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

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, $K_D$, 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 $K_D$ 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 $R_{S/P}$ $\left(\partial \ln V_S/\partial \ln V_P\right)$ including iron partitioning effects resembles quantitatively $R_{S/P}$’s inferred from several tomographic studies down to 2,400 km depth.