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Iron isotopic systematics of UHP eclogites respond to oxidizing fluid during exhumation

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Year:
2016
Type of Publication:
Article
Keywords:
iron isotopes, Mössbauer spectroscopy, oxygen fugacity, subduction zone, ultrahigh pressure
Authors:
Li, D. -Y.; Xiao, Y. L.; Li, W. -Y.; Zhu, X.; Williams, H. M.; Li, Y. -L.
Journal:
Journal of Metamorphic Geology
Volume:
34
Number:
9
Pages:
987-997
ISSN:
1525-1314
BibTex:
Abstract:
The iron stable isotope compositions (δ56Fe) and iron valence states of ultrahigh-pressure eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace the changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe3+/ΣFe ratios of retrograde eclogites, determined by Mössbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites (five samples) display limited mid-ocean ridge basalts (MORB)-like (~0.10‰) δ56Fe values, which are indistinguishable from their protoliths, that is, gabbro cumulates formed through differentiation of mantle-derived basaltic magma. This suggests that Fe isotope fractionation during continental subduction is limited. Garnet separates display limited δ56Fe variation ranging from −0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ56Fe values from 0.15 ± 0.07‰ to 0.47 ± 0.07‰. Omphacite also has highly variable Fe3+/ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe3+/ΣFe ratios (0.54–0.60) compared to that from fresh eclogites (~0.37), whereas garnet displays a narrow range of ferric iron content with Fe3+/ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ56Fe values and Fe3+/ΣFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its Fe-isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe and Mg in garnet and omphacite, and constant Δ26Mgomphacite-garnet values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe3+/ΣFe ratios of omphacite from retrograde eclogites and variant Δ56Feomphacite-garnet values of the studied eclogites (0.13 ± 0.10‰ to 0.48 ± 0.10‰) indicate that oxidized geofluid infiltration resulted in the elevation of δ56Fe values of omphacite during retrograde metamorphism.

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