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Petrology of Serpentinites and Rodingites in the Oceanic Lithosphere

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Petrology of Serpentinites and Rodingites in the Oceanic Lithosphere Petrology of Serpentinites and Rodingites in the Oceanic Lithosphere Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften am Fachbereich Geowissenschaften der Universität Bremen vorgelegt von Frieder Klein Bremen, 2009 Referent: Prof. Dr. Wolfgang Bach Koreferent/in: Prof. Dr. Cornelia Spiegel Tag der mündlichen Prüfung:…………………… Zum Druck genehmigt: Bremen,.......…………… Der Dekan Erklärung Hiermit versichere ich, dass ich 1. die Arbeit ohne unerlaubte fremde Hilfe angefertigt habe, 2. keine anderen als die von mir angegebenen Quellen und Hilfsmittel benutzt habe und 3. die den benutzten Werken wörtlich oder inhaltlich entnommenen Stellen als solche kenntlich gemacht habe. Bremen, den Anmerkungen des Verfassers zur vorliegenden Dissertation Die vorliegende Arbeit stellt zwar eine monographische Dissertation dar, die einzelnen Kapitel, denen die Einleitung vorangestellt ist, sind jedoch bezüglich ihres Aufbaues so konzip- iert, dass sie unabhängig voneinander publiziert werden können bzw. publiziert sind. Durch diesen Umstand ist es zu erklären, dass jedes Kapitel nochmals eine eigene Einleitung, Diskus- sion und ein Literaturverzeichnis enthält. Auch beim Schreibstil, dem Umfang, der Verwend- ung von Abkürzungen sowie der Formatierung von Abbildungen und Tabellen wurde Bereits den Anforderungen unterschiedlicher Fachzeitschriften Rechnung getragen. Diesen Umstand möge der Leser berücksichtigen. Bremen, März 2009 Frieder Klein Table of Contents Zusammenfassung 1 Abstract 3 Prologue 5 Outline 8 1. Introduction 11 1.1. Serpentinized peridotites at mid-ocean ridges 11 1.2. Serpentinized peridotites at active oceanic margins 14 1.3. Hydrothermal systems and serpentinized peridotites 14 1.4. Mineralogical and petrological aspects of serpentinization 16 1.4.1. Serpentinite textures 16 1.4.2. Serpentinization - an isovolumetrical process? 17 1.4.3. Some crystallographic basics concerning serpentine 18 1.4.4. A note on the mineral chemistry of serpentine and its value as a geothermometer 19 1.4.5. The MgO–SiO2–H2O (MSH) system 19 1.4.6. Redox conditions during serpentinization 20 1.5. Rodingitization 24 References 25 %%%% Abstract 37 2.1. Introduction 37 2.2. Geological setting 40 2.3. Analytical methods 41 2.3.1. Microscopy and electron microprobe analysis 41 2.3.2. Thermodynamic calculations 42 2.4. Results 46 2.4.1. Petrography 46 2.4.2. Mineral chemistry 51 2.4.3. Phase diagrams 54 2.5. Discussion 59 !"!#"$ \#& ' 2.5.2. Redox conditions during serpentinization 60 2.5.3. Redox conditions during steatitization 61 2.5.4. Implications for a potential H2S,aq buffer in serpentinite-hosted hydrothermal systems 62 2.5.5. Sulfur metasomatism 63 2.5.6. Possible existence of a free H2-rich vapor phase 66 # * *+&/" : References 69 3. Iron Partitioning and Hydrogen Generation During Serpentinization of Abyssal $"<+/*: Abstract 78 3.1. Introduction 78 3.2. Analytical methods 81 3.2.1. Microscopy and electron microprobe analysis 81 3.2.2. Mößbauer spectroscopy and magnetization measurements 82 3.2.3. Geochemical modeling 82 3.3. Results 85 3.3.1. Petrography 85 3.3.2. Mineral compositions 87 <%@#!!B@#&"/C ' 3.3.4. Geochemical reaction path modeling 92 3.4. Discussion 103 3.4.1. Serpentinization at Hole 1274A and geochemical reaction path models 103 E//!"