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. Results of previous studies of the iron partitioning coefficient between these phases, K_D, still have considerable uncertainty. Here, we investigate this Fe²⁺ partitioning behavior using well-documented <i>ab initio</i> free energy results plus some new updates. Although we focus on Fe²⁺ only, we describe in detail the effect of this iron spin crossover (ISC) on K_D and of the latter on mineral compositions and seismic velocities of an aggregate with typical pyrolitic composition. Our results suggest the velocities of this aggregate are mainly affected by the ISC and much less so by the redistribution of Fe²⁺ between these phases. In contrast, iron partitioning effects manifest visibly in thermally induced velocity heterogeneity ratios. Prediction of the important seismological parameter R_S/P including ISC and iron partitioning effects resembles qualitatively and quantitatively R_S/P's inferred from several tomographic studies.