  Earth's lower mantle is generally believed to be seismically and chemically
homogeneous because most of the key seismic parameters can be explained using a
simplified mineralogical model at expected press-temperature conditions.
However, recent high-resolution tomographic images have revealed seismic and
chemical stratification in the middle-to-lower parts of the lower mantle. Thus
far, the mechanism for the compositional stratification and seismic
inhomogeneity, especially their relationship with the speciation of iron in the
lower mantle, remains poorly understood. We have built a complete and
integrated thermodynamic model of iron and aluminum chemistry for lower mantle
conditions, and from this model has emerged a stratified picture of the
valence, spin and composition profile in the lower mantle. Within this picture
the lower mantle has an upper region with Fe3+ enriched bridgmanite with
high-spin ferropericlase and metallic Fe, and a lower region with low-spin,
iron-enriched ferropericlase coexisting with iron-depleted bridgmanite and
almost no metallic Fe. The transition between the regions occurs at a depth of
around 1600km and is driven by the spin transition in ferropericlase, which
significantly changes the iron partitioning and speciation to one that favors
Fe2+ in ferropericlase and suppresses Fe3+ and metallic iron formation These
changes lead to lowered bulk sound velocity by 3-4% around the mid-lower mantle
and enhanced density by ~1% toward the lowermost mantle. The predicted
chemically and seismically stratified lower mantle differs dramatically from
the traditional homogeneous model.
