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

TL;DR

This study uses ab initio molecular dynamics to investigate how Fe and Al affect the elasticity of MgSiO3 minerals in Earth's lower mantle, providing insights into seismic properties and mineral composition at great depths.

Contribution

It offers new computational data on the thermoelasticity of Fe- and Al-bearing MgSiO3 minerals across various conditions, enhancing understanding of lower mantle composition and seismic behavior.

Findings

Pyrolite-like mineral assemblage fits seismological data well

MgSiO3 post-perovskite with (001) slip explains seismic anisotropy

Results support a mineral composition consistent with seismic observations in D''

Abstract

Fe and Al are two of the most important rock-forming elements other than Mg, Si, and O. Their presence in the lower mantle's most abundant minerals, MgSiO_3 bridgmanite, MgSiO_3 post-perovskite and MgO periclase, alters their elastic properties. However, knowledge on the thermoelasticity of Fe- and Al-bearing MgSiO_3 bridgmanite, and post-perovskite is scarce. In this study, we perform ab initio molecular dynamics to calculate the elastic and seismic properties of pure, Fe^{3+}- and Fe^{2+}-, and Al^{3+}-bearing MgSiO_3 perovskite and post-perovskite, over a wide range of pressures, temperatures, and Fe/Al compositions. Our results show that a mineral assemblage resembling pyrolite fits a 1D seismological model well, down to, at least, a few hundred kilometers above the core-mantle boundary, i.e. the top of the D'' region. In D'', a similar composition is still an excellent fit to the average velocities and fairly approximate to the density. We also implement polycrystal plasticity with a geodynamic model to predict resulting seismic anisotropy, and find post-perovskite with predominant (001) slip across all compositions agrees best with seismic observations in the D''.