Thermoelastic Properties of Olivine and Wadsleyite [Fe_x,Mg_(1-x)]_2SiO_4: Their Relationship to the 410 km Seismic Discontinuity

TL;DR

This study uses advanced computational methods to calculate the thermoelastic properties of olivine and wadsleyite, linking their elastic behavior to the Earth's 410 km seismic discontinuity.

Contribution

It combines DFT, QHA, and VDoS modeling to accurately predict elastic moduli and sound velocities of key mantle minerals under relevant conditions, aligning with experimental data.

Findings

Elastic moduli and velocities match experimental results

Velocity contrasts support seismic observations at 410 km discontinuity

Shear velocity contrast is smaller than compressional, consistent with PREM

Abstract

We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model of vibrational density of states (VDoS) to calculate elastic moduli and sound velocities of alpha- and beta-[Fe_x,Mg_(1-x)]_2SiO_4 (olivine and wadsleyite), the most abundant minerals of the Earth's upper mantle (UM) and upper transition zone (TZ). Comparison with experimental values at room-temperature and high pressure or ambient-pressure and high temperature show good agreement with our first-principles findings. Using our results, we investigate the discontinuities in elastic moduli and velocities associated with the alpha-to-beta-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and temperatures relevant to the 410 km seismic discontinuity. We find the compressional velocity contrast to be smaller than the shear velocity contrast, in agreement with the preliminary reference Earth model (PREM).