  Joint interpretation of bulk and shear wave speeds constrains the chemistry
of the deep mantle. At all depths, the diversity of wave speeds cannot be
explained by an isochemical mantle. Between 1000 and 2500 km depth,
hypothetical mantle models containing an electronic spin crossover in (Mg,Fe)O
provide a significantly better fit to the wave-speed distributions, as well as
more realistic temperatures and silica contents, than models without a spin
crossover. Below 2500 km, wave speed distributions are explained by enrichment
in silica towards the core-mantle-boundary. This silica enrichment may
represent the fractionated remains of an ancient basal magma ocean.