@/#/!C FE 3.4.3. Fe+2%+3 exchange equilibria in serpentinites 104 3.4.4. Geochemical reaction path modeling and serpentinization experiments 106 3.4.5. The formation of brucite and serpentine in mesh-rims 108 # +&/" References 113 Appendix 119 EL!/B$\/N/$"/"! modeling 120 Abstract 120 4.1. Introduction 120 4.2. Method 122 4.3. Results 126 4.3.1. Reaction path models 126 4.3.2. Phase diagrams 137 4.4. Discussion 138 4.4.1. Modeling of rodingitization 139 4.4.2. The critical role of aqueous silica 141 4.4.3. Mass transfer by diffusion or advection 142 EEE"[U#"[EE EE+!V$/"!#\# circulation? 145 E# E E+&/" E* References 148 !CW!XYB"[ /N//$XB"\# supporting a unique microbial ecosystem 155 Abstract 155 5.1. Introduction 155 \/@&/# : 5.3. Analytical methods 158 5.4. Petrography 159 5.5. Discussion 161 5.5.1. Origin of the high H2XY\# LM@V" +B@"$XYV\#"B * E"!$" : # ' *+&/" ' References 171 ^&/#/ * _!!$!"!&`^j Zusammenfassung Die Serpentinisierung von Peridotiten erzeugt große Mengen von Wasserstoff. ^#CVM{/#/#$"#[[ C|+##[@/}WC/_VC# /"_</#!^"!"/_@# von Sauerstoff in Magnetit und Serpentin die Freisetzung von Wasserstoff. Wir haben "B"^VMMMMC#""//#|- f f ziehung in O2,g– S2,g und aH2,aq–aH2S,aq Diagrammen für Temperaturen von 150 bis EFF#"^#&VF</^|C#/#~#""- C#/MMMM##/[#"#/#$$ #$$#/C@$!#/##/V #"[\<&`FR#~[- ^//"`^j[{/F'jVCC#&^!/! Beobachtungen offenbaren eine systematische Abfolge von Mineralvergesellschaftungen +@//& C#" !#// \ ^ /+# Pentlandit + Magnetit bildet sich in partiell serpentinisierten Gesteinen. Die Paragenese <BBB"//@\+# ##%|#C$!@@$$$ #$$#/V/X#&VC#C#//%</}- $$#"C#@#/V+#~#"""_$#/ V#[#"\$"\/CC#$- X#&V#$$#/C#/C# @#/V$#[[C|BB"#${#/ VB#[#$V/#&&!C\C#&- C#$ | |/#/ #[ $[ #/$[CC#"+/$/"\$^ f f _&#/V O2,g und S2[/!#/$/Y2S,aq Iso- !#"!/"##/[$$#/C- YB"\##"[B"/!#$$ ^ C#/ V }$$ !#/ " <%V\"/C#""C#/[V"}\# L"!#@//^|C#/@#~#$"V/- "&!$"`""!#!/""_Uj$# und harzburgitische Gesteinszusammensetzungen untersucht und die Modellergebnisse " <& # <%@#!&&!+B V ! @ C# &"!!^##YC@#/^{/F'V/^ \&+@$/V<V/$#/<- #$~<[C# V[@[|#C`</:Fj#%%~V! `</'j|#C</["#%<V!</- @/^""/</$<%@#!&&!- ##/$!"/<+3U5} VFF@FE:&@&"!!\[V</ enthalten, sind die Fe+3U5}!"/#`F@ F:j_U<#_/\#+3-Serpentin @&/[#"_V#/C![|#C#</[ W#$V_!@##C#&<- 1 #/C/[@@L"!#VF#}&V- #"[{#/VV#/C/|#/V![</- #}$$WX##@/|L"!# }$$&VC#//[#"|#//+# C#"/@L"!##FMF&[|#C@# _#/V}$$[&VVC#![</ #|#C&WX##$^+##/</@- nisse und deren Vergleich mit MgO–FeO–Fe2O3–SiO2–H2O Phasenbeziehungen in Ma- #$|#/"!#!#|#C|#/ *E+VF@F^@}\V/ C # F ^+##/ !/! # !!/ +BC/[!#/|#C/@@V! @|#C@"@#$XVV"@$V ![@"B"L@&$$|#//! |#C#/%#C/#[&@$/ !#/"^$&["!/!- ["/#!&&!##/"#"$"B- "<#/&"@ "#_&@|#/V/\C\V "[##"[\C#/[@"B"- &!$"#$/L"B"&!$""/ #/#/V#\\/"~#//- #/@VCC#&LB!V/$#//- `\#^!j$L"!#VFF#FFV- //^/$#//#[#&#"V &"\@@@\#<C#"&C!- #/\##\@@V/$#/V\# X!BWC##L"@C_!#L"|#/ C/[|#/V<V/$#/##"- #/@V/C@V/V\- #X#&V/@^ <$VXC#"#^$$#Y+ Spezies "/[XC/!C\CV "[C##"[\//XC#"$[ @V\#^!@/X#&V@// ^#XYB"$`XYj`~&j- XCV}$$#X#[&Y4UY2 #$_"/_&#/$#/~#""C#/ #XY&V<"L&[ des KHF vorhanden sind, dar. Petrographische Untersuchungen offenbaren, dass Olivin @V/!