpentinization considered as a constant-volume metasomatic
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Thompson, G. & Melson, W. G. (1970). Boron contents of serpentinites and metabasalts
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+N
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76
References
^!"$_/B`j_/B|+
"!B[{{
§#&V[<["&B[<[^BB&[|+¬^[+`':jYB/
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<#^&B+&"#&[EFE
77
Iron Partitioning and Hydrogen Generation During
! */ %$0 the Mid-Atlantic Ridge
Abstract
Serpentinization of peridotites generates large amounts of aqueous dihydrogen (H2[j[@B!$]B#Q[/[#
!^B/!#!WC
ferrous iron in olivine to ferric iron in secondary magnetite and serpentine. Poorly understood 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$!@#"/QB!"/#"""L"!$
W/!`</'j@#`</:Fj"""!
$"E*"@%`"@$j+@#FF$!U
brucite mesh-rims is trivalent, irrespective of subbasement depth and primary lithology
(harzburgite vs. dunite). Model calculations suggest that both partitioning and oxidation
$VBV"!#&#/!Ction. Serpentine and brucite from Hole 1274A may have formed at temperatures ranging
$"±FF;
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V$"#/"/!C"!#@FF]F;
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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#$Q!#"#B/[
#/#/$"#+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$!"!![!#B/U$"$!//[/Qsequences for rheology, chemistry, and microbial habitability of the oceanic lithosphere
`/[
[''
[''_[''*\[FFE
Iyer et al., 2008). Besides its geophysical and biological peculiarities, distinct chemi78
3.1. Introduction
$#"&B"B"_W/B$
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$ " 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$$#
in primary minerals (e.g., olivine) of the protolith to ferric iron in secondary phases,
["/!"[@B!CB
incorporation of iron in serpentine and magnetite, but also formation of iron-bearing bru`|[FF^+X[FFE_VL""$$['*
<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%#"/$"/"!#%
B"B"[##@B@#B/#"Q&[
/[@`^#V[FFj{/V`"[FF*jB"Q<+/`<+j+#/2[$"{
BB"Q`<+[&"$$WjV@!#[
#/@W"B[@#$$@B!#@B"
MgO–FeO–SiO2–H2O (e.g., Frost and Beard, 2007). In addition, experimental and theo#Q"#@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!"</@#$Q¯XD¤`©FeU©Mgj!¿`©MgU©Fe)Brc] of
F|#"!"$W!$"@#$Q
#""`Y['/['*j<B`'*j#Wperimental study of serpentinization and reported Fe contents in brucite as high as 18
"[!#V@@#
amount of magnetite produced. Along those lines, Bach et al. (2006) suggested that mag79
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^+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</2O3Y2O. The use of geochemical
modeling codes facilitates, at least to a certain extent, the reconstruction of simultaneousB///#!#@$!L#B
$#@#@![@#"/"!
$B/!##/!C[V#!
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|B#"##/
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observations.
80
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,
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&</"`<j[VB$<[+<""!"
$ "! $" Y *E+ <=>@# !!B #
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/"!$"""!#!!Q$L`
!j
"!#`FXj[V$""/C"#
/ "!# #! FF X L "! / "!##B!!Q!!LQ`~
!jL
#""/C"#/"!##!FFX
3.2.3. Geochemical modeling
#$#@#"$#$"[$
##!W"
L'$
@`[''jL
L'@#"B"
$"Y/`'*:j}BV
`FFEj$"[&
Y/`'::j[&`':'''*j}BV
`FFEj$
V/##!#"Q#$"B"$E-[</E$"<
"`FFFj[##+"!W$"
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 thermodynamic data of Fe+3-serpentine [Fe2Si2O5(OH)4°V@#
81
\@@$/B$$"`¢\°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!!#$
(1)
Fe2Si2O5(OH)4 + 3Mg(OH)2¤</3Si2O5(OH)4 + 2Fe(OH)3.
![Fe+3-serpentine ¤
![chrysotile
![Fe(OH)3]
![brucite
Table 1. Calculated thermodynamic data of Fe+3-serpentine
Cp°
a
b x 103
c x 10-5
V°
S°
MgO*
Fe2O3*
Mg3Si2O5(OH)4*
Fe2Si2O5(OH)4‡
9.03
25.04
52.90
50.85
10.18
23.49
75.82
68.77
1.74
18.60
31.60
44.98
-1.48
-3.55
-17.58
-16.69
11.25
30.27
108.50
105.03
6.44
20.94
52.90
54.52
Polyhedral unit
Fe2O3†
SiO2†
H 2O †
g
-185.49
-204.10
-57.34
Mineral
Fe2Si2O5(OH)4§ Gf°
-708.36
*
Helgeson et al. (1978); †Chermak and Rimstidt (1989); ‡Fe2Si2O5(OH)4 = Mg3Si2O5(OH)4 + Fe2O3 - 3MgO; §Fe2Si2O5(OH)4 = Fe2O3
+ 2SiO2 + 2H2O; Cp° = heat capacity (cal mol-1K-1'%

<
%3 mol-1); S° (cal mol-1 K-1); g
(kcal mol-1); Gf° (kcal mol-1)
Y!B$@#B&$"Y/`'*:j
those of Fe(OH)3&$"L`
['':jL"tropy (S°) of Fe+3!"#//#"/"`Y/['*:"Y/[''Ej$`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 $Q $ V / ! # |#"[|^@BY&!"$"
}BV
`FFEj+VB$Q#"@#B$#
![W!$!/#![$VB$Q$
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
Antigorite is common in alpine-type serpentinites but rarely found in abyssal serpen+@B_V`FFEj|`FF*j$$@
"B"!!$BCV$
chrysotile to represent the Mg-endmember of serpentine (cf. Wilson et al., 2006). To ac#$+![&¯+2Si2O5(OH)4] instead of amesite
(Mg2Al2SiO5(OH)4j#"[@#_!B!@
/"W/$#|V["!$
QW</'FC@#/"`$<
"|[FF'j[
#VB$BV"!/
L#$"@#@[#!!/[["["[ "/[ /[ BWB!C[ #[ [ /#[ <V[#$#$@@BB/#!!
#@`!¤F<j!"$B!
LB!"#$"!#$"EFF;
&
U$LB!$"#$V@U
$"V#BÁF"Table 2. Mineral composition used as
peratures of 150, 200, 250, 300 and 350
starting materials in reaction path models
;
Ol
Opx Cpx
$% "& ' # –
Atoms
Si
Al
Fe
Mg
Ca
O
Mg#
1.00
0.20
1.80
4.00
90
1.98
0.04
0.19
1.75
0.04
6.00
90
Table 3.\
'
Na+
ClHCO3Ca2+
2+
K+
SiO2,aq
}
2+
Al3+
SO42O2,aq
^
464.
546.
2.34
10.2
53.
9.8
0.11
0.0000015
0.000037
28.2
0.25
7.8
1.99
0.03
0.09
0.89
1.01
6.00
91
Although serpentinization is temperaturedependent, as mineral stabilities and com! VB /B "!#[
temperature gradients appear to be minimal in deep crustal levels and a change of
temperature can thus be ruled out as the
driving force for serpentinization. The adV/$"!#"!#
as the only variable in a reaction path is
the ease of examining the temperature dependence of heterogeneous phase equilib$&"!$L
compositions selected in this study are (1)
# `FF V[ </ 'Fj `j
C@#/`N!WN
!W¤:FNNV
L@$""!j
Each computation consists of several
!N+@//$_U#[_#!&/
$`L@$"!j;
}"B
83
"!# $ B" EFF ;
[ #/ #@#" @#$"%#!"!#$$["
#@#""!$"%#$@#&B"$#$"!##"#/!"BB#"
$"BB#"[$$VUV$"#B#@#"
"@/[$"F*"!#F/"!#
$%()"&*# – These models emulate the entrainment of heated
$!QW"!#!#"B!
##`V</'FjC@#//"+V/UB"B"@V!!W"#B#
"#"#//$U"#!#@$"V#ally incipient to complete serpentinization. It must be borne in mind that at the incipient
/$!CUQ"BB
/$/%#LVB$Q#@B|
equation are unreliable above ionic strengths > 3 molal. For this reason, all reaction paths
"UÃF
L!#$"$N}!&/$
;
#/_}!&#!!#/_
""#$!#///"!#`F[FF[
F[FFF;
jL$""#$!
"#/U/C@$B
!"!L%#[W!""#QU$!!W"BFL!##W"
#@#"@#@#$"%#!$#$
&
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 (~
F]Vj
!`F]VjL!"B""Wplicitly 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`YB[''j]#QB
!B"#!$]]
]]!
"$X|`FF'j
$/$#!/!B$BB#cates, oxides and hydroxides in pseudomorphic textures and veins. Due to the intimate
/$!@#[""/#@
! "! L V" $Q#B # "!@
B`_<+j$"Q
+ ] !B !C &[ "W#
V" ¡!@#""
typically found. In samples from Hole 1274A mesh-rims commonly reveal a distinct
C/$"@@#$V[$@BC$
! @# "/ QB ¡! "/ 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
a)
b)
Mgt
r
oo
PAM
S
i-p
Brc
N
e-
F
Ol
Ol
Brc
Si d)
c)
Mgt
rp
vein
Mgt
Srp
Fe e)
S
Ni f)
Ol
Ol
Ol
20μm
20μm
20μm
Ol
Brc
20μm
increasing intensity
g) Ol
h)
Mgt
Ol
Brc
Srp
Mgt
Brc
Brc
0.3 mm
Fe-poor Srp
picrolite vein
0.2 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) Pseudomorphic 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//$"¹""F¹"&$#!#
"$@@#V[@#@#
/C$""#////!/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 & $"
Hole 1274A (cf. Seyler et al., 2007) and commonly preserved.
Veins]/VV$!QB!
"/[@#@##@Q#/"/"!_<+
V/B2 contents at the olivine interface and may point to the presence
$@#[@#V@&"/W$@##
##VB @ VQ /# ! V ##B @# "/ &$# !# "L VB /
`ÅF¹"j"!QB!!#"`Å¹"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 /# # </ @# @ B
"[[!@#`j
"!Q#/!#"!
! " # V/ !# $ V "
@#"/![@#"/`/Ej/"/
87
100
a)
b)
c)
d)
e)
f)
Mg#
90
80
70
60
50
100
Mg#
90
80
70
60
50
100
Mg#
90
80
70
60
50
0
10
20
30
40
SiO2 [wt. %]
50
0
10
20
30
40
50
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.
]V</@'F'
$
FF]F
/$"FEFEFFFFF
[!VB+V/"B#C!BW</
88
3.3. Results
Si
+ H 2O
+ O2
Fe
Mg
Si
Mg3Si4O10(OH)2
Fe2 Si O
2 5 (OH)
4
Fe3Si2O5(OH)4
Mg2.85Fe0.15Si2O5(OH)4
Mg3Si2O5(OH)4
)4
(OH
5
O
e
SiF
Fe
Mg
2
Fe
Mg0.8Fe0.2(OH)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
contains both Fe+2 and Fe+3 as some analyses plot along lines to Fe+3-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.
$:]'
$]E@#+2O3 and 1.0

