Iron partitioning in natural lower-mantle minerals: Toward a chemically heterogeneous lower mantle

Abstract

The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep lower mantle are iron-rich and differ markedly from a pyrolitic composition. In the lowermost lower mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association [ K D Bridg-fPer = ( [ Fe / Mg ] Bridg ) / ( [ Fe / Mg ] fPer ) at in juvenile diamond inclusions is as low as 0.1–0.2. During ascent of the diamonds with their inclusions to the surface, the K D Bridg-fPer eventually increases to values of 0.4–0.5 and even as high as 0.7. The details of the element partitioning between natural Bridg and fPer in the lower mantle are as follows: iron in Bridg is ferrous Fe 2+ in the A site, substituting for Mg 2+ ; almost all iron in fPer is ferrous Fe 2+ ; the share of ferric Fe 3+ iron in fPer is Fe 3+ /ΣFe = 8–12 at%; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the lower part of the lower mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse K D Bridg-fPer values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe 2+ in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the lower mantle.