Formation of metallic Fe in bridgmanite under shallow lower mantle conditions

An experimental study was conducted to understand the processes of formation of metallic Fe in bridgmanite. The starting materials are (Mg-0.9,Fe-0.1)SiO3 orthopyroxene containing 0.5 to similar to 5 wt% Al2O3 prepared at pressures P = 3 GPa and T = 1473 K under the oxygen fugacity controlled by the Mo-MoO2 buffer. Fe-57 enriched Fe was used to enhance the signals in the Mossbauer spectroscopy. High-pressure annealing experiments were conducted using a multi-anvil press under the shallow lower mantle conditions (P = 24.5-29 GPa, T = 2000 K) using a Au capsule. In case of Al-rich samples, the run products contain all three forms of Fe (metallic Fe, Fe2+ and Fe3+). Concentrations of those Fe-related species are determined using a combination of electron-probe microanalysis, scanning electron microscopy and synchrotron Mossbauer spectroscopy as well as the mass balance calculations. At the lowest pressure explored (24.5 GPa), relatively large amount of metallic Fe and Fe3+ are observed and we found that the molar ratio of ferric to metallic Fe is nearly 2:1 supporting the model of disproportionation of ferrous Fe, 3Fe(2+) = 2Fe(3+) + Fe. At higher pressures, metallic Fe content is estimated from Fe3+ content assuming this ratio. It is found that the amount of both Fe3+ and metallic Fe decreases with pressure. This result is explained by a model showing that the formation of metallic Fe from bridgmanite is associated with a volume expansion and implies that the regions of relatively large amount of metallic Fe are limited to the shallow lower mantle. Possible implications of the present results are discussed including the partial melting in the shallow lower mantle.