Pressure-volume-temperature equation of state of MgSiO3 perovskite from molecular dynamics and constraints on lower mantle composition

摘要

The composition of the lower mantle can be investigated by examiningdensities and seismic velocities of compositional models as functions of depth.In order to do this it is necessary to know the volumes and thermoelasticproperties of the compositional constituents under lower mantle conditions. Wedetermined the thermal equation of state (EOS) of MgSiO3 perovskite using thenonempirical variational induced breathing (VIB) interatomic potential withmolecular dynamics simulations at pressures and temperatures of the lowermantle. We fit our pressure-volume-temperature results to a thermal EOS of theform P(V,T) = P0(V,T0) + Delta Pth(T), where T0 = 300 K and P0 is theisothermal Universal EOS. The thermal pressure Delta Pth can be represented bya linear relationship Delta Pth = a + b T. We find V0 = 165.40 A^3, KT0 = 273GPa, K'T0 = 3.86, a = -1.99 GPa, and b = 0.00664 GPa K^-1 for pressures of0-140 GPa and temperatures of 300-3000 K. By fixing V0 to the experimentallydetermined value of 162.49 A^3 and calculating density and bulk sound velocityprofiles along a lower mantle geotherm we find that the lower mantle cannotconsist solely of (Mg,Fe)SiO3 perovskite with XMg ranging from 0.9-1.0. Usingpyrolitic compositions of 67 vol % perovskite (XMg = 0.93-0.96) and 33 vol %magnesiowustite (XMg = 0.82-0.86), however, we obtained density and velocityprofiles that are in excellent agreement with seismological models for areasonable geotherm.