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

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.