Publication Date:
2016-08-10
Description:
The iron stable isotope compositions ( δ 56 Fe) and iron valence states of ultrahigh-pressure (UHP) eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe 3+ /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 MORB-like (~0.10 ‰) δ 56 Fe values, which are indistinguishable from their protoliths, i.e., 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 δ 56 Fe variation ranging from -0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ 56 Fe values from 0.15 ± 0.07 ‰ to 0.47 ± 0.07 ‰. Omphacite also has highly variable Fe 3+ /ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe 3+ /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 Fe 3+ /ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ 56 Fe values and Fe 3+ /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 & Mg in garnet and omphacite, and constant Δ 26 Mg omphacite-garnet values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe 3+ /ΣFe ratios of omphacite from retrograde eclogites and variant Δ 56 Fe omphacite-garnet values of the studied eclogites (0.13 ± 0.10 ‰ to 0.48 ± 0.10 ‰) indicate that oxidized geofuild infiltration resulted in elevation of δ 56 Fe values of omphacite during retrograde metamorphism. This article is protected by copyright. All rights reserved.
Print ISSN:
0263-4929
Electronic ISSN:
1525-1314
Topics:
Geosciences
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