2O3
!BW!`</']'Ej@#
Al2O3. The composition of clinopyroxene exsolution lamellae could not be determined.
Brucite – Regression analyses of brucite-serpentine mixtures in mesh-rims reveal
</$@#@::E`/j[W!$"!*E+E[
FEF"`</j*E+*['"`</*j</V$"
/!B$@#`2±j#[
#/V#B#/V#
$V#B["[/BV<`j[
`Fj
and Al2O3`F'j_V#$#`/V3 in Table A1) are most
&B]#Q!@#"W[#
89
SiO2 [wt. %]
SiO2 [wt. %]
80
Ol
Srp
70
Brc
SiO2
0
10
20
Brc + Srp
30
60
40
96
94
92
90
88
86
84
82
80
78
76
b)
Mg#
SiO2
Ol
Srp
Brc
0
10
Brc + Srp
20
30
40
50
60
70
94
c)
SiO2 [wt. %]
SiO2
92
Mg#
90
Ol
88
86
Mg#
45
40
35
30
25
20
15
10
5
0
90
Mg#
Mg#
45
40
35
30
25
20
15
10
5
0
100
a)
Mg#
45
40
35
30
25
20
15
10
5
0
84
Brc
Brc + Srp
Brc
Brc + Srp
82
80
0
20
40
80
100
120
94
SiO2 [wt. %]
d)
92
90
88
Srp
86
84
Mg#
82
80
Brc + Srp
15
20
25
30
e)
78
94
SiO2 [wt. %]
92
90
88
Brc + Srp
25
86
84
82
Brc
20
Mg#
45
Ol
40
35
30
25
20
15
Mg#
10
Brc
5 SiO2
0
0
5
10
45
40
35
SiO2
30
25
Mg#
Ol
20
15
10
5
0
0
5
10
15
60
30
35
80
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
$@#&B@"`FF]Fj
in Table A1.
– The composition of serpentine varies depending on its precursor
mineral and textural context. Regression analyses reveal that serpentine in mesh-rims is
""/V[//$"</'E'`/j
/B@FL+2O3"B@F[@#"B@
#!F[!@BV$"/!
!VB#$#[#///!$]#Q
serpentine matrix.
#"!!$!BW`@j<///$":
'/$
`±*j[+2O3`±j[
2O3`±
j[<`±Fj$`±Fj"!"!`/[L@+j!"/!/#V</@'E':[B/"V&/
"/[</'
+2O3 contents are similar to those of mesh
!!!/#V/]#QB
W"B±FF
3.3.3. Mößbauer spectroscopy and bulk-magnetization
<"$!B$#B!C!!@B"
drilling to investigate Fe+3UV#$!@#!$
"!W$!C|#&"/CB#
to relate Fe+3UV#"/$""|#&"/Cyses and thin-section petrography revealed a positive correlation of magnetite content
W$!C}V!["/
/QB"!$#B!C&&B!C!Table 4. Mößbauer spectroscopy and bulk magnetization results of micro-drilled mesh-rims
Titration* Magnetization
Mößbauer
Sample
1274A-6R2, 128-135 cm
1274A-10R1, 3-10 cm
1274A-15R1, 106-114 cm
1274A-17R1, 121-129 cm
1274A-20R1, 121-126 cm
1274A-22R1, 24-32 cm
1274A-27R2, 5-11 cm
1274A-22R1, 24-32 cm‡
% Fe304
Fe+2
Fe+3
Fe+3}
Fe+3}
% Fe304
0
14
23
21
52
0
11
0
52
27
26
43
17
70
43
66
48
59
51
36
31
30
46
34
0.48
0.68
0.66
0.50
0.66
0.30
0.53
0.34
0.45
0.62
0.68
0.56
0.68
0.57
0.51
0.57
0.33
0.79
1.38
1.20
2.80
0.16
0.79
0.43
* data from Paulick et al. (2006); ‡ micro-drilled bastite
91
["//B![<=>@#!V+3UV#
@FFFE:$B#B!`L@Ej/B$#B!tinized peridotites hosting abundant magnetite, Fe+3UV#B/
/$"FF:"!`*E+[E"jBC
@#&"/C+3U$!@</CBQ"
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"$"
#V!#@B"</2Y2O. Univariant phase equilibria
#@W"$&$&"![@#!!!VV#$!$/%##"[%#$#!#@!@/B/
!![V#/#!W@"!
/!#[/Q$]</W/#@@!
and brucite has been emphasized recently by Evans (2008). Sleep et al. (2004) examined
the effect of Fe partitioning into serpentine and brucite on hydrogen production during
!C[#"/QW]</@#$Q$FL!!
constitutes a useful extension of pure endmember phase equilibria, but still impose too
"B$#"$%#&#@\"
!"/#_U`}B[''''@}B^V[''
}B&[FFj@#/!/V#$%#
&"!#/!CV#V#!
"/V/V#!$%#&#/!C
`+B$[FF<
"|[FF'[FFEB$
[FF*}C&[FFFjYV[$""$!V#
/" ! "/ # #!& $ +3 into serpentine
@[#/#"!/Q"#$+3`|#['*'\CÆC<[FFYB
^B[''/[':C['*'B$[FF*}&
}&['*FjL!"!""#W!@B
$!V##BBW!#!&$+3 in serpentine,
92
3.3. Results
/Q"!B//#/!!##&$&//+3#!&@B
brucite during serpentinization. Although it appears possible that brucite contains Fe+3,
#B#$#"$@#
,"-./01#%(211*34()5'"-'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!$V`</'Fj!`</''j@#`</'j
"/ F ;
`/ @j Y[ V # "!B @#
becomes part of the stable equilibrium mineral assemblage according to the generalized
N
(3)
olivine + H2£!@#"/Y2,aq.
"/$@#$#B@&$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"#$!"Bing temperature (because the amount is constrained by the amount of SiO2@#&
93
10
a)
olivine
minerals (moles)
1
serpentine
0.1
magnetite
brucite
0.01
b)
Mg# mineral
0.4
brucite
80
0.2
Fe+3/6Fe in serpentine
chlorite
80
brucite
H2,aq (mmolal)
200
brucite in
200
magnetite out
100
mol% H2,aq related to mineral
d)
80
60
magnetite
40
20
0
e)
aH2O
pH
0.986
10
0.984
9
0.978
0.976
Cl
Mg
Na
K
Ca
-4
-5
Fe
-6
-7
Si
Al
25
100
175
250
Temperature (°C)
325
400
0.988
k)
aH2O
pH
0.986
0.984
8
0.982
7
0.980
-2
-3
0
11
4
magnetite
20
12
5
0
f)
-1
serpentine
40
0.988
0.982
6
60
0.990
aH2O
7
j)
80
aH2O
8
100
pH
serpentine
0.980
6
0.978
5
4
Log concentration (molal)
mol% H2,aq related to mineral
brucite in
0
9
pH
i)
300
0
Log concentration (molal)
serpentine
90
400
magnetite out
tremolite 1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
clinopyroxene
70
c)
100
h)
60
300
-8
-9
chlorite
0.01
0.0
70
10
magnetite
trem.
0.6
olivine
H2,aq (mmolal)
0.1
100
0.8
90
100
brucite
clinopyroxene
1.0
serpentine
400
olivine
serpentine
0.001
0.001
100
g)
1
Mg# mineral
minerals (moles)
10
0.976
Cl
Na
0
l)
-2
Ca
K
Al
-4
Si
-6
Mg
-8
-10
-12
25
Fe
100
175
250
325
400
Temperature (°C)
Fig. 5.[
<\
>
1 kg of seawater was equilibrated with 1 kg of dunite (a-f) and harzburgite (g-l), respectively, at temperatures between 25–400 °C. See text for discussion.
94
3.3. Results
B"j["#$@#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)
(5)
(6)
Fe3O4 + H2O + H2,aq + 2SiO2[¤3Si2O5(OH)4
Fe3O4 + 2H2O + H2[¤`Yj2
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 %#& #@ ` @#$$ @B !Q
""@/[/[!@#jV"&`#"jBgen is implicitly acounted for in the reaction path model. As (R4–6) proceed to the right
/"!#["/"!BW#@;
the system is entirely controlled by exchange equilibria among serpentine and brucite
(Fig. 5a). The H2[VB!@B$+3-serpentine (Fig. 5b,
[jN
(7)
(8)
2Fe3Si2O5(OH)4 + 2SiO2,aq + 5H2¤2Si2O5(OH)4 + 3H2,aq
2Fe(OH)2 + 2SiO2,aq + H2¤2Si2O5(OH)4 + H2,aq.
+/ `]:j "!# @B $ +3-serpentine, and thus the
activity of H2[["@B2[VB[%B@#$$@BN
(9)
Mg3Si2O5(OH)4 + H2¤2,aq + 3Mg(OH)2.
With the exhaustion of magnetite, serpentine becomes increasingly magnesian as tem!#![@#@"/B`/@:]'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
<//B"!/%#
`/$jL$W@/"!#!B//"!#[%/@#$$/@B//!
assemblages described earlier.
95
,%.%(211*34()5'"-'7#
L&"!#/"##!
"!![[C@#/`N!WN
!W¤:FNNVj+#/
!!![/##[@#
remains largely unaltered in most samples from Hole 1274A (cf. Klein and Bach, 2009).
Figures 5g–l summarize the model results in terms of equilibrium mineral assemblages
"!$"%#$#$"!#+EFF;
#@#"""@/$V[![""
amounts magnetite (Fig. 5g). The predicted concentration of H2,aq is higher compared to
model 1A, because orthopyroxene provides the SiO2 needed to produce serpentine and
"/`/j//CN
(10)
olivine + orthopyroxene + H2£!"/Y2,aq.
At temperatures above the quasi-invariant point of olivine-serpentine–brucite equilibrium, concentrations of SiO2["!"+`/j[@#"#
more serpentine is produced according to (R10). Diopsidic clinopyroxene forms at the
W!$"W#"!#@'F;
"&@B[
VB"@#@#$"/B/"!#
È;
VF;
jL"/$@#$$"$"/
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
È]j#W#L±E;
+B"!#
of H2,aq and SiO2[@#$$V#@B!@##@#"`/
j/!/$"2,aq to HSiO3- at pH ~ 8.5 the concentration
of SiO2["L±;
`/&jL/!Y"!"
+@&$!"B!BW!BW`
L"&`
Lj"!jL@/Q"#$
%#+"!#@E;
[@#!BW!#@#"
!Y[B"!#!
$V
"/#"V"!#/@#$$@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
Srp
Brc
1
Cpx
0.1
Brc
Chl
Srp
Mgt
0.01
150 °C
150 °C
0.6
Brc
+3 /6
70
0.1
Brc
Mgt
Srp
Chl
0.01
Srp
0.4
0.2
(k)
150 °C
Srp
Mg# mineral
Cpx
Fe
Fe
0.8
90
0.6
80
Brc
rp
0.4
eS
+3 /6F
Fe
70
0.2
Mgt
200 °C
200 °C
(h)
Cpx
0.1
Brc
Mgt
Chl
Srp
0.01
Mgt
250 °C
250 °C
Srp
Brc
(d)
(i)
0.1
Brc
Srp
Cpx
Chl
0.01
Mgt
300 °C
0.001
300 °C
Ol
Srp
Ol
(j)
Cpx
0.1
Chl
Srp
Mgt
0.01
rp
0.4
eS
+3 /6F
Fe
70
0.2
(m)
250 °C
90
0.8
Brc
0.6
80
60
100
0.4
rp
eS
+3 /6F
70
Fe
0.2
(n)
300 °C
Srp
1
10
water to rock ratio
100
0
1
10
100
0.0
1.0
0.8
90
Fe+3/6Fe
Srp
0.6
80
0.4
70
0.2
Mgt
350 °C
350 °C
0.001
0
0.0
1.0
Srp
Mg# mineral
(e)
1
0.6
Brc
60
(o)
0.1
water to rock ratio
350 °C
1
10
Fe+3/6Fe serpentine
10
80
100
Mg# mineral
Mgt
0.8
90
60
1
0.0
1.0
Fe+3/6Fe serpentine
0.001
10
200 °C
Srp
Mg# mineral
1
60
100
(l)
Fe+3/6Fe serpentine
Srp
Brc
(c)
0.0
1.0
Fe+3/6Fe serpentine
moles minerals
0.8
80
(g)
1
0.001
10
moles minerals
Srp
90
60
100
Srp
Brc
(b)
moles minerals
1.0
100
Mgt
0.001
10
moles minerals
(f)
Mg# mineral
moles minerals
(a)
Fe+3/6Fe serpentine
10
0.0
100
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
$//&Ùj%#&#@L#[
#"#$&&/$[#V#/
"/B%#"!QV"!#`F[FF[F[FFF
;
j+/#V`</'FjC@#/`N!WN
!W¤:FNNVj
/"/[/U/$$
97
log concentration (molal)
0
-1
-2
-4
-8
0
-1
-2
-3
Ca
-3
-4
-5
Si
H+
-6
-6
200 °C
r)
-2
-3
-4
-5
-6
0
Cl
Na
Mg
K
w)
-1
-2
Ca
Fe
-3
Al
Fe
Si
Mg
Si
H+
Al
Si
H+
-5
-3
Al
0
Cl
Na
Mg
K
-1
x)
-2
-6
-7
300 °C
-3
Al
300 °C
-9
0
Cl
Na
Mg
K
-1
y)
-2
Fe
Ca
Si
-3
-4
-5
H+
-5
-6
-7
-7
-8
-8
Cl
Na
Ca
K
Mg
K
H+
Ca
Si
Si
Al
Mg
-4
-6
350 °C
-9
0.1
Fe
Si
H+
H+
Fe
-8
-8
-2
K
Al
Si
Mg
-5
-7
t)
Mg
Ca
-4
-6
0
Cl
Na
Ca
K
-3
Si
H+
-5
250 °C
-9
Fe
Ca
-4
Fe
Si
H+
H+
Fe
-8
-2
Mg
K
-6
250 °C
s)
Cl
Na
Ca
K
Al
Si
Mg
-7
0
200 °C
Ca
-4
-8
-1
Mg
K
Ca
-9
-7
-9
Cl
Na
Ca
K
H+
Fe
-8
-8
-1
150 °C
-7
-7
-9
v)
-2
Fe
-5
H+
Fe
-9
Cl
Na
Mg
K
-4
Si
Fe
Al
-7
150 °C
q)
H+
Si
Mg
-6
Si
H+
-8
0
Mg
K
Ca
Al
-5
-7
-1
Cl
Na
Ca
K
-3
-6
-1
u)
-2
-5
-9
-1
Fe
-4
0
0
Cl
Na
Mg
K
Ca
-3
-9
p)
Fe
H+
Fe
Al
350 °C
-9
1
10
water to rock ratio
100
0.1
1
10
100
water to rock ratio
Fig. 6. (continued)[
\
>
*'
\
>%<'
\
composition for model 2B. See text for discussion.
"#$!B#!
#B[
$VV"B/U%#
compositions, mineral assemblages and solid solution compositions of serpentine and
@##/$#$U
98
3.3. Results
,"-./01#"-*6#
+F;
!#@#""@/"B$#"
amounts of serpentine and brucite (Fig. 6a). Trace amounts of magnetite are present only
@UÃEF|#"!/BV//U[
</$"'/U*U`/&jL</$!
$"'*/'EU+/UV#B!
!@V[UBF"$$`/
&jL/+3U$!/2,aq and H2O acV/Y2[VU`/!/*$*j}
!!$"/U[Y2,aq concentration are no longer buffered by
equilibria (R4-6), causing a change in slope in H2V#U`/*j_#@`*j
`:j#/VY2 activities, and although Fe+3!`/&j[
H2[@#B"@""&"L!Y/Btrolled 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/`/@]j[""F;
L"$$"/@&U±+VB
"!#!"[!@#@"/B
magnesian at higher temperatures, promoting magnetite formation. The molar amount
$"/B/U[@"#"#
$!@#L%@BV!!$"
#@#"B"!/"!#!"
1A (Fig. 5), predicted concentrations of H2[@B$"FFF;
`/*j<V[U[$VY2 increase almost expoB/U
/$V"VB
"$F;
"[#/@#$$`/!]j
!#[$V[+/B/
$V"`W!$"#V"!#!"+j!/U
+F;
#@#"!"@/!"B$V""#$!"/`/j[%/
@B$VLF;
#B[Y2[$V"V"!$V"
temperatures.
99
800
300 °C
H2,aq (mmolal)
700
dunite
harzburgite
600
500
250 °C
400
300
200
200 °C
150 °C
100
0
0.1
350 °C
1
10
100
water to rock ratio
Fig. 7. Predicted dihydrogen generation for serpentinization of dunite (grey lines) and harzburgite (black
lines) at constant temperature as a fuction of water-to-rock ratio. See text for discussion.
,%."*7#
+ F ;
U E ! #@#" "@/ !# @B B$C@#/"B$!@#""/
Ù±EF[/$j"#$+UÅEB!BW
@"!$#@#""@/</@&UÃF[[
"U"!"+LV#$#
"!$!@#"$"+`/&j
!BWB!#!
!!"@/!"B$+[@#"&#!B"$$!#"!
VV$"!#/U"$`F"!j
UYB/B/"!"+[@#
again, H2[!B@&$"/[%/
H2[@#$$/@B!`/*jULB/Wdition of Si provided by dissolution of orthopyroxene according to (R10) supporting (R6).
L$!BW"@#&"!/!!$[
//$$!#@#""@/!
##@#[#/"#$"/[
and thus higher H2,aq at equilibrium.
+/U!Y"@B%#$</2+ due to high concentra$</}!/VB$C@#/[!
brucite buffer the amount of dissolved Mg2+W/%#W/B
100
3.3. Results
0
300 °C
50 MPa
-1
H2O H2
pyrite
pyrrhotite
ite
vaesite
po
ly
dy
m
pentlandite
-3
er
ill
t.%
S
w
.12
0
aw
heazlewoodite
hematite
-6
-6
0
-1
aru
-5
0.0
4w
t.%
S
-4
-5
-4
-3
-2
0
-1
1
200 °C
50 MPa
H2O H2
pyrite
-2
vaesite
pyrrhotite
er
ite
-3
pentlandite
ill
-4
m
-5
-8
-8
-7
-6
heazlewoodite
te
aw
hematite
t.%
magnetite
4w
-7
S
S
-6
wt.%
aru
i
0.12
0.0
Log a H2S(aq)
ite
ite
magnetite
m
Log a H2S(aq)
-2
-5
-4
-3
-2
-1
0
1
Log a H2(aq)
Fig. 8. Fe–Ni–O–S phase relations in H2-H2<*
\
arrows pointing the direction of increasing water-to-rock ratios. Note the different paths taken for rock with
low (0.04 wt. %) and moderate (0.12 wt. %) sulfur contents.
101
V#
#"@@B#$!"B!BW
L"!nent of orthopyroxene is only partly incorporated into secondary clinopyroxene, leading
2+"`!$"+j&!Y$:U
+VB/U`j[
$"&"!BV%#
+FF;
[F;
FF;
#@#"""@/B
""F;
[V["$$/
$"#$"/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&
$"Y*E+!¯`[[
j9S8°#`3Fe) + magnetite, fol@B!C`3S2) + magnetite in partly to completely serpenC&`X|[FF'j+/B#/[!
#$#C # U C[ #Q!`j !/ $#/B
/"!/@V$X|`FF'j[#!"!#$#C$!#$#$#/B!#UC
#@B`':j[
!#@V/!B$B"+!@#$#CB"!C
#"/["/!
$""@//C"/
Minimum H2[#@C!#
"/"@/$"F"<F;
"E<EFF;
`X
and Bach, 2009). The modeling results reveal that serpentinization of dunite and harzbur/B#$QY2[@VF;
@C"@/`$/
*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!$]</#@@![@#"/L"!
!!/"#$/"!#
U`/j/"/@#!|#@V"
@" /B / U "!# YV[ "$@#"!#"#"!#$pentine. The temperature dependency of Fe–Mg equilibria is mirrored by the temperature
dependency of redox conditions during serpentinization, and concomitantly monitored
@B
!Y[@$"]</#@$W/pentine, brucite and magnetite can be adopted for samples from Hole 1274A, constraints
on serpentinization temperatures are required.
!C"!#V@"$&$"Y*E+#/!!`|[FFEX|[FF'j&WB/
isotope compositions (Alt et al., 2007). Bach et al. (2004) interpreted the replacement of
olivine by serpentine, brucite and magnetite in the presence of fresh clinopyroxene from
#!!$$Y*E+[@V$!C"!#[!@@B±FFF;
|/¨18`#!:»j$&$"Y
*E+[+`FF*j!!"!#`±F;
jX|
`FF'j$##/]&@!""@"!
$"Y*E+[WB"!#@FF;
`X
[':jL&/"!#"#[#!!
@#"!VB//""!$!
brucite from Hole 1274A (Figs. 5 and 6).
The evaluation of predicted and measured mineral compositions in combination
"!#""@V[#!!W"&#/!C`/j+F;
#C@#/"!U$E[!VB[$!</'@#</:F+FF
;
!/U!@B@"YV[&
ratio values for serpentinization of peridotites from Hole 1274A are not available from the
literature, our predictions should be regarded as provisional.
103
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"!/
@#@##!"@Q/[#"
#$"@"@/@#B@F;
+@V"!##@@#$QB#/["/##V$B""@
#@@!/!#!`/:j+!$"
W"@B#@</22O3Y2O system,
@#&#$#$&"!@!$
#@#""@/U$@V{#!
#$#C"@/C#"/@@`
Fig. 8, cf. Peretti et al., 1992). In contrast, even at highly reducing conditions pentlandite
#$#CB![$VY/#$"
W!$![!!WC
"/YV[@$&!`&j$C@VL"#$#$#$!"W##/C@V
!![C#V!![$#B/B
$#B!C!`X|[FF'jV!!/$C
$"#!#"/W"@B/B
#/$#$"@#@#"$!# "/ #!! $" $ C # # "#V!!/$C#[#"!!
!$#@#""@/V"!#/U[/
##/L±F;
3.4.3. Fe+2+3 exchange equilibria in serpentinites
104
_V`FF:jV!#$#/W
3.4. Discussion
of Fe and Mg+2Fe-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!YB^B`''j
!<=>@##`j/Q+3 in serpentinite (about 50 % of
j`j/Q+3[/@#F`$|#
['*'\CÆC<[FF/[':C['*'}&
}&['*Fj#_<+V'*$"##`@$*
oxygens) at the tetrahedral site of serpentine in mesh-rims is occupied by Si and aluminum 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!$/Q"#$+3
in hydrous secondary minerals.
""$&B!C!+3U@#FF
FE:YV[#!&$+3 by brucite cannot be excluded so that the actual Fe+3U
$!"B@"""$/B$#B!C
peridotites the Fe+3U$B#B!@FF:[
@B/&B!C!L/+3U"B
@&Y2[[@##/!B$&
#/$$#VUVV$Y2,aq (see R7) or both. Increasing aSiO2,aq is un&B!"/+3U2[@#$$W/BV#
"!#@B`'j
| F F ;
` W! "!# / $ !C
Y*E`+[FF*|[FFX|[FF'j!
Fe+3U$!!!/BV!#&"/B@#V/QB/U"!#`/j+F;
U[@#!V!"!
# #V $" Y *E+ `</ $ @# ¤ :F </ $ ! ¤
95), the Fe+3UF[["!@+3U$!B!C
!+FF;
U$F[!@#!V
"!##V[!+3UF[
"!#"!YV[@###
$!/[!!!$+3 in
serpentine have to be considered as minimal values. With increasing temperatures the
"" @ @V ! $#[ V!C$&$"Y*E+&!"!#@
F;
U
105
3.4.4. Geochemical reaction path modeling and serpentinization experiments
Seyfried et al. (2007) conducted serpentinization experiments of a spinel-lherCFF;
F<LB!</$'+3U$W!mentally derived hydrous alteration products, i.e., serpentine is ~ 0.42 and thus consis+3U$!$!B!C!$"Y*E+L
"!W!"##/"!"/#
V&WU[U"$!#V"$
B$`FF*jW!"EE/QEF
/!C#$W!"@#$/
"`#V/$&j!@#[#QU
L#+3U$!$"#V`</'FjC@#/
"FUL$$@#W!"B
derived Fe+3UV#`FEj@#`j!$$@#[
(2) incorporation of Fe+3$![`j&$
thermodynamic data and solid solution models. Another factor is that the models rep#@#"V#@#"V
W!"V["!$/Q#!&$+3 into serpentine is
W!"L&$"/W!"@BB$
`FF*j"@B#!$FF;
U¤#"!
"/"B$""!#@F;
U±L#"B
"B"V$"/$"FF;
W!"
$B"[!!""/#
^B/#!**"<W!"$B$
`FF*j["$#"!/"
path model (350 mM) that emulates the serpentinization of a lherzolite (62 vol. % Ol, 26
V!W[FV
!W[V!j[//#<
"
|`FF'j"!C!/$@B"W
/%#"#$Y2,aq generated during serpentinization. The huge
$$@!@V Y2,aq concentration is obviously related to
&`$""@j$!@##!
"$B$`FF*j`[@!!
@#j<
"|`FF'j#B!/$
!@#"W!"B`<B['*B$[
FF*j@""#VB!"
#$"L#!$/!V!$Bdrogen 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 concenB[@#"!!#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;
[[!]]"/[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
`</ :'j EFF ;
F < @ " $ /
"!##"QB"B"}#W!"B"%#"!&B!$#!C
model 1A (see Fig.6). Predicted and measured concentrations of dissolved Mg (predicted
'"<"#*"<j[`"<F"<j[`"<"<j[
`F"<
0.3 mM) and H2[`"<"<j//"`/j[#/+B$`FFj#Q/$
"#%#"/$"@B"Q<V["#
!Y`;
j$*[#V#!Y$E'/"#
!!Y$E:
"#[W!"@B+
B$`FFj#"@/##BV"#V"!#
$EFF;
!#$F<
|`''j#!CW!"$V`</::j
FF;
F<L@V"/B$!![@#"/[/"#"/#`/[
!</'*@#</'jYV[UBC%#"!B#![@W!$B
# / %# / " V @ V[
$`!¹<"#¹<jY2,aq (146 mM versus 158
mM) are very similar, indicating buffering by serpentine, brucite and magnetite (Figs. 6
*j"W!"@B&BB$`':j[B#
Q`#V##"j
107
$QW
4W]
7OF
/RJD6L2
2S[ U[Q
7OF
6US
)R
(Q
)R
7OF (Q
6US
PHWDVWDEOH2O%UF GHYHORSLQJ
ZLWKLQ wJUDGLHQWLQ6L2
6US )R
%UF
2O U[Q
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[@VVtive 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
"/QB ¡!"/#"`/[
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[ ! @# </]2–H2O system are
/V@B$/`/'jN
(11)
(12)
(13)
(14)
3 Mg2SiO4 + SiO2,aq +4H2¤</3Si2O5(OH)4
2 Mg2SiO4 + 3H2¤</3Si2O5(OH)4 + Mg(OH)2
Mg2SiO4 + 2H2¤2,aq + 2Mg(OH)2
Mg(OH)2 + 2SiO2[¤</3Si2O5(OH)4 +H2O.
+@VF;
[V#@V!$/`j$
W # $ `/[ $" !BW @&j[ log a SiO2,aq
-3
Tlc
-4
-5
-6
Berndt et al. 1996
Fo
Ol
Brc
Srp
aH2O
= 1.0
aH2O = 0.5
Srp rc
+B
l
O
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-serpentinebrucite 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
W!B"#!+[V@#]!]
$[##`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 pH2O < ptotal[/$#V$#!#@
the MgO–SiO2–H2B"`/Fj[VB"B@#
!C$[#"_"$
$#@W!`/[$[':j@#@V&$"Y
*E++[@#C@V!Vpentine-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@@&#@`jÈj[
@ & W! " C/ "" $ `j
&![#[%##"V!@BQ[
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 / V V!/ 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`</::j/"$#!"!#!
![ @# "/+/[ #"/ $# #@#" FF ;
` aH2¤jV#$""!#B!@#[
Q/#"/##$W!"[V[Q#@#B
110
3.5. Conclusions
Fig. 10 illustrates the isothermal reaction path of SiO2[È:$!/j@@#V!@#B$Èj"@@$`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$`j&/!@$Èj[
ultimately control the SiO2VB$B"VW#<
!C W!" #$QB # 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|F]F;
B@B!]"/]@##@}V!QB
!/$V@#VVpentine.
Model calculations reveal that both partitioning and oxidation state of iron is very
V"!#&#/!C+"! $ W/ ! `</ 'j @# `</ :Fj " " "! $" Y *E[ &B VB "
|#"/#[!!!@#$"
"!# // $" ± F F ;
!/ @#& & /$"±F+#"#V$"#/"/!C
"!#@FF]F;
U±[B/$#/"W"
&"!!!@$&B"!$##/$!ing during serpentinization. Serpentinization of orthopyroxene generates more dihydro/!C$V/"!#`F;
j[L
@# V @& ! "/ ! V@B $
W#+"!#@F;
@#@W
silica source is no longer required to facilitate the formation serpentine and magnetite.
+/B[B/!&!!W"BF]F;
`!/
!"/Bj@#"/"!#/@#
![B/B@B!C$VW@B$Bdrogen yield by serpentinization of orthopyroxene.
111
+/Q"#$V!@#Vhydrogen generation. Textural evidence indicates that olivine is replaced by brucite (and
not serpentine) along the grain boundaries so that the formation of brucite appears to be
the initial step of a serpentinization reaction sequence. We propose that brucite is meta@@BQ$!##/"B"V$!
nucleation is available.
The formation of Fe+3!"&B@#B//[
!#"!#&<=>@#!!
#@#FF$!U@#""Vlent, 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
#®L_B"
112
References
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>©
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<**
*}*??='*
<
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
1
2
1
1
1
2
1
2
1
1
1
2
104-105
128-135
3-10
121-129
24-32
5-11
104-105
128-135
3-10
121-129
24-32
5-11
147.65
Depth (cm)
22.79
32.78
49.33
89.51
122.34
147.65
22.79
32.78
49.33
89.51
122.34
Rock type
Depth (mbsf)
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
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
Wt. %
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
K 2O
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
4
4
4
4
4
4
7
7
7
7
7
7
Oxygens

