Abstract

We report Mössbauer spectra of γ-Fe2SiO4, γ-Mg1.8Fe0.2SiO4, β-Mg1.8Fe0.2SiO4, and Mg0.85Fe0.15SiO3 garnet synthesized in equilibrium with excess Fe metal and SiO2 at high pressure and high temperature in a multianvil press. Spectra of γ-Fe2SiO4 and γ-Mg1.8Fe0.2SiO4 show one Fe2+ doublet corresponding to the single octahedral site in the spinel structure and one Fe3+ doublet with Fe3+/Fetot = 0.06 and 0.04 for γ-Fe2SiO4 and γ-Mg1.8Fe0.2SiO4, respectively. The spectrum for β-Mg1.8Fe0.2SiO4 includes two Fe2+ doublets tentatively assigned to the [6]M1-M3 and [6]M2 sites and one Fe3+ doublet with Fe3+/Fetot = 0.03. The spectrum of Mg0.85Fe0.15SiO3 garnet shows two Fe2+ doublets corresponding to the dodecahedral and octahedral sites and one Fe3+ doublet with Fe3+/Fetot = 0.07. No Fe3+ was present in the starting materials within the limits of detection, and the amounts of Fe3+ measured in the experimental products represent the minimum possible for these phases at the given P-T conditions and Fe-Mg composition. The ability of transition zone phases to accommodate significant Fe3+ in equilibrium with Fe metal implies that, in an isochemical mantle, the fO2 of the transition zone must be lower than that of the upper mantle.

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