Thermoelasticity of Fe³⁺- and Al-bearing bridgmanite: Effects of iron spin crossover

We report ab initio (LDA + U_sc) calculations of thermoelastic properties of ferric iron (Fe³⁺)- and aluminum (Al)-bearing bridgmanite (MgSiO₃ perovskite), the main Earth forming phase, at relevant pressure and temperature conditions and compositions. Three coupled substitutions, namely, [Al]_Mg-[Al]_Si, [Fe³⁺]_Mg-[Fe³⁺]_Si, and [Fe³⁺]_Mg-[Al]_Si have been investigated. Aggregate elastic moduli and sound velocities are successfully compared with limited experimental data available. In the case of [Fe³⁺]_Mg-[Fe³⁺]_Si substitution, the high-spin (S = 5/2) to low-spin (S = 1/2) crossover in [Fe³⁺]_Si induces a volume collapse and elastic anomalies across the transition region. However, the associated anomalies should disappear in the presence of aluminum in the most favorable substitution, i.e., [Fe³⁺]_Mg-[Al]_Si. Calculated elastic properties along a lower mantle model geotherm suggest that the elastic behavior of bridgmanite with simultaneous substitution of Fe₂O₃ and Al₂O₃ in equal proportions or with Al₂O₃ in excess should be similar to that of (Mg,Fe²⁺)SiO₃ bridgmanite. However, excess of Fe₂O₃ should produce elastic anomalies in the crossover pressure region.