Disproportionation of Iron in the Lower Mantle

Abstract:

It has been proposed that metallic iron exists as an accessory phase in the lower mantle, as a result of a disproportionation of Fe²⁺ to Fe⁰ and Fe³⁺. If correct, this has significant implications for the siderophile element geochemistry of the lower mantle, notably through its impact on isotopic tracers and platinum group element distributions. Metallic iron could also serve as a likely host for volatile elements in the lower mantle, such as C, S, and H, impacting the mantle’s carbon and hydrogen budgets. It is understood that bridgmanite is the dominant phase in the lower mantle, and it has been shown that the presence of Al promotes the partitioning of Fe³⁺ into the perovskite structure as an FeAlO₃ component, charge balanced by metallic iron. Frost et al. [2004] proposed that this disproportionation process occurs in the lower mantle, where the formation of aluminous perovskite implies the precipitation of approximately 1 wt% metallic Fe-rich alloy. However, there has been only sporadic subsequent study to confirm this process at deeper lower mantle conditions. Here we report laser-heated diamond anvil cell experiments on almandine-pyrope garnet and peridotite at lower mantle conditions ranging from 40-50 GPa. X-ray diffraction and SEM analyses both indicate that reaction products contained multiple silicates including perovskites and stishovite. X-ray diffraction measurements were insensitive to iron metal, but TEM/SEM/EDS examination of FIB/SEM recovered samples reveals iron metal grains dispersed throughout the samples, providing evidence that the iron disproportionation reaction did occur.