Mössbauer spectroscopy of quenched high-pressure phases: Investigating the Earth's interior

Abstract

New phases formed at high pressure and temperature can be successfully quenched to ambient conditions if the kinetics of the back transformation are slow. One important application of this technique is to mineral physics — the combination of microscopic measurements of the structural, physical and chemical properties of high-pressure minerals measured in the laboratory with macroscopic geophysical and geochemical data to produce a unified description of the Earth's interior. The dominant high-pressure phases inferred to be present in the Earth's mantle have been synthesised using a multi-anvil press to correlate structural and chemical information obtained from Mössbauer data with bulk geophysical and geochemical data to determine properties of the mantle, such as the oxidation state. Results from experiments on the major transition zone minerals β−(Mg, Fe)₂SiO₄, (Mg, Fe)SiO₃ garnet and γ−(Mg, Fe)₂SiO₄ spinel include the discovery of significant Fe³⁺ in these phases synthesised in equilibrium with metallic iron and excess silica, implying that the oxygen fugacity (fO₂) of the transition zone must be substantially lower than the upper mantlefO₂. Mössbauer spectra of the lower mantle phase (Mg, Fe)SiO₃ perovskite show that Fe²⁺ occupies the large distorted site almost exclusively, and that significant Fe³⁺ is present at the minimum fO₂ stability limit. Low temperature spectra indicate a phase transition possibly corresponding to a distortion of the perovskite structure. Mössbauer spectra of Fe_xO quenched from high pressure indicate that the Fe³⁺ content of samples in equilibrium with metallic iron is small, implying that the minimum amount of Fe³⁺ in the lower mantle (Mg, Fe)O at lower mantle conditions is also small. Mössbauer spectra from samples of (Mg, Fe)O synthesised at different temperatures andfO₂ conditions show that the Fe³⁺ content can be reliably measured, and that it varies significantly withfO₂. Implications of these results to properties of the Earth's interior are discussed.