P– V– T equation of state of MgSiO 3 perovskite and MgO periclase: Implication for lower mantle composition

Abstract

The P– V– T equation of state (EOS) for magnesium silicate perovskite (MgSiO 3) and periclase (MgO) was obtained by fitting measured P, V, T data. The EOS of MgSiO 3 perovskite was determined by applying the Mie–Grüneisen–Debye approach, based on three alternative bulk modulus values ( K T0 = 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 ((∂ K T /∂ T) P ). Combined with the EOS of magnesiowüstite, we calculated the density ( ρ), bulk modulus ( K S) and seismic parameter ( ϕ) for simplified pyrolitic lower mantle along a plausible adiabat (temperature at 670 km, T a = 1900 K), and compared these values to those of a seismological model (PREM). The results based on K T0 of 261 GPa, which has been commonly used, showed good agreement with PREM, within 0.5% for ρ, and 2–3% for K S and ϕ, throughout the lower mantle. The EOS obtained using the recent K T0 data (240–250 GPa), substantially overestimated K S 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.