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Some key thermoelastic properties of MgSiO - 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 ) 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 ). The discrepancy between calculated K 's and K 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 = (∂lnK /∂lnρ) , 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. 3 S PREM S PREM s s P

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Geophysical Research Letters

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