'

\{

^__

119
<&=
Insights from geochemical reaction path modeling
Abstract
L"B" ! " !! !V /
$"$/"QU#"Q@#L"#!
V/$%#]&#@%#"V$"!#//
!C/@@@BV/
$/W$$!C%#/@@FF;
[FF
;
[EFF;
"@/B!$/`/#!j
!$"FF;
FF;
[@#B%#B#$$@B/@@[[#"Q&
Q#Q!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$$#$
Y+ species,
!VB!/"Q]#"Q
@#B/
!@#$/@B$!/
2 activities. Our model calculations further predict the formation of diopsidite
$"/@@!BW!#&FF;
FF;
[#///
VB/"!#$:FF;
#$"$!sidite veins.
4.1. Introduction
/ @V $" "Q $ & %#
V@$$@B#"Q&`LB['
"['*
['*_V['**[':YB[''j!C
# / %# @" &[ /
`B$ ^@@[':F&BB$[':B$[FF*j#%#&
$""//`XB[FFXB[FFj[!`
120
4.1. Intoduction
[ ' | ['* |
[ '' /[
':+@['::j[$
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[ /
@ ` [ ':' YB [''+[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
$ $ #" `
-
Figure 1. Photomicrographs of rodingitized gabbro
from ODP Leg 209 at the MAR 15°N (Kelemen et
al., 2007). (A) Patchy replacement of plagioclase by
prehnite (prh) (sample 1274A-11R-1, 46–49 cm).
(B) Clinozoisite (czo) replacing plagioclase (sample
1274A-21R-1, 12–17 cm). (C) Euhedral grossular
(grs) crystal (sample 1274A-21R-1, 12–17 cm). All
*!#
mm wide.
"['*j #B[ B
mineral assemblages in rodingites com!/$
]+[#/
C[ ![ /#UB/#lar, and vesuvianite. Other phases are usually diopside and chlorite. In many cases,
rodingitization is multistage (e.g., Schandl
[ ':' YB [ '' { [ FFE " }"[
2007) or rodingites are overprinted by later metamorphic events (e.g., Frost, 1975).
During later stages of rodingitization, the
direction of mass transfer may be reversed
`[
"B@$"&jL
121
Table 1. Comparison of major element compositions of rodingites from the MAR with average compositions of
peridotite and basalt
Depleted mantle
MAR N-MORB
MAR rod. 1
MAR rod. 2
Alpine rod. 1
Alpine rod. 2
Munro rod.
39.68
SiO2
44.90
50.01
36.45
35.25
37.43
37.68
TiO2
0.13
1.11
0.32
3.88
0.19
0.06
0.57
Al2O3
4.28
6.31
20.00
8.07
15.34
18.81
11.68
FeO
8.07
9.73
4.67
11.85
2.92
3.19
5.80
MgO
38.22
8.67
16.30
10.38
29.32
13.58
8.15
28.86
CaO
3.50
11.75
10.53
17.83
29.32
21.74
Na2O
0.29
2.52
0.54
0.34
0.08
0.10
0.28
Source
Salters and Stracke
(2004)
Klein (2004)
Honnorez and Kirst
(1975)
Honnorez and Kirst
(1975)
Li et al. (2004)
Li et al. (2004)
Schandl et al.
(1989)
!"&$Q#!!"$!Q"]%#
equilibria. The situation is less complicated in rodingites from mid-ocean ridge settings,
@#"!$%#`![/@@[j$
_W"!$B!""@//V$"%
!/
"!$/!VL@"!$!#!!"!"/@`<|j
from the mid-Atlantic ridge (MAR). Oceanic rodingites from the MAR (Honnorez and
X[ '*j[ !/$" ~" `{ [ FFEj[ / $" +/@`[':'j""/#Vably, rodingites are depleted in SiO22[$"!
@/BV@
##B/B[!#/
!B/
#B[/C""B/
"""[#$
B!C@!BW@&
#/!C`/[
"['*jL"B"V/$$/CB!C"$"/
V$!C/%#`
"['*{[FFEj"@$
[</[V`YB
[''j|`FF*jVB//"!$
activities in rodingite formation.
The intention of this communication is to revisit the problem of driving force for
/C@BW"/%#]"!@#BC$
"Q#"Q/L"!@B!"[
"Q&%#@#$$B@B!C
4.2. Method
122
"!#$#@#"$W[$#-
4.2. Method
ZU
ZU
[= 0
ZU
'LIIXVLYH WUDQVSRUW
[= 1
[= 1
)OXLG IORZ
ZU
3HULGRWLWH
[= 0
*DEEUR
[= 1
ZU
ZU
'LIIXVLYH WUDQVSRUW
,QILQLWHVL]H
[= 0
3HULGRWLWH
,QILQLWHVL]H
Figure 2. Schematic representation of the rationale
behind using titration models to describe metaso" ! `"Q $" [ ''*j Ç is
the reaction progress and ranges from zero at the
$!%#"!
@B W ! "# $ & @ %#[ #@ & A titration model is appropriate in assessing the
qualitative phase relations developing either in an
VV /" `/ %# % !j $$#V/"[&@BB$"
%#Q Q# `+j B $" @#B/#W%#
composition (B).
ous species and dissolution of minerals in
B"</]
]]]+]]
]]
Y # #/
L'
@`[''j
L $
L' @ the thermodynamic data set from Helge`'*:j$"[&
Y/ `'::j[ & Y/ `''Fj & `''*j $
dissolved inorganic aqueous species. The
@##!/$
slop98.dat and spec02.dat databases (see
Wolery, 2004 for details). Data for Fe-ser![@#[&
$"<
"|`!j#"!$E,
MgSO4o$"`<
"[FFFj
as aqueous Al complex data from Tagirov
and Schott (2001). The data base consists
of standard-state thermodynamic param[ <]XB $Q[ equation of state parameters for miner-
Table 2.VV$!"
#
Reacting solid
/%#
Temperature
}]&
Ç
1
Peridotite
]FF;
1
0 to 1 (T)
2
Peridotite
]FF;
1
0 to 1 (T)
3
Peridotite
]EFF;
1
0 to 1 (T)
1a
Plagioclase
Fluid 1
FF;
ÁFFF
F`}Uj
2a
Plagioclase
Fluid 2
FF;
ÁFFF
F`}Uj
3a
Plagioclase
Fluid 3
EFF;
ÁFFF
F`}Uj
1b
!BW
Fluid 1
FF;
ÁFFF
F`}Uj
2b
!BW
Fluid 2
FF;
ÁFFF
F`}Uj
3b
!BW
Fluid 3
EFF;
ÁFFF
F`}Uj
1c
Gabbro
Fluid 1
FF;
ÁFFF
F`}Uj
2c
Gabbro
Fluid 2
FF;
ÁFFF
F`}Uj
3c
Gabbro
Fluid 3
EFF;
ÁFFF
F`}Uj
1c‘
Gabbro
Fluid 1
FF;
ÁF
F`}Uj
2c‘
Gabbro
Fluid 2
FF;
ÁF
F`}Uj
3c‘
Gabbro
Fluid 3
EFF;
ÁF
F`}Uj
123
Table 3. Composition of peridotite starting material
Minerals
wt. %
composition
Olivine
77
Fo90
Orthopyroxene
18
En90
Clinopyroxene
3
Di90
Spinel
2
pure
Oxides
wt.%
SiO2
45.24
Al2O3
0.91
MgO
44.02
FeO
8.77
CaO
1.06
Composition of gabbro starting material
Minerals
wt. %
composition
Plagioclase
50
An80
Clinopyroxene
50
Di85En8.5CaTs6.5
Oxides
wt. %
SiO2
50.61
Al2O3
18.09
MgO
7.79
FeO
1.39
CaO
20.83
Na2O
1.18
als and aqueous species that are used to
"!# #@#" `{/ Xj
for temperatures and pressures up to 1000
;
FF < ` [ ''j
We calculated equilibrium constants for
50 MPa and temperatures from 0 to 400
;
;
""@/
X@$#_UL"!#_U%#!
"#@B#
as reaction path geochemical modeling of
%#]& `}B &[
2003). We used the B-dot equation for
#$VB$Q$V / ! | W ^@B]Y& !" " $" }B
`FFEj ^V # ! / #B VB $Q[ W!!/#![$
2VB$Q$"^#""
`':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, anj[!`C[!
2FeAl2Si3O12(OH)), and chlorite (clinochlo[!j!"!@"!V</'F
"#""$
B#"!!/#!"[@#$$@"B"
!!$BCV`_V[FFE|[FF*j
antigorite is rare in oceanic serpentinites. The garnet phase in rodingites is commonly hydrogrossular. Having [(OH)4]E substituted for [SiO4]E[B/##@W!$""V/#L&$"B"
$&"!!"##B/##-
124
4.2. Method
#"#YV[
hydrogrossular in most rodingites has
B]Y2O (Honnorez and Kirst,
'* [ ':' YB 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 andalusite, antigorite, boehmite, corundum,
diaspore, dolomite, gibbsite, huntite, hyWB!C[&[&B[[
magnesite, margarite, monticellite, paragonite, pyrophyllite, and sillimanite. Also
#!! # $ #$
Figure 3. Results of reaction path model calculations,
in which 1 kg of harzburgite (Table 2) was reacted
with 1 kg of seawater at temperature between 25 °C
and 400 °C. Shown are: (A) variations in the compo\'
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[$QB$B¤/@#""!
$QB$!!!#B#//@
}&/ #"! /C !C & !
"!#B[$/!!#"#/CN
}#!$&/$;
#/_[#/
_[##!!%#&/$!!"#[]&B"`jFF;
[
`jFF;
[`jEFF;
@BF<}#!!
%#!/`+:Fj[!BW`^:_:
Lj[
"/@@`F!/[F!BWL@j[/!
/"#$&QW"#$%#<#W"
/!#/!/V@`Çj[$#$"!#Q$#$#$&L
&/!/B"&"
VV#/QW"#$%#`&/jL
for using this type of titration model for metasomatic processes is provided in Fig. 2 (cf.
[''*j"""[%#"!@#B$B"
@BW!`Ç¤Fj+B$"@#B[/%!"!B
/ @#B[ %# "! @#$$ @B &`Ç¤j
4.3. Results
4.3.1. Reaction path models
+$!"#`L@jLQ#""#//$%#/!`L@
jFF;
[FF;
[EFF;
L%#/!/[
clinopyroxene, and gabbro (Table 3) to simulate the interaction of gabbro and gabbroic
"!%#!#@B!"!#$FF;
[
FF;
[EFF;
`/E]j"[FF/$/@@
&/$%#$"!]"W""W%#/VB&`/*j
126
Figure 4. Results of
plagioclase
titration
* tion progress in terms
of plagioclase added,

