Reduction of iron with no systematic isotope fractionation during continental subduction observed in metamorphic rocks from the Dabie-Sulu orogen, China

Abstract

• No resolvable Fe isotope fractionation in metabasites during prograde dehydration. • Eclogites have Fe 3+ /∑Fe systematically lower than the other metabasites. • Fe reduction likely occurred during eclogitization with simultaneous S oxidization. In order to better understand the mobility and charge balance of Fe in subducted slabs, here we report precise Fe isotopic data, supplemented by Fe 3+ /∑Fe and C-N-S elemental contents, in a well-characterized suite of metamorphic rocks from the Dabie-Sulu orogen, China, including mafic to felsic greenschists, amphibolites and eclogites. There is no systematic Fe isotopic difference among the mafic schists (δ 56 Fe = 0.08 ± 0.08‰, 2SD, N = 13), amphibolites (δ 56 Fe = 0.09 ± 0.04‰, 2SD, N = 19) and eclogites (δ 56 Fe = 0.07 ± 0.06‰, 2SD, N = 27), implying limited Fe mobility during prograde metamorphic dehydration. The overall δ 56 Fe variation of the metabasites and felsic schists ranging from 0.02 ± 0.03‰ to 0.20 ± 0.03‰ most likely mirrors heterogeneity in their Neoproterozoic magmatic protoliths, and can be interpreted by derivation from diverse sources and isotope fractionation during magma differentiation. In a contrary to limited Fe mobility, a decrease in Fe 3+ /∑Fe ratios has been identified for metabasites during eclogitization (on average 0.43 for greenschists, 0.42 for amphibolites and 0.24 for eclogites respectively). Reduction of Fe could have been compensated by oxidation of S where a concurrent expulsion of sulfate seems to be necessary. This study thus supports the hypothesis that Fe-S redox coupling in subducted slabs promotes transfer of oxidizing agents into the mantle wedge in subduction zones.