ThicknessandClapeyronslopeofthepost-perovskite boundary

coulddecrease the pPv boundary thickness, the boundary may remainthick in a pyrolitic composition because of the effects of Aland the rapid temperature increase in the D99 layer. The pPvboundary would be particularly thick in regions with an elevatedAl content and/or a low Mg/Si ratio, reducing the effects of thelarge positive Clapeyron slope on the buoyancy of thermalanomalies and stabilizing compositional heterogeneities in thelowermost mantle. If the pPv transition is the source of the D99discontinuity, regions with sharp discontinuities may requiredistinct compositions, such as a higher Mg/Si ratio or a lower Alcontent.The lowest 200–400km of the mantle (the D99 layer) is known tohave seismic properties distinct from those of the overlying mantle,including a laterally varying discontinuity at the top of the layer (theD99 discontinuity)