The composition and redox state of hydrous partial melts generated at the top of the lower mantle

Seismic low-velocity zones, that are proposed to form through hydrous partial melting, have been observed at the top of the lower mantle. The buoyancy of such melts will depend significantly on their iron content, but little is known about how iron partitions into melts under these conditions. In this study, ferrous and ferric iron partitioning between bridgmanite and melt has been studied at 25 GPa and 1973 K. A model, based on these data, indicates that increasing oxygen fugacity and the Al content of bridgmanite can change the mineral-melt partitioning of iron from incompatible to compatible. Hydrous partial melts at the top of a pyrolitic lower mantle are calculated to have an Fe3+/EFe ratio of 0.26-0.5 and a total iron content of 5.9-7.8 wt. % in the fO2 range between IW+2 and IW+4. Such melts would be less dense than the surrounding lower mantle and would migrate upwards into the transition zone. This could facilitate the formation of superdeep diamonds and crystallize an assemblage that also contains ferropericlase as the melts lose water to the surrounding ringwoodite.