Investigation of blueschist and serpentinized harzburgite from the Mariana forearc [Elektronische Ressource] : insights into the mechanisms of element mobilization in subduction zones and storage of fluid mobile elements in the mantle wedge / vorgelegt von Sonja Pabst

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2009
• Nombre de lectures: 63
• Langue: English
• Poids de l'ouvrage: 72 Mo

Extrait

Investigation of blueschist and serpentinized
harzburgite from the Mariana forearc: Insights into the
mechanisms of element mobilization in subduction
zones and storage of fluid-mobile elements
in the mantle wedge
INAUGURAL – DISSERTATION
zur Erlangung der Doktorwürde
der Naturwissenschaftlich-Mathematischen Gesamtfakultät
der Ruprecht-Karls-Universität
Heidelberg
vorgelegt von
Diplom-Geowissenschaftlerin
Sonja Pabst
aus Nordenham
Heidelberg, im Oktober 2009
Gutachter:
PD Dr. Thomas Zack (Universität Mainz)
Prof. Dr. Alan Woodland (Universität Frankfurt)
Tag der mündlichen Prüfung: 11.12.2009Contents
Abstract v
Zusammenfassung vii
Chapter 1 1
Introduction
1.1 Aims and scope of the thesis 1
1.2 Thesis outline 2
1.3 Li, Be and B geochemistry 3
1.4 Boron isotope fractionation 4
1.5 Light elements in the Earth 6
1.5.1 Light elements in the subduction cycle 6
1.6 Geological setting 10
1.6.1 The geography of the Izu-Bonin-Mariana (IBM) subduction zone 10
1.6.2 History of the IBM arc system 12
1.6.3 Current subduction in the Mariana Arc 13
1.6.4 Serpentine mud volcanism and sample location 16
Chapter 2 19
Analytical Techniques
2.1 Sample documentation 19
2.2 Electron probe micro analysis (EPMA) 21
2.3 Secondary ion mass spectrometry (SIMS) 22
2.4 Determination of boron isotope ratios by SIMS 23
112.5 Matrix correction for B analysis by SIMS 23
2.6 Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) 25
2.7 Micro-Raman spectroscopy 25
Chapter 3 29
Evidence for boron incorporation into the serpentine crystal structure
3.1 Introduction 29
3.2 Analytical Techniques 29
3.3 Petrography and mineral compositions 30
3.4 Intragrain boron distribution in serpentine 32
3.5 Discussion and Conclusion 34
Chapter 4 37
Serpentinization of the Mariana forearc mantle wedge: light elements
as tracers for the hydration history
4.1 Introduction 37
4.2 Petrography of serpentinites and serpentine textures 37
i
Contents
4.2.1 Identification and discrimination of serpentine polymorphs and brucite 44
4.2.2 Serpentine polymorphs and brucite correlated with their textural position 45
4.3 Major element contents of serpentinite forming minerals 52
4.3.1 Olivine 52
4.3.2 Pyroxenes 53
4.3.3 Spinel 54
4.3.4 Brucite / Amakinite 55
4.3.5 Serpentine 56
4.3.6 Ca-(OH)-rich ‘phase’ in serpentinites 59
4.3.7 ‘Black serpentine’ 60
4.4 Light element (Li, Be, B) contents of rock-forming minerals 61
4.4.1 Light element mappings by ToF-SIMS 61
4.4.2 Light element SIMS analyses 64
114.5 Boron isotope composition ( B) of serpentine by in-situ SIMS 67
4.6 Interpretation of serpentine textures and mineral chemistry 68
4.6.1 Estimates of the degree of partial melting 69
4.6.2 Melt impregnation (refertilization / ‘metasomatism’) 70
4.6.3.1 Fe-distribution between serpentine-brucite-magnetite:
evidence for serpentinization conditions 70
4.6.3.2 Serpentine polymorph distribution: evidence for variable fluid-rock ratios 73
4.6.4 Correlation between serpentine textures and their light element composition 74
4.6.5 Boron isotope evolution (serpentinizing fluid pulses) 76
Chapter 5 79
Metamafic blueschist-facies rocks from the Mariana forearc:
reactions at the slab-mantle interface
5.1 Introduction 79
5.2 Mineral Chemistry (major elements) 80
5.2.1 Amphibole 80
5.2.2 Phengite 81
5.2.3 Chlorite 82
5.2.4 Talc 83
5.2.5 Epidote and Allanite 83
5.2.6 Pyroxene 84
5.2.7 Garnet 85
5.2.8 Ti-phases (titanite, rutile, ilmenite, titano-magnetite) 86
5.2.9 Other phases (pumpellyite, apatite, quartz, plagioclase, zircon) 86
5.3 Petrography 86
5.3.1 Amphibole-Talc-Chlorite-Schists 88
5.3.1a) metasomatic rind of ultra-mafite, i.e., serpentinized peridotite 89
5.3.1b) metasomatic rind of mafic slab 92
5.3.2 Chlorite-Epidote-Rocks 95
5.3.2a) blueschist-facies meta-basalt / endmember 95
ii
Contents
5.3.3 Am-Chl-Phe-schists 99
5.3.3a) meta-sediment 99
5.3.3b) rind of meta-sediment 101
5.3.4 Rocks of magmatic origin 101
5.3.5 Andradite-bearing serpentinites (Grt + Srp) and Bt-Chl-rocks 103
115.4 Light element (Li,Be,B) contents and boron isotopic composition ( B)
of (rock forming) minerals 104
5.4.1 ToF-SIMS element maps 104
5.4.2 SIMS analyses: budgets of Li, Be and B 110
115.4.3 SIMS analyses: B 111
5.5 Geothermobarometry 112
5.6 Calculated modal compositions: bulk rock and potential source material 115
