The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites

Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chlor...

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Published in:Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2017-07, Vol.18 (7), p.2562-2579
Main Authors: Inglis, Edward C., Debret, Baptiste, Burton, Kevin W., Millet, Marc‐Alban, Pons, Marie‐Laure, Dale, Christopher W., Bouilhol, Pierre, Cooper, Matthew, Nowell, Geoff M., McCoy‐West, Alex J., Williams, Helen M.
Format: Article
Language:English
Subjects:
Alpine ophiolites
Carbonates
Case studies
Components
Dehydration
Earth Sciences
Facies
Fe isotopes
Fluids
Fractionation
History
Infiltration
Iron
Isotope composition
Isotope fractionation
Isotopes
Ligands
Magma
Metamorphic facies
Metamorphism
metasomatism
Oceanic crust
Oxidation
Petrography
Ratios
Sciences of the Universe
Sediment
Stable isotopes
Subduction
Subduction zones
Sulphides
Zinc
Zn isotopes
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Summary:Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidized or oxidizing components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride, and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex, and the Zermatt‐Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P‐T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S‐bearing and C‐bearing fluids, this suggests that relatively small amounts of Zn are mobilized from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction. Key Points Iron and zinc stable isotope and elemental data are presented for a prograde suite of metabasalts and metagabbros from Western Alpine ophiolite complexes Bulk rock δ56Fe and δ66Zn do not vary across metamorphic facies and the eclogitic samples show a MORB‐like isotope composition Blueschist facies metagabbros preserve evidence for infiltration of sediment derived fluids that impart a light δ56Fe isotope composition to the gabbro
ISSN:1525-2027
1525-2027
DOI:10.1002/2016GC006735