1 g of plagioclase add
\*
\ 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
\ plagioclase is added,
the middle three panels
show the equilibrium
mineral assemblages,
and the lower three
panels indicate the
solid solution compositions.
4.3. Results
127
Figure 5. Results of
clinopyroxene titration
models. Ç is the reaction progress in terms
of clinopyroxene added,
where Ç= 1 represents 1
g of clinopyroxene add
\*
\ tions were calculated in
reaction path models depicted in Fig. 2. 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
\ clinopyroxene is added,
mineral
assemblages,
and the lower three panels indicate the solid solution compositions.
128
Figure 6. Results of gabbro titration models. Ç is
the reaction progress in
terms of gabbro added,
where Ç= 1 represents 1
g of gabbro added to 1 kg
\*
\
compositions were calculated in reaction path
models depicted in Fig.
2. Temperatures are 200
°C (panels A–C), 300 °C
(panels D–F) and 400 °C
(panels G–I). The upper three panels plot the
\ sition as gabbro is added,
mineral assemblages, and
the lower three panels indicate the solid solution
compositions.
4.3. Results
129
Figure 7. Results of gabbro titration models. Ç is
the reaction progress in
terms of gabbro added,
where Ç = 1 represents
100 g of gabbro added
\*
grams start in the left with
1 g of gabbro added (Log
Ç $' anextension of Fig. 5 to
\ *

\sitions were calculated in
reaction path models depicted in Fig. 2. Temperatures are 200 °C (panels
A–C), 300 °C (panels D–
F) and 400 °C (panels G–
I). The upper three panels
\
composition as gabbro is
added, the middle three
panels show the equilibrium mineral assemblages,
and the lower three panels
indicate the solid solution
compositions.
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
/QB$$"!$!C%#L%#!Y!
@##`!Y¤[!XWEFF;
j"#$V
$""U&/`"<j
$%#!@/
(35 – 40 mM) throughout the entire temperature range. In contrast, Si concentrations of
%#!"&B/"!#L!$@B
$V!@F;'F;
["L±F
;
@#@B!]@#"@/[@#$$/##
activity [aSiO2`j°V#+[!Y*:
FF;
!XW$F'L%#@""&/"!#[$"F/#@V#BEFF;
/#@V
#BFF;
[/#@V#BFF;
""!#
/[#@#"`{/Xj$`2(aq) + H2¤
HSiO3 + H+j$"FF:*[!!Y2(aq)
VB$%#V"W!@B!!
/+#["!$%#!##/!]
/B "!# ! / $" &[ 2(aq) at
FF;
#"#["2`jEFF;
[V//
#/#</#/B$$[+!
@#/#`±´<j<"!"/[W!$@#[
@"/QB"$"!#
Reaction of plagioclase
#$!/]%#FF;
[FF;
[EFF;
!B
/E}/!/Ç#$"$"/
!BW£/!£!!£!!/
FF;
L"%!V#
$!VB$%#|/@
131
@#!B//VB!+
and aH+&!!!@B!+/
"Ç[@#VV"
reactant plagioclase is added in the model. Secondary plagioclase is predicted to appear
""!!/L"$#
suggests that garnet and epidote-ss are close to the Al-endmembers in composition, and
clinopyroxene is diopsidic.
+FF;
[W!#!BW/£/
!£!!/+/Y+ co-evolve to
/V#/Ç[@//&/!/appears and secondary plagioclase appears. Al and Si concentrations are similar to each
[VV"!FF;
#""!"#$FF;
#[@#B!/"!/
L!#$B"EFF;
£!/
+ chlorite. There are subtle but steady increases in Si concentration and proton activity.
+!@#/#+
"!#[+/Ç[@#V/
FF;
B!/!!!"EFF;
"!#+Ç$F[!B""@/$
$"/FF;
/!FF;
EFF;
=,+
#$!BW]%#!"/+FF
;
FF;
W!B"/B#$"B!BW/+@"!#[#$"[+!V+!/]%#!"[
!V/%#"!/FF;
+EFF;
[##$B""&B$$N
" "/ £ !BW " "/ |!Y+%#[
L#B""!"/[#@
/Ç
Reaction of gabbro
#$/@@]%#!"!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 Ç, while the thick lines represent reactions predicted to run at lower Ç (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??#?[

\
at high pH and low SiO2. Gabbro (cf. Table 3) is predicted to form tremolite–albite–clinozoisite–quartz
\2+/a2H+) of about 7.1 and log aSiO2$*X*>
_\
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
'*\

not make rodingite along its evolution to equilibration with gabbro (short-dashed line).
" `/ E jL # $ B " "@/ FF ;
!BW/£!BW/£!BW/
!£!BW!"[@#B!B@"@!!
$$VB@#$$!!//"
""@/L#/#"#[#"&
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134
4.3. Results
Table 4.

\
|
different mineral assemblages at 300 °C and 50 MPa
pH

Ca2+
Srp–Di–Brc
Srp–Tr–Tlc
Tr–Ab–Czo–Qtz
7.31
12.5
1.51
5.81
11.2
1.55
5.30
27.4
3.82
CaCl+
8.62
8.86
21.69
CaCl2(aq)
0.74
0.76
1.84
CaOH+
1.58
0.01
0.04

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=
2+
2
+
Log (aCa /a H )
10.73
7.74
7.11

