Fe2+ Partitioning in Al‐Free Pyrolite: Consequences for Seismic Velocities and Heterogeneities

Abstract

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. Previously reported iron partitioning coefficient between these phases, KD, still have considerable uncertainty. Here, we investigate the Fe2+ partitioning behavior using ab initio free energy results. Although we focus on Fe2+ only, we describe the effect of this iron spin crossover (ISC) on KD 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 RS/P ∂lnVS/∂lnVP including iron partitioning effects resembles quantitatively RS/P’s inferred from several tomographic studies down to 2,400 km depth.

Keywords

• lower mantle
• velocity heterogeneities
• iron spin crossover
• iron partitioning
• seismic velocities