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 [ KDBridg-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 KDBridg-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 Fe2+ in the A site, substituting for Mg2+; almost all iron in fPer is ferrous Fe2+; the share of ferric Fe3+ iron in fPer is Fe3+/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 KDBridg-fPer values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe2+ 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.
- lower mantle
- iron partitioning