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

Abstract

Arc lavas display elevated Fe 31 /RFe 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 d 56 Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.

Zinc isotope analyses of singularly small samples (<5 ng Zn): Investigating chondrule-matrix complementarity in Leoville

“…Both geological standards are within error of reported average literature values of δ 66 Zn (0.25±0.08 for BCR-2, 0.29±0.06 for AGV2; Moynier et al, 2017). Our analyses of CP1 metasediments show that the obtained Zn isotope data (δ 66 Zn = -0.07±0.10 , δ 68 Zn = -0.17±0.11 ) are also in agreement with previously reported data (δ 66 Zn = 0.00±0.08 , Inglis et al, 2017). All data lie on the mass-dependent correlation line ( Fig.…”

“…We have sampled multiple terrestrial standards with varying bulk compositions and with known Zn isotope signatures. Per standard, five samples were weighed (50-200 µg per sample) containing approximately 5-10 ng Zn each, from bulk powders of BCR-2 (USGS basalt), AGV2 (USGS andesite) and CP1 (metasediment from the Queyras Schiste lustrs Complex, Inglis et al, 2017). In addition, we have selected larger aliquots of BHVO-2 (USGS basalt) and CV chondrite NWA 12523 (5 mg), with totals of ∼600 ng Zn each.…”

“…Table 2). The shaded areas are (recommended) literature values for CP1 (purple, Inglis et al, 2017) and BHVO-2, BCR-2 and AGV2 (orange, Moynier et al, 2017). We note that literature only reports the δ 66 Zn values.…”

Geochemical characteristics of basalts from Andaman subduction zone: Implications on magma genesis at intraoceanic back‐arc spreading centres

“…Melt extraction and mantle depletion processes at MORs and BABs with flux‐induced metasomatic enrichment of the mantle at ocean–ocean and ocean–continent convergent margins collectively attribute to the pervasive chemical heterogeneity in the asthenospheric mantle triggering magmatism of diverse compositions. A geochemically depleted asthenospheric mantle rendered by melt extraction episodes at mid‐oceanic ridge (MOR) systems and back‐arc basins (BAB) is re‐injected and replenished at ocean–ocean and ocean–continent subduction zones by hydration, metasomatism, and hybridization of asthenospheric mantle wedge through release of fluids, sediments, and melts from dehydration and melting of subducting oceanic slab, respectively (Brandl et al, ; Inglis et al, ; Masuda & Fryer, ). Magmas produced in mid‐oceanic ridge settings and island‐arc systems reflect distinct geochemical features that link respective magmatic processes and geodynamic conditions.…”

“…A geochemically depleted asthenospheric mantle rendered by melt extraction episodes at mid-oceanic ridge (MOR) systems and back-arc basins (BAB) is re-injected and replenished at ocean-ocean and oceancontinent subduction zones by hydration, metasomatism, and hybridization of asthenospheric mantle wedge through release of fluids, sediments, and melts from dehydration and melting of subducting oceanic slab, respectively (Brandl et al, 2017;Inglis et al, 2017;Masuda & Fryer, 2015). Magmas produced in mid-oceanic ridge settings and island-arc systems reflect distinct geochemical features that link respective magmatic processes and geodynamic conditions.…”