Effect of Al on the sharpness of the MgSiO3 perovskite to post‐perovskite phase transition

Abstract

By means of static ab‐initio computations we investigate the influence of Al on the recently discovered perovskite to post‐perovskite phase transition in MgSiO3. We examine three substitution mechanisms for Al in the two structures: MgSi → AlAl; SiSiO → AlAl□; and Si → AlH. The substitutions introducing oxygen vacancies (highly unfavorable, energetically) and water (favorable) both lower the 0 Kelvin transition pressure, whereas charge coupled substitution increases it relative to 105 GPa for pure MgSiO3. From the transition pressures for 0, 6.25, and 100 mol% charge coupled Al2O3 incorporation and simple solution theories, we estimate the phase diagram of Al‐bearing MgSiO3 at low Al concentrations. Assuming the Clapeyron slope is independent of Al concentration, we find the perovskite‐to‐post‐perovskite transition region to span 127–140 GPa, at 6.25 mol% Al2O3. When the upper pressure limit is bounded by the core‐mantle boundary, the phase coexistence region has width 150 km.