*><
*?{2 and 0.085 for Ca2+.
j
$!VB#@BB["+$BB#F"<
!BW"
"/[//©Al in the course of the model run.
+$$!#!$EFF;
[!B"/BB"@BB[!"Q
$"#L##£!
!BW£!!BW"</
and aH+$"%#!!BW!!![
Al concentrations increase continuously. The secondary minerals are magnesian, epidote$B@#V!"C"!tion progress.
/*!#$%#]"#@#"##"&
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135
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prehnite + tremolite + chlorite + talc +
plagioclase. Si concentrations are pre@B[!Broxene 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,
Figure 10. Summary of the mineralogical changes
within a gabbro dike away from the contact with a peridotite undergoing serpentinization at 200 °C (A), 300
°C (B), and 400 °C (C). At Ç?
\
<
controlled by serpentinization reactions (W/R = 105).
At Ç = 1 the W/R is 103, and at Ç = 2 it is 10. The mineralogical succession is discussed in detail in the text.
The dominant reactions in the order of decreasing Ç,
i.e., with decreasing distance to the gabbro/peridotite
contact, are (A) = 200 °C: tremolite + prehnite £ diopside + chlorite, (B) = 300 °C: prehnite + tremolite
£ cpx + epidote-ss £ garnet + chlorite, and (C) =
400 °C: plagioclase + clinopyroxene £ epidote-ss +
tremolite £ chlorite + clinopyroxene.
136
+"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%).
LW!#FF;
!BW!£
clinopyroxene + epidote-ss + tremolite
£ !BW ! " ! £ ! "
! £ ! "
prehnite + plagioclase. Mg concentrations
and pH remain fairly constant throughout
#
steps and plateau close to quartz satura}U`{/UX¤F}U
4.3. Results
¤Fj}]</!"/#/#[!![
secondary plagioclase is intermediate in composition (An 62 – 64 mol. %).
+EFF;
[!/V"//N!"
!BW£!"!BW!/{&
temperature runs, Si concentrations are predicted to increase initially and then plateau,
@##C"##`{/UX¤FE}U¤FjEFF;
"!#
$@+</"[Y+ increases
/B # B " "! "/ [ "[
!BW[!©
~ from 0.7 to 0.8 and calcic secondary
plagioclase (An ~ 80 mol. %).
"!VW"$!!/!$/
"/V@#@$`
"['*['*_V['**
[':YB[''{[FFE|[FF*j
V@B!$!!L!/"/:![@cause it helps understanding the results of the reaction path models presented in Section
!B"
]</]+2O3–SiO2–H2O and
MgO–SiO2–H2O (blue dashed lines) as a function of temperature and silica activity. The
<+Y ! ! / :+ V $ !B ! #V[!/#`[j[!/CU
!#ÙE[*U:jLVB$
/%#}ULV#$VB
%#/!#$/"!#!]
%#[V$%#$V]!pentine–brucite boundaries and actually deviate from the boundaries into the serpentine
Q@#@""$"!#`/
jV"!#/@FFEFF;
[$$V@%#
@B ! @B /@@
"&B"!#L%#$
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}U`/#jL%#@B/@@V/
V#@#C#`$EjL"
]+
@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!
"B@#$$
2+U2H+ and aSiO2`j!C%##/@@ V VB V@ # !/ %# @#$$ #[@#$$@B![![@#B[$"!
VV//#]!@#B//@@[%#@#$$@B"[![/@@@B"&/
"+ " %# @#$$ @B "]!]!
"B"&!/#[@#/"#/$!%#
+/ !@ ![ 2+U2H+ and aSiO2(aq) change, but the relative
$
2+, H+, and SiO2`j//V/$$#&
LW"#[!#@B##W"
$$ ! #@B %# @#$$ @B $$
#"Q "Q "@/ `L@ Ej | 2+U2H+ and aSiO2(aq) differ by
#/B $ "/# @ !]@#]! #"Q
@#$$"@/"]C]@]#C"Q@#$$"@/
L/
2+U2H+[V["W#VB#$$!Y`*
V#j|
2+V"&@B""/
buffer assemblages considered. These results indicate that rodingitization reactions can@V@B$$
2+ activities. Instead, the differences in the activities of
SiO2(aq) and H+ generate most of the thermodynamic driving force for rodingitization
reactions to proceed.
4.4. Discussion
The reactions during serpentinization have been discussed in numerous recent
W!"#`}C&[FFF+B$[FF
138
4.4. Discussion
B$[FFE[FFEB$[FF*|[
FF*<
"|[!j#!C"#@
Q/$##$!C"!V#/!C""!BV!#!$!V/#%#
are hypothesized to cause rodingitization in intercalated gabbroic material.
4.4.1. Modeling of rodingitization
L"#"@/B!$/$"/@@%#/#/!CFFFF;
}
%# "! @B /@@ `[ Ç £ j[ B!
greenschist-facies mineralogy is predicted to develop. In the geochemical models pre[!/V@Ç"#$"
&@%#+/Ç$"F[&
`/}Uj$"`/E]j$"/*$#[#/
& !BB #@ $ !C]/C
B"!$"$$!$!B#$"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@Ç#@
V!WB$$"/@@!/@@`$
Fig. 2).
+VV$#"///@@$#
$!W"B!/F[@#
$"!#"CFF+!]"]]!/"@/$"//ÇFF;
`/F+j!
139
@!@B!]!BW]QB]!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!["[
![ !/} / Ç[ " ! ! become replaced by epidote-ss and clinopyroxene. Proximal to the contact, epidote-ss
!BW/VB/[!@B!BW
at the contact.
+EFF;
/Ç`[%#"!@B
/@@j[!/!BWW"+Ç
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j+!!/`Ç£Fj[!"/VBB!BWQB""\!!
$"EFF;
#""B["!$/Ç[[
//@@U![!/!BW
£!"£!BWEFF;
[!"£
!BW!£/FF;
["!£
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We suggest that our simulations provide valuable information on the sequences
$""@/V!/@/@@!
/@@[/@@/Ç|##
"#V&!B![Ç|##V!"Q]#"Q@#B@"!
!/!@V
,%
Gabbroic veins and screens from fracture zones on the equatorial Mid-Atlantic
//B/W$/C`YCX['*j!V#@Q!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;
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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%#!$"#"Q"Q/#/CW/$#/@@@B#$"\@@"!Y^!WB
this type of relation. The least altered samples are characterized by tremolite + chlorite +
!!/!"@/[!/C!
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V/V!`Y&[''\[''j"B[/@@&V$"\#~;_Q
"$@#!V/`|#[''j
\ ~ & B/# [ V !@B!"/`|#[''j}!Q/
#$V]![&$@#$$
##V##//$"
4.4.2. The critical role of aqueous silica
Our model results highlight the critical importance silica activities play in rodingitization. That silica activity gradients play a role in the formation of rodingites and
@&#"Q&@!!@$|B`'**j
noted that diffusion imposed chemical potential gradients mainly of silica and magne#"/V"$@#$#"Q@YCX
`'*j!!!"$!@BB/!"@B
consumption of silica in the serpentinization reactions. Frost and Beard (2007) realized
that the silica activity of brucite–serpentine equilibrium oversteps the reaction transform/!/#/#V-
141
$!C%#!B/CL!
!B#/:@!/$$"
garnet, diopside or chlorite, the activities of aqueous silica set by brucite–serpentine equi@#"!V"B"V$&!L$$
"!$##/"!#@FF]F;
+
higher temperatures, olivine is stable and brucite is absent. The aqueous silica activity
$V]!@#$$/B"!#[EFF;
@V[VB//C&!`
F<j+B$`FFj!!%#$"V!
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L!/'%#@B]"!Y[
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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$
##!"$XC&`':j[!$B"@
"!"$L"!`'*FjX"!L"!
B @#$$ @B W ! `N !Cj "V$B!%#B"[W</
+[!VB/[!#FF;
FF;
`/E]*j["B
$$#!%#$&[/
VB$
%#W"B"`/E]*[L@Ej^$$#V"
!$Q[}}&`''*jV""/C
$ " ! $ #/ / %# !@
V//CCB!B"`/[LB[
'
"['*+#"V&[FF:j$&##2 diffusivi$FF]FF;
"!#/$F: to 10'm2 s`}}&[''*j
142
4.4. Discussion
!B$F[$$V@#&@#$Q[//#B[
10 to 10 m2 s}$$#VB[//#B&[
V/$$#V$##"]F&BL"
easily matched by estimates for the minimal life span of peridotite-hosted hydrothermal
systems (Früh-Green et al., 2003).
8
#!/#$#V/!!&$//
2+V!%#"Q]#"Q`jLQ/
"!$
"""V@B
%#/#/!C`/[
"['*{[FFEj}#//#$##"##!$
L!#
L@E
Y+ 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%#Q["$
/$"F:
`$L@j##""#"&$FF[$"/%#
F""U&/
##"!
/L!@"/%#W$%#/@@"&/
`FFEjV[FF;
[
!@B$"/#!Y$":]!B"&
$"FF+]!
$"FF#@/`
2+U2H+j$%#@B
/#L!/'B#%##
@@!#/L#//Q"""
B@%#]"@B#"&/$
%#@B!]&/!
&L!#@$#B#//
""!" %# ! #/ # `/[} V[':§B[''j!@"["!#
@//#$&"!CB"
""!VV#$/C"&W"B#&B
}!!V$/"#$
%#-
143
#/C<&B["$/B$$#"
@B//
!L!$!C"!##FF]FF;
#//V/$$
rodingitization. The thermodynamic constraints discussed here demonstrate that rodingiC`
"""j#"BV@B//!
##
"$%##"![
#/
!@&"!#@FF;
B
facilitate rodingite formation as suggested by Frost and Beard (2007).
9#9#9#>
L"#W!""!"@V"Q]
#"Q C L"!# !! V / $$ ! "@/{/"@B!/""
V/`/[YCX['*[
':'B[FF*j
V$""@#B
!//$#!#!!`XC&[':j
L!$""V!/@#BEFF;
F
< "B W! $" $ $ @& `/[ [ '* { al., 2004). These calculation results also explain the common alteration rim of chlorite
#!/%!
^$$#"$"Q]#"Q@@""
!"//#"Q&@"$//CL!/:#//B
! / %# @B /@@ `[
{/}U¤j##[""!
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$/`/[|[FFEj!/[
[VW@[@W!@#@
""/[#/B"!$!
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$$"
the solid phase assemblage (e.g., reactions 3, 6, 7, 10, and 13 in Fig. 8). In fact, the diop]/"@/!$"/&B#
`BB#[$/B/j[!V$"
"!VB@#///"!#`:FF;
j
$ $"`B [ FF*j ^! # $ V %# VB
/"!##!#!Q$##!
the accretion of the igneous oceanic crust (e.g., Dunn et al., 2000). There may indeed be
petrological and geochemical evidence for deep and high-temperature circulation in the
"!`/[|[FFE\/BLB[':j+/$"
"!#$!V#!!"""#
@ /Q ! @/ ![ /"!# # } high-temperature origin of the diopsidite veins (Python et al. 2007) is possible, our model
##//!&#$""!#FF]
FF;
@B""$/@@!BW%#B
externally buffered by serpentinization reactions. The model calculation of clinopyroxene
FF;
FF;
`/j!
L@/"!$!"B!BW@"!/[B
clinopyroxene is virtually pure diopside (Fig. 5). The diopside in the Oman diopsidite
!+[[
W!$$""!#<V[
small amounts of garnet commonly accompany diopside in the veins (Python et al., 2007),
#!
!BW!
!@!@B"]"/]"!#$EFF;
+B/"!#[VB$##%#W/V
#@/#$#C#`$/|[FF*j[
!@/VB!#/$"##
B"/B$"@B%#[##
145
VB@#$$W"BV@B!]@##@#"|#
@#W!$"$V@&"!#@VF;
[
$!/#/@@BB`FF*j/$""!#$:FF;
|"!
$V!!$/$"!#$
formation for the Oman diopside veins.
4.5. Conclusions
(1)
(2)
Titration reaction path models can be employed successfully to reproduce the
mineral assemblages commonly formed in rodingites and provide crucial insights
into the main driving forces of rodingitization reactions.
At a pressure of 50 MPa, rodingitization can proceed at temperature around 200
FF;
[!C%#@B@#]!]!#@#"+"!#$EFF;
/[@#@
%#@#$$@B]"]!#%#[&$
`j
Èj
`j
146
! @& "& %#[ $" / //@@@$/@@"
(some are rodingitized, others are not) can thus be related to the temperature-de!%#]"#@!
/C"&B#$$$#"""LVB
/!#@#"Q"Q
/!LVB/
2+ is virtually
^$$#V"$$
/&B$$#$
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@/#B[/%#%#W&B!#!
VB/#V/C#
!$
$"B"
L$$#V$$"&#//V/C#
V@"""#"Q&[@W#$
@#[C"B$"![##
@B!/#V/@@
L$"$!V""B&!"!#$FF]FF;
$"/@@!BW!B/
"!#`:FF;
j#$"$"!B#
assemblages.
4.6. Acknowledgements
9#;#*+
}&§/#$V@#!#<
YL"<
"!V#!"@/"B"
@B"#V&$#V""
"#/#[YÉ&+#"[="#
!!L&#!!@B^##//"$`^\j/
|FU}|@B^\
U_W
#L
_B"
147
References
+@[L+[#[
[|&[X[{B[\{[[[V[
<[
':: <B/ / ![ ~"@ ![ !!N ! /
"\/B*[]
+/[[
[<[''*WB/!!C!
#"Q&$"<+/`;jNX[+[
[
<[<[^[_[^`_j[/$^//"[
Q#[V[!!:]::
+/[ [ [ \[ |[ <['' </ WB/ ! $# $ ! V $" U @+@BN}"[|`_j[/$^/
/"[Q#[VE'[!!E]
+/[[Y&[[|#[^[VB[<[''Y@!"!$##!!"&W!Y^!
\!/L!#_B{[:]'
+[^_[B$[}_[FF
"!V%#$"#"QB"B""/NW!"#BEFF
;
[FF@\"
""+*`:j[]E
+[
[ & [ }
[ |[ }[ #&[ Y[ \[ [ |#[ \[ FF*
Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro
W!@B"$#/<+/[;F`^{/
F'jN#$#WB/!#B\"B[\!B[\B"
`:j[F:FF[NFF'UFF*\
FF*
+[X[_V[<[|&$[<_[FF\"B$#"Q]/
&!C^WV/@[#
\#$+$_E`j[]E
+#"[[{#@[Y['*L<+/E;[©[!C
#"Q#
#$_:[]
+#"[Y[V&[L[FF:/CB$!
V!{&![B{FE[**]':
|[}[\[
[YVB[[#&[Y[[<[FFE@]!]Q/$"^{/F'[<+;\"B[\!B[\B"`'j[F'[NFF'UFFE\
FFF*EE
|[[{"[
[Y""@/[\['*\"V$!B
serpentinization. Science 156, 830–832.
|[[®[[''L!@%#"$!B!
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|/[+[B$[}[':^V!"$/@&![_{/#[B
@#</B/B:E[E]
151.
|[[#/[^[[+X[FF\@@!/"#[@+@B[^{/*[F*FN"/""#/$"V$/%!/#$/BE`j[::]'F
|&[<[<XC[^[':*L!$"@B%##/""!"
@#</B/B'[:E]'
|#[^[Y@[[Y&[[
[<[''<"!"$!
&$"\$L$"`_Q;j#nal
of Geophysical Research 96, 10,079–10,099.
|[^["[<[|[[[+[^#[<[+/[[FFE^!
/"!#B"#"!À!/!V#$/BE`j[:]F:
|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
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^#[+[L"B[^["[
[FFFL""##
!B!!$#"@_Q
';F#$\!BF`|Fj[[*][
_V[|}['**<"!"$!!!+#V
of Earth and Planetary Sciences 5, 398–447.
_V[|}[FFEL!"#B"VNB"@\/BVE[E*']F
B[<[^!![\<[''#%["["""\ogy 19, 211–214.
[Y[/[<[@[_['+#B[
$[!/$##
"\"
""+[*]:
149
[|['*
""!"$![@&/B\_B[
[}/#$/B
16, 272–313.
[|[|[[FF*VB!C#$ogy 48, 1351–1368.
\[\{[XB[^|[|[<[X[+[{#/[X+[|#Q[ ^+[ |[ [ &#&[ \[ FF F[FFF B $ B"
VB{
BVQF[E']E':
\[ \{[ [+[ ^®+/[ {[ '' </ @ !
$!B!$#!!#/@@$_Q
`Y^![:'EjN<V[
[\[X<[+[[<B[`_j[
/$^//"[Q#[VE*[!!]
254.
\[\{[[+[{#B[
[''@/@!
B" !C $ _ " Y^![:'N<V[
[\[X<[+[[<B[`_j[
/$^//"[Q#[VE*[!!F']
163.
\/B[L[LB[Y[':+WB/!!Q$
#
#["!["N_V$¨:@#$$/$
@B!`&"jB"#"/#
of Geophysical Research 86, 2737–2755.
Y&[ [ |#[ ^[ #[ [ "[ {[ +"[ [ {[ [
|#"[ [ '' /B $ _ Q # #!! " W!Y^!`#Qj#$\!B
98, 8069–8094.
Y/[Y
[^B[<[@[Y}[|[^X[ '*:#""B#
$"B"!!$&$"/"+"#$
Science 278A, 1–229.
YC[[X[['*/B$/$"#<+
$# C / /Q @# </B
and Petrology 49, 233–257.
&B[^[B$[}_[':YB"!C$!#NW!"V/$"/B""
"B\"
""+F[*]*:
[ }[ &[ _Y[ Y/[ Y
[ ''
L'N $ !&/
150
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##![$"]FFF@F]FFF;
"!#¬
Geosciences 18, 899–947.
X"[ |[ X&[ _[ <[ ^[ {/ F' !@ B[ FF* {/
F' #""BN ! F&"& #V @#B B @
<+/[E;]; N X"[ |[ X&[ _[ <[ ^
`_j[/$^//"[Q#[VF'[
pp. 1–33.
XB[^[X[+[|&"[^X[\[\{[|#Q[^+[{B[
<^[[_[&[<[[XX[{@[\L[VCC/[[+LF
!@B[FF+$$WB"VQ<+
/F;#E[*]:
XB[ ^[ X[ +[ \[ \{[§/[ ^[ &[L<[ |#Q[
^+[ YB[ <[ &[ <[ [ _[ &#&[ \[ &#@[ <[
|B[+[{[|[{#/[X[\&[^[|#&"[X[|B[+[
|C[ }[ [ X[ _[ <[ ^#[ +[ |[ <[ {B[
<^[|[+[#""[_[BV[[FF+!B"N{
BYB"F*[E:]EE
X[_<[FFE\"B$/#
#NY[Y^[L#&[XX`_j[L\"B_V[+""[!!E]
EXC&[^[':LB$""C/<#"^posita 3, 222–231.
XC[ [ ': B"@ $ &$"/ "+" </ :[ **]
279.
{#B[
[\##[\[''WB/#"!"!$Y^!
/@@`Y:'E:'E\jN/"!#$
#BN<V[
[\[X<[+[[<B[`_j[
/$^//"[Q#[VE*[!!*]
234.
{[©[~/[{[}[
[+[§[
[[FF*/B$/V$"
/L[
#$<"!/B[]
382.
{[©[[<[|#[X[FFE<"!!/$~"!\/BVE[:]
<
"[L<[FFF\"!"B!#VB#@"
hydrothermal vent plumes. Deep-sea Research. Part 1. Oceanographic Research
Papers 47, 85–101.
<
"[L<[|[}[!L"B"B//
151
#/!C$#"Q&\"
""+
NFFU/FF:FF
<
"[L<[&[_{['':#]&#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']E'
<#C[[ [ [ FF* # /" / $ /@/ /
from the Karabash alpine-type ultrabasic massif, Southern Ural. Geochemistry
International 45 (10), 998–1011.
[ [ /[ \[ ': YB/ / $" " # & "
_B{[]F
"[
[}"[+_[FF*B""/$/$"NV$"/%##<+@"[+@[#@\"L`:j[NF:UE*
4866-8-11.
®YB[^[[_[}&[[''L/$/$"
[
|
#"@[#"L`Yj#/!C
\"
""+['*]F:
[ {[ [ <Y[ FFE \" " $ """ #"Q
B"N!C[/C[%@"B!!\"
""+:`j[]
B[<[
#[\[[§[|[+[+[[FF*"!N
/B / $ VB / "!# B" # "!@!/_B
{[:']F
[<Y[''*YB"!%#"!
N|[Y{`_j[\"B$YB"^!}B
¬[§&[!!F]
[ <[ ': <"!" $ /N ! ! $
B" ]</]+]]
]Y+" # $ 283B, 121–150.
152
References
[<[&[+[FFE
"!$!"\"\!B\B[FFFENFF'UFF\
FFF'*
[_[®YB[^[}&[[':'/!C#"Q
&$+@@\@[
</*[*']'
B$[}_[^@@[}_[':F]!FF;
FF@N"!$/$!\"
""+EE[F']
B$[}_[##&[^[+[^_[FFE"QB"
B""/N"!B!Y[W
@#N\"[
[{[[[{<`_j[<
/NYB"|{!
+"\!B[}/[^
[!!*]:E
B$ [}_[ ##&[ ^[ #[ [ FF* W V# " $
#/!CNW!"#BFF;
[FF@
"!$#"QB"B""/\"
""+*[:*]::
&[ _{[ Y/[ Y
[ ':: # $ "B" !
!!$##!/!#"!#N/"$!#$!&@FFF;
\"
""+[FF']F
&[ _{[ Y/[ Y
[ ''F # $ "B" !
!!$##!/!#"!#N!
"!!$/!\"
""+EÈj[
915–945.
&[_{[[^
[}[<[V&B[^+[''*/!// %#N "/ " "B" !! $
##BW"!W\"
""+[
907–950.
L/V[|[[[FF+#"#"!#%#V\"
""+[']''
Thayer, T.P., 1966. Serpentinization considered as a constant-volume process. American
Mineralogist 51, 685–710.
L"![+|['*E
]$$#C@"@!
#$/B[E]E
L"![|['*F\"!B"\"
"chimica Acta 34, 529–551.
}[[V[<[':!#!/""-
153
!"
@#</B/B*'[]*
}[ _|[ }&[ ^+[ ''* ^$$# $ V Y \[ "!$"! # #!! " @# Mineralogy and Petrology 130, 66–80.
}C[{[&[_{[FFF^/#/#"Q$"@#@"
B"B"@B"!/#V%#"!#
of Geophysical Research 105, 8319–8340.
}B[L[ FFE #Q $ "B" $ /" "/ $
"]#B"N^!"$_/B`_j|+
"!B[{{
}B[L[&[{[FF$®<#_U`V:Fj{@[+@###[<W
§B[|[|[Y[
$$[+[''_V$/%#V
#/"#""!"#E'[]E
154
Serpentinized troctolites exposed near the Kairei
BEJ=J
L&
unique microbial ecosystem
Abstract
L V %# $ X YB" `XYj //#CL!#B!LBVVB/Y2
[ VB / [ "&@B Y4UY2 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
L X YB" `XYj / `
j /#CL!#`LjV+#/#FFFQB
@VB"V`\"[FFY"
[FFjL""!$V%#[#/"V!`/[[
[</[X[4j/`
Y4[
2, H2j[Q!@B\"
`FFj[/X%#/B"B"%#
$"B!"/Q+#@#V/[V[VX%#V###B/$
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!{<_j W #@% V" $ XY `L& [
2004). This microbial ecosystem is sustained by the primary production of hydrogenotrophic, hyperthermophilic methanogens, utilizing H2
2 as the primary energy and
carbon sources. The H2
2 are completely photosynthesis-independent substances,
!VB@B//`B"j!["!B/YB!{<_
&B"/#$B_B"!!B`L&
al., 2006).
YB//"!#B"V%#"X%#V@!$"VB"VQ$@[
{/V[+C[@#//<+/
`<+j`
#[FF[FF*<[FF:jL<+Y2-rich hydro"V%#V/B@@#!C$@B!@B#!$"!B"VQ
`
#[FF^#V[FFj[@//Vdence indicating the involvement of peridotite in the generation of H2-rich hydrothermal
V%#XY`^V[FF\^""[FFj[#/!B"%#/BW@/
Y4 and
"!@`/[
#[FFj[
XB"%#V
Y4 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
A
30 N
15 N
AFRICA PLATE
S2
RCI
Central
Indian Ridge
(CIR)
0
AUSTRALIA PLATE
15 S
Rodriguez
Triple Junction
(RTJ)
Southwestern
Indian Ridge
(SWIR)
30 S
B
Southeastern
Indian Ridge (SEIR)
45 E
60 E
75 E
90 E
Kairei Hydrothermal Field
S1
RCI
ANTARCTICA PLATE
45 S
25o00'S
95 E
25o30'S
25o30'S
RTJ
IR
SE
IR
SW
70o08'E
70o10'E
0
B
3800
3800
00
0
00
40
00
30
00
340
0
0
300
3000
2900
00
0
0
0
31
00
00
27
00
4
00
35
00
25
2900
35
3200
0
0
31
2900
28
00
30
00
28
00
29
00
0
0
0
2900
3000
2800
3100
300
400
310
0
3000
00
0
00
0
25
5
3000
00
300
0
29
0
00
3000
0
-4500 m
350
70o00'E
2500
00
40
0
-5000 m
00
South Hill
00
25
00
S1
40
0
0
0
300
0
0
00
00
0
30
3
69o50'E
28
3000
3
00
27
30
00
30
2800
00
00
29
2900
00
R-
35
00
00
32
0
00
30
CI
40
28
29
30
29
0
0
4000
4000
3500
0
3000
300
0
North Hill
00
00
3
0
31
28
25
C
00
30
0
28
0
0
28
50
350
Hakuho Knoll
00
35
0
35
3100
00
32
270
00
00
300
00
25o20'S
35
0
3300
3200
0
330
Uraniwa-Hills
00
0
00
00
32
00
35
35
40
30
320
29
0
00
25
0
00
5
40
0
3600
3400
300
350
3500
C
00
33
3600
25
00
2
25o10'S
3700
3700
0
30
00
00
70o12'E
330
00
30
25
35
70o10'E
3900
00
70o00'E
70o30'E
3400
69o50'E
70o00'E
69o30'E
31
69o00'E
70o10'E
-4000 m
3100
70o08'E
-3500 m
-3000 m
70o10'E
-2500 m
70o12'E
-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
&$@B/#/@#"B$
B"%#XY
5.2. Geological background
LXYQ/"$
""B$L
`;'[*F;FE_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
&"$XY`;'[*F;FE_X#"/[FF:j`/@jL
Y"[Y#Y[V@BFF
"!_}/VB`/jL!B"!/B$"
##/ @B W/ /! `}_j `/ j
L#/##!!#/WV!#
Y`/jLY/ " # Y[#/#/##@V#`/j^#/#V
Y[V&!B!W#"
#+#/#!$"!"!$!&
#[!/!BW!#$!&#//
Y!
"!W`
j`/[|&"['':j
""!$"BC@B_@<
+BC`_<+N_{©+::FF#!!@jXCVBLB
!$"#/V/$&@"#$F+[#/
´""@"#B"_{$#/
~+"!B$"#$_<+B
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;
!#$FF@+VB$Q$##!##/|#`Y/[
1969).
5.4. Petrography
}#EV/@@&"@B$"Y!/#['[$#V/@@$
&B!#![#@W!$"!"/
#!!"[@#B&B#$!!$
#U#"@#B`/[^&[FFFj"/V
B @ ! $" + `/ ;F #
~L&C[FF*+<$[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</#"@¯¤FF</U`</
Fetotal) atomic ratio] of the olivine cores in the plagioclase dunite and troctolites ranges
$"'::"V##::[$!"V@//$":F'*"V##'|#["""
![@V[!/V!L
! #@ # ! VB # "! igneous minerals. The plagioclase in the troctolites has experienced varying degrees of alL!#!/"B"!$VBQ
/[!""#$/#UB/#+`Yj
Small veins consisting of prehnite and chlorite commonly cut highly altered parts.
The olivine gabbros are also affected by serpentinization, although the degree of
159
(a)
Mt
Ol
Serp
Serp
Ol
Serp
(b)
Vn
Serp
Ol
Mt
Mt
Serp
Serp
(c)
Cpx
Mt
Ol
Serp
Cpx
Serp
Pl
Figure 2. Photomicrographs of the olivinerich gabbroic rocks from the Uraniwa-Hills.
(a) Mesh texture composed of serpentine and
magnetite in a plagioclase dunite. Relict olivine crystals are partly recognizable. (b) Mesh
texture composed of serpentine and magnetite
in a troctolite. Relict olivine crystals, as well
as serpentine + magnetite vein, are also iden
*'{
<<
<
serpentine and magnetite in an olivine gabbro.
Clinopyroxene and plagioclase are relatively
unaltered. Mineral abbreviations: Serp = serpentine, Mt = magnetite, Ol = olivine, Cpx =
clinopyroxene, Pl = plagioclase, Vn = serpentine + magnetite vein. Scale bar represents 0.5
mm.
alteration is less than in the plagioclase dunite and troctolites. Most of the olivine (85 - 82
$</j!B!@B!"/[#$
adjacent to plagioclase. Microfractures in the olivine crystals are lined by serpentine to/Q/"/`/j
!BW!//B
!VV/@@"#$"!/!Broxene rims and plagioclase.
/#!V@#"!["$V"/netite (Fig. 2). This type of veining is commonly observed in serpentinized abyssal pe`/[+[FF*jV/@@&`[FF:j[
&B$""#$VB!C`|[FFj
!C[!/##@/%
/[#/V!"$@WBBW
160
5.5. Discussion
5.5. Discussion
5.5.1. Origin of the high H2LB&
The exceptionally high H2XY%#"&@
@V%#"B/V$"/#
Y2C/"""B$XY%#
#@[\^""`FFj!!"
basaltB!@$%#"B[#usually high concentration of H2/@B!!$"#Q
"[#N
(1)
`j
Fe2+ + 2 H2£2 + 2 H+ +H2,
#+ + Fe2+ + 2 H2£
#2 + 0.5 H2 + 3 H+.
Table 1. Selected chemical and thermodynamic parameter
Temperature (°C)
Fe2+
log aFe2+
log aH2S
pH (@25 °C)
in situ pH
JH2
observed H2,aq
Log K (rxn1)
predicted H2,aq
Log K (PPM)
predicted H2,aq
Log K (methan.)
£+methanogenesis
Kairei 6
Edmond 12
Comment
365
6.0
-6.19
-2.41
3.44
4.42
1.214
7.9
-2.17
0.055
-3.68
0.86
8.59
-20.0
370
13.1
-6.17
-2.33
3.13
4.17
1.301
0.25
-2.20
0.027
-3.64
0.99
8.35
57.3
a
a, d
b
a
a
b
c
a, d
e
b, d
f
b, d
g
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
e: = J* 10^[log K + 2*pH + 2*log aH2S+log aFe2+]
f: = J* 10^[(log K + 2*log aPo)/(4/3)]
g: CO2,aq + 4 H2,aq = CH4,aq + 2 H2O

