Thermoelasticity of Fe2+-bearing bridgmanite

TL;DR

This study uses first-principles calculations to investigate the high-temperature elastic properties of Fe2+-bearing bridgmanite, clarifying the effects of iron displacement and comparing results with experimental and Earth model data.

Contribution

It demonstrates that iron displacement, not spin crossover, explains thermoelastic changes in bridgmanite, providing insights into Earth's lower mantle properties.

Findings

Elastic moduli agree with experimental data for pure bridgmanite.

Iron displacement explains thermoelastic changes better than spin crossover.

Calculated velocities align with Earth model data in the lower mantle.

Abstract

We present LDA+U calculations of high temperature elastic properties of bridgmanite with composition (Mg$_{(1-x)}$Fe$_{x}^{2+}$)SiO$_3$ for $0\le{x}\le0.125$. Results of elastic moduli and acoustic velocities for the Mg-end member (x=0) agree very well with the latest high pressure and high temperature experimental measurements. In the iron-bearing system, we focus particularly on the change in thermoelastic parameters across the state change that occurs in ferrous iron above $\sim$30 GPa, often attributed to a high-spin (HS) to intermediate spin (IS) crossover but explained by first principles calculations as a lateral displacement of substitutional iron in the perovskite cage. We show that the measured effect of this change on the equation of state of this system can be explained by the lateral displacement of substitutional iron, not by the HS to IS crossover. The calculated elastic properties of (Mg$_{0.875}$Fe$_{0.125}^{2+}$)SiO$_3$ along an adiabatic mantle geotherm, somewhat overestimate longitudinal velocities but produce densities and shear velocities quite consistent with Preliminary Reference Earth Model data throughout most of the lower mantle.