Fe2+ partitioning in Al-free pyrolite: consequences for seismic velocities and heterogeneities

TL;DR

This study investigates how Fe2+ partitioning and spin crossover in the lower mantle affect seismic velocities and heterogeneities, providing insights into Earth's deep interior composition and dynamics.

Contribution

It offers a detailed analysis of Fe2+ partitioning behavior using ab initio calculations, highlighting the impact of spin crossover on seismic heterogeneity ratios.

Findings

Seismic velocities are mainly influenced by the iron spin crossover.

Iron partitioning contributes to thermally induced velocity heterogeneity.

Predicted $R_{S/P}$ ratios align with tomographic observations down to 2,400 km depth.

Abstract

Iron partitioning among the main lower mantle phases, bridgmanite (Bm) and ferropericlase (Fp), has non-monotonic behavior owing to the high-spin to low-spin crossover in ferrous iron (Fe2+) in Fp. Results of previous studies of the iron partitioning coefficient between these phases, $K_D$, still have considerable uncertainty. Here, we investigate the Fe2+ partitioning behavior using well-documented ab initio free energy results plus new updates. Although we focus on Fe2+ only, we describe the effect of this iron spin crossover (ISC) on $K_D$ and of the latter on compositions and seismic velocities in a pyrolitic aggregate. Our results suggest that its velocities are mainly affected by the ISC and less so by the Fe2+ partitioning. In contrast, iron partitioning manifests in thermally induced velocity heterogeneity ratios. Prediction of the seismological parameter $R_{S/P}$ ($\partial \ln V_S/\partial \ln V_P$) including iron partitioning effects resembles quantitatively $R_{S/P}$'s inferred from several tomographic studies down to 2,400 km depth.