Tracing fluid transfer across subduction zones using iron and zinc stable isotopes
|Dates:||7 November 2018|
|Times:||13:00 - 14:00|
|What is it:||Seminar|
|Organiser:||Department of Earth and Environmental Sciences|
|Who is it for:||University staff, Current University students|
|Speaker:||Dr Helen Williams|
Our speaker is Dr Helen Williams from the Department of Earth Sciences at the University of Cambridge.
Subduction zones are the main site of volatile element transfer between the downgoing plate, the overriding mantle
wedge and the Earth’s deep interior. The breakdown of serpentine minerals within the downgoing slab and the fluids
released play a fundamental role in volatile cycling as well as the redox evolution of the sub-arc mantle. Constraining
subduction-related serpentinite devolatilisation is essential in order to better understand of the nature and composition
of slab-derived fluids and fluid/rock interactions. Iron and Zn stable isotopes are recently-established geochemical tracers
can trace fluid composition and speciation as isotope partitioning is driven by changes in oxidation state, coordination, and
bonding environment. In the case of serpentinite devolatilisation, Fe isotope fractionation should reflect changes in Fe
redox state and the formation of chloride and sulfide complexes; Zn isotope fractionation should be sensitive to complexation
with carbonate, sulfide and sulfate anions. This study involved targeting samples from Western Alps ophiolite complexes,
interpreted as remnants of serpentinized oceanic lithosphere metamorphosed and devolatilized during subduction. A striking
negative correlation is present between bulk serpentinite Fe isotope composition and proportion of ferric iron, with the
highest grade samples displaying the heaviest Fe isotope compositions and proportion of oxidised iron. The same samples
also display a corresponding variation in Zn isotopes, with the highest grade samples displaying isotopically light compositions.
The negative correlation between Fe and Zn isotopes and decrease in ferric iron content can explained by serpentinite sulphide
breakdown and the release of fluids enriched in isotopically light Fe and heavy Zn sulphate complexes. The migration of these
highly oxidizing sulfate-bearing fluids from the slab to the slab-mantle interface or mantle wedge has important implications
for the redox evolution of the sub-arc mantle and the transport of metals from the subducting slab.
Coffee and tea will be available after the seminar in the first floor foyer of the Williamson Building.
Dr Helen Williams
Role: Lecturer in Geochemistry, Petrology and Cosmochemistry
Organisation: University of Cambridge
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