High-pressure, temperature elasticity of Fe- and Al-bearing MgSiO3: implications for the Earth’s lower mantle

Abstract:

A good knowledge on the thermoelasticity of the major constitutional minerals, Fe- and Al-bearing MgSiO₃ bridgmanite and post-perovskite (pPv), is one of the keys for understanding the lower mantle. However, the available data are scarce. By means of first-principles molecular dynamics simulations that treat all anharmonic effects accurately, we extensively explore the effects of the existence of Fe, including both Fe²⁺ and Fe³⁺, and Al on the elastic properties of MgSiO₃, at different compositions and atomic distributions, mineral phases, and pressure and temperature conditions that are relevant to the Earth’s lower mantle. Based on these results, we find that the laterally averaged (1D) density and seismic velocities support a pyrolitic composition across the whole lower mantle; we also study the transverse anisotropy of these minerals in a subducting slab near the core-mantle boundary, and confirm that the pPv phase with predominant (001) slip plane matches the seismic anisotropy in the D′′ region independent of the amount of Fe or Al. These results provide a useful resource to investigate the seismic properties of mineral assemblages in the deep mantle and to explore the mineralogical origin of heterogeneities in the lower mantle.