#[#$_#/V}$$ <#/C/[}$$#X#- &CYB"\##$!#/ XY@[L&#%B"+"| #@XY"/C#&C#$ 2 Abstract Serpentinization of peridotite generates large amounts of dihydrogen (H2,aq), in- @B!$MB#$#$#/B#[[/# ![!YB/!#$#V WC@B$B"/!}V"- !V#"B"$MMMM!"!#! f f relations in O2,g– S2,g and aH2,aq–aH2[/"$"!#@F EFFF<}#"!$MMMM! trace changes in oxygen and sulfur fugacities during progressive serpentinization and C$!$"<+/FR#~ `^//"[{/F'j/!@VB"- /$"#"/!C"/ pentlandite assemblages forming in the early stages of serpentinization to millerite + !B!BB"""@/C&+#W#VB @V@#@/!B!C&+!!B[@#$$/$- VV#@B!$@#$$"$/"# $B/[$"$#+!" #$#C$![#["V$"&#// of serpentinization. In contrast, steatitization indicates increased silica activities and that /#$# $#/B #[[ # !BB" !B V #[ form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under [#[#$#C@#!!#$#$ f f &LV#$ O2,g– S2[/B"$!$ H2S,aq, indicating that H2V\#@#$$ YB//#/!C/B!@#&& "![&"!#LW"- "B"!"`#/_U"!#j#- C@#/&"!L"#"!"!@ B[@#&"/C"#"[<%@#!!B$!B $#B!C#C@#/$"^//"{/F'[< +/L"!V""V#C- /[/@#`</:Fj$V[C$! `</'j@#/"/[[B!"/#"" ""!"/[<%@#!V+3U5V#@- FFFE:$!"/"""VB"!B !C&@#"/[+3U5V#$!"/! B//$"FF:_U#[! solution model that includes greenalite and Fe+3!#V// @#$@!"/W!- #""!#!@VFV#B[ the dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen requires an external source of silica. At these temperatures, hydrogen fugacities are too $#!@@}"!#!@FMF [@#@"@B//$[@#$ olivine to serpentine, magnetite and brucite requires no external silica. The MgO–FeO– Fe2O3–SiO2–H2O phase relations observed in the mesh rims indicate that serpentine and @#$"Y*E+&B$""!#@FF- 3 &@F_V#/!/!!!/ #@#$"B"@@B[$! #"B"V/$$!$"#$[@C the assemblage serpentine + brucite. Our study indicates that unprecedented details about the reaction sequences during serpentinization may be obtained from merging careful !/!["/[!!B"!V"B" modeling. L"B"!"!!!V/ $"$/"[U#"[@#L"#!- V/ $ \#M& #@ \# "V $" !#// serpentinization into a gabbroic body. Phase assemblages typical of rodingite (grossular !/j!$"FFFF[@#B \#B#$$@B/@@+\#@"" $$@B/@@[!!!/" replaces clinopyroxene. Our model results support the hypothesis that rodingites form #/!CB\#@B!C- actions are present. Our calculations further indicate that the formation of mineral as- "@/!"/V@B/$"\ "BV@B!VB/!##!\# <$$#"&B@B$$#$Y+![! VB!/"[M#"[@#B/- #"!@#$/@B$!/ activities. LV\#$XYB"`XYj /V/$B/[Y4UY2 ratio. We sug- / $ V XY[!V !@W!$"!$XY\#/!BV V!B"!B!C[//$Y2. Model calculations predict that high H2$B"\#@ attributed to serpentinization of the troctolites and subsequent hydrothermal reactions @&#XY 4 Prologue "[&"&#!_R!/!$- !_"/"!$&R#$ "[&$#B#$#`+#[ '*j[!`/[+@[':'+WY![''"[ '_VL""$$['*FB[FF:{#['*{#_[ '*'Y!['::['*Ej[/"/`/[+#- "{#@['*|[FFE|['*E['' [':*['E'j["/`/[+/[''|[':'j $/`/[^R+X[FFEB<['' [FF&[''Fj B#!!#V[B#$[#"[&#- //B@B[/LB$V! `C@#/[C#j["@##"[&["!- ##V#"!VV!"$V !BW@B"@/"@B!/#!"`"[ '[':<B[''<B['*/['*['*' $[':}&}&['**j Serpentinites contain minerals and mineral assemblages that occur almost no- _`|[FF*j!/#!"[ the alteration assemblage consists mainly of magnetite, and brucite or talc (depending on !"!"!#j[""#$"[[ !UB/B#$##["@/[/[- #[[!C[B$#"[- #"#!`+@['*:^&['*E_&['* [':&[''"['Fj L"/!#$!\@B$ /\#!$"W"/"V" _YB"B"\#@B!CC@B/B reducing conditions,
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