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, KD ${K}_{D}$, 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 KD ${K}_{D}$ 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 RS/P ${R}_{S/P}$ partial derivative lnVS/partial derivative lnVP $\left(\partial \,\mathrm{ln}{V}_{S}/\partial \,\mathrm{ln}{V}_{P}\right)$ including iron partitioning effects resembles quantitatively RS/P ${R}_{S/P}$'s inferred from several tomographic studies down to 2,400 km depth.