Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.

Dorfman, S.M.; Nabiei, F.; Boukaré, C.-E.; Prakapenka, V.B.; Cantoni, M.; Badro, J.; Gillet, P. Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities. Minerals 202111, 512. abstract

Diffraction patterns for phase assemblages synthesized from Fa72 at 33–89 GPa. Bdg = bridgmanite, pPv = post-perovskite, Wus = wüstite/magnesiowüstite, Sti = stishovite, N = NaCl medium. Due to overlap between oxide and NaCl peaks, we cannot resolve whether the structure is B1 cubic or rhombohedral, though EDX results indicate 92–98% FeO. Reference stick patterns for Bdg and Sti are provided below. For these experiments, λ = 0.3344 Å.