161
We calculated the equilibrium concentration of H2 predicted for reaction (1) to as$"#Q!!V@W!$@VY2 concentraXV%#LY2 concentrations controlled by the PPM (pyrrhotite!B"/j@#$$`!BEUY2¤U"/!BEUY2j
# # # $ V %# $" X _"QV""!#`L@jY2 concentrations for
`j±F"<@%#[<@#$$!/VY2
$""<L"W!"Q/
!$@EFFE;
`B$
[FFj$W!/V$Y2X%#`:
mM), and reaction (1) falls short of supplying enough H2 by a factor of 140. These simple
##//[$["#Q!!VB#&B!sible for the unusually high concentration of H2XB"%#
YV/##!@BB/!#/@
B"!#:"<Y2[WW"$ & W! VB $ XY !V V@ W! $ high H2"#XB"%#&
!C$"!["@BV[/Wtremely H2B"%#`/[|[''}C&[FFF
+B$[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
@#$"!`![Table 2. Composition of rocks used
in reaction path models
162
wt. %
Troctolite
Basalt
SiO2
Al2O3
FeO
MgO
CaO
Na2O
42.3
6.4
8.3
39.0
3.6
0.4
51.5
16.1
8.4
8.5
11.4
3.0
5.5. Discussion
!BWj/$"#@#$@#L
"#!C$&
of unity can produce more than 16 mM of H2["Y2 concentrations in
V%##/$"@""@B!`
#[FF"
et al., 2007).
$#$#"#<N/@
L!C$V!#/B/@N
(3)
(Mg0.9Fe0.1)2SiO4EUFY2£
U</3Si2O5(OH)4UF</`Yj2U3O4UY2.
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£</3Si2O5(OH)4 + H2O
@###£!
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
-1
A
Q
-2
log aSiO2
Kairei soln.
on
tz saturati
troctolite calc.
Tlc
Ol
Tlc
Srp
-3
p
Sr l
O
peridotite calc.
-4
Ol
Brc
Srp
Brc
-5
250
300
350
450
400
Temperature(oC)
log moles dissolved species
-1
B
Qtz saturation
-2
-3
H2
SiO2
-4
H+
-5
-6
0
2
4
6
8
10
grams of basalt encountered
Figure 3.'
\
{{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
164
+2Si2O8 + 1.25 Mg2SiO4 + 2.5 H2£
F
2 Al 2 Si 3 O 10 (OH) 2 + 0.5 Mg 5 Al 2 Si 3 O 10 (OH) 8 + 0.25 SiO 2 (aq)
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¤</2SiO4 + 2.5 SiO2(aq) + H2O
¤$!##
!#@#"[[#!"$
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$V]!@#$$/B"!# `<
" |[ FF:j |# !@@B # ` V @B
j#@#/#$"L"!#/
C$XY%#[@B"/B$
V %# !!} # $ & "!#"#W"#@#"!
B"EFF;
"%#"!"!
$V%#L"##//"<##2
/%#[/B/!#`F"<V/}C&[FFFj2 concentra@VXB"%#`*"<j["#C
#["#/!
"`/j@V!
can explain the high H2XY%#[B$#$/
SiO2L"!$[2 must
V@"/%#%!$XB"B"
165
0.5
0.4
NiO, wt%
mantle
olivine
array
0.3
0.2
Pl-dunite
troctolite
Ol-gabbro
0.1
0.0
0.82
0.84
0.86
0.90
0.92
F o 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)
H2 O
Aw
0
log aH2 (aq)
Pn
–1
Po
–2
Hz
–3
Rainbow
Kairei
Mt
Py
–4
Bu
–5
–5
Mi
Hm
–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 troctoliteseawater and peridotite-seawater reactions in early stage of the serpentinization are shown as light and dark
<
<*

