Is the lower mantle chemically distinct from the upper mantle?
In plain words
Sinking plates reach the deep mantle, so the mantle is not split into two sealed layers. Yet the deep mantle may still differ in composition, for example holding more of the silicon-rich mineral bridgmanite.
Precise statement
Determine whether the bulk lower mantle (660 to about 2700 km depth) has the same composition as the upper-mantle source of mid-ocean-ridge basalts (pyrolite) or is chemically distinct, for example Si-enriched with higher bridgmanite fraction or with bridgmanite-enriched domains. An answer is a lower-mantle composition with uncertainties that fits seismic velocity and density profiles through mineral-physics equations of state and is consistent with geochemical mass balance.
What would settle it
Inversion of seismic velocity and density profiles against mineral-physics equations of state at lower-mantle pressures and temperatures, with resolved trade-offs between temperature and composition.