Oxidation state of iron in hydrous mantle phases: implications for subduction and mantle oxygen fugacity

Samples of olivine, wadsleyite, ringwoodite and majorite with Mg/(Mg+Fe)=0.85–0.90 were synthesised at high pressure and temperature using a multianvil press under nominally anhydrous conditions as well as hydrous conditions with varying conditions of oxygen fugacity, and examined ex situ using Mössbauer spectroscopy to determine Fe3+/ΣFe, and either Fourier Transform Infrared Spectroscopy or Secondary Ion Mass Spectrometry to determine H concentration. Under oxidising conditions, the relative concentration of Fe3+ increases with water content in olivine, wadselyite and ringwoodite. Majorite does not incorporate significant water, but shows high Fe3+/ΣFe values when synthesised under oxidising conditions in the presence of water. Fe3+/ΣFe is strongly influenced by oxygen fugacity for all phases studied, and the mechanism of hydration is likely coupled to Fe3+ incorporation in the crystal structure. Enhanced Fe3+/ΣFe in mantle-derived olivine and orthopyroxene has a negligible effect on oxygen fugacities estimated using olivine–orthopyroxene–garnet oxybarometers, while increased Fe3+/ΣFe in garnet due to dehydrogenation could lead to an overestimation of oxygen fugacity in exhumed samples. Subduction zones are likely oxidised relative to the rest of the mantle, and hence represent regions where not only water is concentrated, but also Fe3+.