"\*
166
5.5. Discussion
$#$#8#*/LB@&
}V#@W!
$"V"!$V%#[
W!"&@/Y2$XY%#LXB"Q@$&BV%#
@B%!%# /#/
/ #/ @ V %# "! $ @ /
L$#W!%#"B[/"!
"[#/&""!$@`L@j
"!#[/"!#
EFF;
`&"$j[#/%#!/VB
@`Q&"$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.
Uraniwa-Hills
ks
oc
ic r
Ridge axis
f
ma
2
mafic rocks
1
Hydrothermal circulation
Heat source
s
k
oc
Olivine-rich r
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<
\
^}*'
|
<
KHF could cause the high concentration of aqueous SiO2

<
\*
167
$#$#9#J
$#!#B$XY%#"@W!
Y4
LB! ! B" B" & W@
/
Y4`#"<j/Y2-enrichment (Donval et al.,
''*
#[FFj&/[
Y4$XY%#
#@B@_"Y%#[$$F
H2`^V[FFL&[FFE\^""[FFX#"/
et al., 2008). In H2!B"B"[
Y4 is considered to be
!#@B#$
2N
`:j
2 + 4 H2£
Y4 + H2O.
& & $ # $ 2 Y4 are
#// # B! B" ` [ FFj L$[ L! B! `LLj @@ B " B U
W "B !B BC/ $ $" $ Y4 in hydro" %# `/[ Y |[ ''' ##& B$[ FFEj
B[ # #[ !! @ W B $ 2 conver Y4 during serpentinization of peridotite (e.g., Horita and Berndt, 1999).
LB!"!&@"#$["B/VB[$VV
/@@ "! /QB `/ Ej |# "# $ B
B$$&$#$
2
Y4 (Horita and Berndt,
'''j[$##B$LLBY&[#/$
Y4 relative to H2
XYV%#[B@B
$VB$2V$#$#!`[':X|[
2009). Assuming that fO2 is mainly controlled by SiO2 activity during serpentinization
(Frost and Beard, 2007), relatively high-SiO2 activity of serpentinization of the troctolitic
&`@Vj##VB/fO2 conditions compared to ser!C$B!@B!L&W!V
higher S content than typical mantle peridotite due to its high incompatibility (Puchelt
et al., 1996). Both high fO2/VB/#$#
V ! $ $ & B V!
# [ #Q `C "j @"
@`/jLW!@$B!C
!@#[#/#B"![!B!VBC/"$"$"
2 reduction, so that
"$"/Q"##/%#[
168
5.7. Acknowledgements
#!!B!+VB["$%#
/"!#C$/Q"/
L !/ # !! %# !
"W!X B"%# 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&+$!/&2
%# #/ !C[ # # VB / fO2 condition
compared to serpentinization of typical abyssal peridotite. The higher fO2 condition, as
/#$#VB#/#$#$&[##
#$B&WB$LL
@@BL##!!$
Y4 formation, resulting
###B
Y4UY2$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!"<
$U§&#&$&$##!![!@
Q!B$V#@@#/§XF
#
#V""@B{#
#|/B"!V#B$
"#!L#BQB#!!@B\+$Q
$" <B $ _#[ ##[ ![ L/B $
169
!`<_©Lj`EFFj[+<L_
"#!B!"/
+$"$#/_B[/"$
"!V"$_V"$§#/$"!
#$"/L/B[""@B<_©L
170
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#"Q B" B" " /N+ W!"
#BEFF;
[FF@\"
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BCVN
B!!
\"\!B\B:[FF[NFF'UFF\
FF*
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V/ # $ !C !/!B[ "
"B[!!B$!$"<+;`^{/F'[
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|[ <_[ +[ ^_[ B$[ }_[ [ '' # $ #/
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#[{[^V[[X[
[|[^[#&V[[|![[##[
§[QB$__L_#[FF:Y/B/@
B@$"#"QB"@;;
<+/#$__L_#_L+\[
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X_"[;;[
/\"
\!B\B*[FF:[NFF'UFF\
FFF*
\"[L[
@[Y[§"&[L[&#[L[Y"[[L#[[@[
[X&[[L#/[[&"#[X[[§[[[FF
"
$ B V @& "& %# B" !#" /#C L! #[ /
_{'[**'
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[EF:!
}B[ L[ '' _U[+ $ !&/ $ /" "/ $ ##
B"N &/ VV /# ` *Fj[ {
{V"{@[{V"[
$
}B[ L[ FFE #Q $ L"B" ^ $ \" </
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`j|+
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175
Danksagung
+/@"^&""^&V[$}$//|[
"{$$`"j$CL"Ê}\&/&_YV"$#/
C#&#/|C#&_"L&
!/#&!$"#//$["@
"#LC##"Ë$"&=}C#$/"#"C#"/>L"C#V&[V/
+@C#/&""
<^&/@^=[V//@#&"!#^&#!$$[@#
$L"Y$@$<#&!C#@@[$[ ! # $" C#w/& #$C#C/ # "
/X""C#$/["C#@//#wV/den Arbeit zu verbessern.
<Y"=^&[$V/+@ellen thermodynamische Daten von hunderten aquatischen Spezies berechnet hat. Ohne
w +@ C# /&""+#>" "= " ^&[
"`""C#j/##{#"Y#"C#"
#[/C#
"C#/#$@##/\!w</+@/#!!Ê/C&#/&""
$$$/|/#@"+@&"/$$["
"&/$@
&^
\#^Y/#&$C#|#/
\!@V^{/F'&$$!^&#@@C#/!|@<#^+!!
"=$$#C#/@<&B&&
#$^
!/&$Lw/&~/#
<@^&/"#[#$"C@""V&
~#$C#^&V!%@"_<#Y"X[
meinem Bruder Wieland Klein, ohne deren Zuneigung und Hilfe die vorliegende Arbeit
"C#/&""w
^V/+@#V^##//"$QC
`^\& |+FU[ |+FU # ^\_WCC&# Ê^ C "
System Erde“).

Petrology of Serpentinites and Rodingites in the Oceanic - E-LIB

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