Is the Earths mantle Heterogeneous or Homogeneous?
Actually I would think that strong convection would lead more to homogeneity than inhomogeneity. Convection leads to mixing, mixing leads to homogeneity. The convection is not fast enough to homogenize the system. The descending crustal slabs persist as a distinct mass deep into the mantle, according to geophysics. The composition fo crustal material is very different from the composition of the bulk mantle. Elsewhere, partial melting and migration of liquid leaves residuals of distinct compositions different from bulk mass as well; these episodes are not uniformly distributed throughout the mantle and how the fractionation occurs depends on local conditions. The pressure-temperature gradient also impact mineralogical stabilities and leads to a more or less similar depth-composition profile throughout the mantle, which is essentially what you refer to as layering but isn’t strictly speaking occurring in layers, although there are distinct transformation zones at relatively constant dep
