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

Inglis, Edward C., Debret, Baptiste, Burton, Kevin W., Millet, Marc-Albarn, Pons, Marie-Laure, Dale, Christopher W., Bouilhol, Pierre, Cooper, Matthew, Nowell, Geoff M., McCoy-West, Alexander, and Williams, Helen M. (2017) The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: a case study from Western Alpine ophiolites. Geochemistry, Geophysics, Geosystems, 18 (7). pp. 2562-2579.

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Abstract

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.

Item ID: 65073
Item Type: Article (Research - C1)
ISSN: 1525-2027
Keywords: Alpine ophiolites, Fe isotopes, metamorphism, metasomatism, subduction, Zn isotopes
Copyright Information: (C) 2017. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Funders: European Research Council (ERC), National Environment Research Council (NERC), French Government Laboratory of Excellence (LABEX), Durham University
Projects and Grants: ERC HabitablePlanet 306655, NERC Deep Volatiles Consortium grant NE/M0003/1, ERC MASE 279828, LABEX ANR-10-LABX-0006
Date Deposited: 23 Dec 2020 04:02
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370503 Igneous and metamorphic petrology @ 50%
37 EARTH SCIENCES > 3703 Geochemistry > 370303 Isotope geochemistry @ 50%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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