Mössbauer spectroscopy of mantle transition zone phases and determination of minimum Fe3+ content

We report Mössbauer spectra of γ-Fe₂SiO₄, γ-Mg_1.8Fe_0.2SiO₄, β-Mg_1.8Fe_0.2SiO₄, and Mg_0.85Fe_0.15SiO₃ 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-Mg_1.8Fe_0.2SiO₄ show one Fe²⁺ doublet corresponding to the single octahedral site in the spinel structure and one Fe³⁺ doublet with Fe³⁺/Fe_tot = 0.06 and 0.04 for γ-Fe₂SiO₄ and γ-Mg_1.8Fe_0.2SiO₄, respectively. The spectrum for β-Mg_1.8Fe_0.2SiO₄ includes two Fe²⁺ doublets tentatively assigned to the ^[6]M1-M3 and ^[6]M2 sites and one Fe³⁺ doublet with Fe³⁺/Fe_tot = 0.03. The spectrum of Mg_0.85Fe_0.15SiO₃ garnet shows two Fe²⁺ doublets corresponding to the dodecahedral and octahedral sites and one Fe³⁺ doublet with Fe³⁺/Fe_tot = 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 f_O2 of the transition zone must be lower than that of the upper mantle.