Elemental and isotopic insights into fluid mobility accompanying the subduction of oceanic ...

186 Pages·2017·25.27 MB·English

Durham E-Theses
Elemental and isotopic insights into (cid:29)uid mobility
accompanying the subduction of oceanic lithosphere
INGLIS, EDWARD,CHRISTOPHER
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Elemental and isotopic insights into
fluid mobility accompanying the
subduction of oceanic lithosphere

Edward Christopher Inglis

A thesis submitted in partial fulfilment of the requirements for the degree of:

Doctor of Philosophy

Durham University, UK

Department of Earth Sciences
Durham University
October 2017

Abstract
Abstract

This study presents a detailed elemental and isotopic investigation of element mobility
related to high-pressure, subduction related metamorphism of oceanic lithosphere.
Particular respect is paid to the mobility of key redox mediating components (e.g.
multivalent elements such as iron (Fe) and sulphur (S)). The overarching aim of this work
is to elucidate the contributions of different subducting lithologies to the redox budget of
the sub-arc mantle. Furthermore, a comprehensive petrological characterisation of
different meta-mafic and meta-ultramafic lithologies is presented.

The primary geochemical tools utilised here are iron (Fe) and zinc (Zn) stable isotopes.
Iron stable isotopes are sensitive to small losses of Fe and, as such, serve as excellent
tracers of Fe mobility within a high-pressure subduction setting. In addition they are
fractionated in the presence of S in the form of Fe-SO bearing complexes.
x(aqueous)
Consequently Fe isotopes can be utilised to trace the mobility of oxidising S bearing
fluids during subduction zone metamorphism and slab dehydration. Zinc stable isotopes
are strongly fractioned by SO and CO bearing fluids, and have been utilised here to
x x
trace the mobility of S and/or C bearing fluids within subduction zones.

Whole-rock metabasalts and metagabbros data presented here demonstrate that there is no
resolvable fractionation of either Fe or Zn isotopes across prograde metamorphic facies
(greenschist à blueschist à eclogite). Rather our Fe and Zn isotope data for basaltic
eclogites (mean δ56Fe = +0.12 ± 0.11 ‰ and δ66Zn = +0.27 ± 0.09 ‰) overlaps with both
measurements of low-grade, seafloor metabasites (mean δ56Fe = +0.11 ± 0.04 ‰ and
δ66Zn = +0.20 ± 0.04 ‰) and estimates of MORB (δ56Fe = +0.11 ± 0.17 ‰ and δ66Zn =
+0.28 ± 0.03 ‰; Teng et al., 2013 and Wang et al., 2017 respectively. It is noteworthy
that the blueschist facies samples show trace element evidence in the form of strong
enrichments in alkali fluid mobile elements, for interactions with external, sediment
derived fluid(s), which has driven the Fe isotope compositions of these metagabbros
towards light values. These factors together suggest that the prograde dehydration of the
mafic oceanic crust within subduction zones is associated with minimal release of S or C
bearing fluids. It is also apparent that the associated meta-gabbro/basalt dehydration
fluids are relatively poor carriers of dissolved Fe.

In order to understand the role of subducting slab serpentinites on the redox budget of
subduction zones, an exhumed section of eclogite-facies serpentinised ultramafics from
the Zermatt-Saas ophiolite have been studied, and as part of this investigation a network
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Abstract
of high-pressure metamorphic veins have also been documented. These veins
predominantly comprise of olivine and Ti-clinohumite, and are interpreted to be high
permeability zones of fluid release and migration formed during the partial dehydration of
the antigorite serpentinite host.

Building on the petrographic investigation of these serpentinite dehydration features it is
shown that whole rock antigorite + olivine bearing serpentinites, which are representative
of serpentinites undergoing partial dehydration, display a strong negative correlation
between δ56Fe values and S contents. This suggests that the release of S from these rocks
that controls their Fe isotope composition. Furthermore, both vein forming olivine and
Ti-clinohumite display markedly light δ56Fe values (δ56Fe = -0.24 ± 0.28 ‰ and -0.12 ±
0.11 ‰), demonstrating the mobility of isotopically light Fe within serpentinite-derived
fluids in subduction zones. We thus conclude that serpentinite dehydration is associated
with the liberation and mobilization of SO bearing fluids, which are isotopically light
X
with respect to Fe.

An entire segment of oceanic lithosphere, from mafic crustal units to underlying
serpentinised peridotite, has been studied. The data presented here has enabled a greater
understanding of the principal controls on fluid release and element mobility during
subduction zone metamorphism. While the dehydration of metabasites clearly involve the
release of fluid mobile elements it is likely that the dehydration of these lithologies play a
relatively small role in the transfer of redox mediating components during subduction. Of
greater significance is the process of serpentine mineral breakdown and fluid release,
which is observed to play a key role in the release of highly oxidizing species within
associated serpentinite derived fluids.

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Declaration
I declare that this thesis, which I submit for the degree of Doctor of Philosophy at Durham
University, is my own work and not substantially the same as any which has previously been
submitted at this or any other university. Edward C. Inglis
Department of Earth Sciences, Durham University, October 2017
© The copyright of this thesis rests with the author. No quotation from it should be published
without prior written consent and information derived from it should be acknowledged.

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