5.7 Slab-mantle-interaction: evidence for an active subduction zone mélange
formed by tectonic and metasomatic mixing 119
5.8 Light element behavior during dehydration and mélange metasomatism 124
115.9 Boron fractionation ( B) during slab dehydration and slab-mantle-interaction
(fractionation along the slab-mantle interface) 126
Chapter 6 129
Synthesis: Fluid-rock interaction and light element recycling
in the Mariana forearc
6.1 Subduction input - metamafic high-pressure rocks 130
6.2 Serpentinites of the forearc mantle wedge 132
6.3 Boron isotope evolution along the slab-mantle interface in the Mariana forearc 136
References 139
Appendix A 159
A1: Documentation of reference minerals for d11B SIMS analyses
and matrix correction 160
A2: Composition of reference minerals 162
A3: List of mineral abbreviations 165
Appendix B: Serpentinites 167
B1: Sample BSE-images with EPMA, SIMS and Raman spots 168
B2: Electron probe micro analyses 169
B2.1: Serpentine and brucite minerals 169
B2.2: Spinel 170
B2.3: Olivine 170
B2.4: Clinopyroxene and orthopyroxene 171
B3: SIMS data 173
B4: Light element variation diagrams for different serpentinite samples 182
B5: Micro-Raman spectra of serpentine and brucite 185
iii
Contents
Appendix C: Metamafic rocks 195
C1: Sample BSE-images with EPMA, SIMS and Raman spots 196
C2: Electron probe micro analyses 197
C2.1: Amphibole 197
C2.2: Chlorite 201
C2.3: Talc 202
C2.4: Pumpellyite, epidote and allanite 203
C2.5: Phengite and Biotite 205
C2.6: Garnet 206
C2.7: Pyroxene 207
C2.8: Spinel phases, titanite, rutile 208
C2.9: Apatite and zircon 209
C2.10: Serpentine (antigorite) 211
C3: SIMS data 212
C4: Micro-Raman spectra of andradite 218
Appendix D: Supplementary Results 219
D1: Sr - light element variation in blueschist minerals 220
D2: Sr - light element variation in serpentinites 220
D3: U concentration in rutile – a possibility for dating 221
D3.1 Analytic 222
Danksagung
Eidestattliche Erklärung
Electronic Appendix (CD)
ivAbstract
Numerous serpentinite seamounts on the forearc of the Izu-Bonin-Mariana (IBM) subduction zone
present the only known locations worldwide where mantle wedge serpentinites and blueschist-facies
metamafic fragments can be directly sampled. These fragments have been transported diapirically in
a low temperature fluid-mud matrix from within this active subduction zone from a depth of >20 km
below seafloor, i.e., directly from the slab-mantle-interface. At South Chamorro Seamount (ODP Leg
195), ~85km distal from the trench axis, the slab surface is at ~27km depth, where estimated
temperatures are <350 °C, typical for blueschist-facies, sub-forearc subduction zone environments.
11This is the first study which combines high-resolution results on light element (Li, Be, B) and B
distribution of both slab-derived metamafic rocks and serpentinized mantle rocks from an active
subduction zone. Mobile in aqueous fluids and sensitive as tracers of fluid source and mobilization,
Li, Be and B in (Na-)amphibole, phengite, chlorite and serpentine provide (i) information to quantify
devolatilization of the subducting mafic oceanic crust in shallow regions and (ii) information about
fluid infiltration into the forearc mantle peridotite due to fluid transfer from the dehydrating slab into
the overlying mantle wedge.
Analyses of Li, Be and B contents and B isotope ratios were performed using secondary ion mass
spectrometry (SIMS). Light element distribution maps were made using Time-of-Flight SIMS.
Micro-Raman was used to identify serpentine polymorphs and brucite in serpentinites.
The fine-grained metamafic fragments (<5 mm in diameter) comprise a large variety of mineral
assemblages. These assemblages indicate a range of protoliths that have been subjected to mechanical
mixing and metasomatism within a mélange zone at surprisingly shallow depths. Minerals such as
chlorite, Na- and Ca-amphibole, phengite, epidote and Na-pyroxene in paragenesis with pumpellyite
correlate with blueschist-facies conditions at ~27 km depth (at ~300 °C). The main Li, Be and B
carriers are phengite > chlorite + amphibole. Estimated concentrations of light elements in bulk rocks
are in the same range as in altered oceanic crust and subducting sediments, demonstrating that the
major amount remains in the subducting slab and is not released with fluids. However, moderate B
11loss is suggested by the light B values of phengite, chlorite and amphibole (–6 ± 4 ‰). As B
fractionation is most effective at low temperatures, this light B isotope signature can be explained by
11low fluid losses from the shallow slab, which originally had a slightly positive ave