P–V–T equation of state of MgSiO3 perovskite and MgO periclase: Implication for lower mantle composition

The P–V–T equation of state (EOS) for magnesium silicate perovskite (MgSiO3) and periclase (MgO) was obtained by fitting measured P, V, T data. The EOS of MgSiO3 perovskite was determined by applying the Mie–Grüneisen–Debye approach, based on three alternative bulk modulus values (KT0 = 240, 250 and 261 GPa), yielding substantially different values for high-order thermoelastic parameters such as the Grüneisen parameter (γ) and the temperature derivative of bulk modulus ((∂KT/∂T)P). Combined with the EOS of magnesiowüstite, we calculated the density (ρ), bulk modulus (KS) and seismic parameter (ϕ) for simplified pyrolitic lower mantle along a plausible adiabat (temperature at 670 km, Ta = 1900 K), and compared these values to those of a seismological model (PREM). The results based on KT0 of 261 GPa, which has been commonly used, showed good agreement with PREM, within 0.5% for ρ, and 2–3% for KS and ϕ, throughout the lower mantle. The EOS obtained using the recent KT0 data (240–250 GPa), substantially overestimated KS and ϕ by 5–10%, requiring a progressive enrichment in (Mg,Fe)O with increasing depth. In addition, a more realistic vibrational density of states (VDOS) model (Kieffer-type model) was applied to calculate the EOS. The effect of VDOS on thermoelastic parameters was not large enough to modify those conclusions.