First principles thermoelasticity of MgSiO₃-perovskite: Consequences for the inferred properties of the lower mantle

Some key thermoelastic properties of MgSiO₃- perovskite (pv) have been determined at lower mantle (LM) pressures and temperatures using the quasi-harmonic approximation in conjunction with first principles phonon dispersions. The adiabatic bulk moduli (K_S) of pv and of an assemblage of 80 vol% pv and 20 vol% MgO were obtained along the thermodynamically inferred adiabat and compared with the seismic counterpart given by the preliminary reference Earth model (K_PREM). The discrepancy between calculated K_S's and K_PREM in the deep LM suggests a super-adiabatic gradient, or subtle changes of composition, or phase, or all beginning at about 1200 km. The Anderson Grüneisen parameter, δ_s = (∂lnK_s/∂lnρ)_P, was predicted to decrease rapidly with a depth (from 2.7 to 1.2 across the LM) supporting the thermal origin for the lateral heterogeneities throughout most of the LM.