On the interaction of the geotherm with a post-perovskite phase transition in the deep mantle

The energy balance in the presence of a perovskite (Pv) to post-perovskite (pPv) transition within Earth's D” layer is examined in order to explore the relationship between changes in seismic velocity associated with this phase change, the extent of the two-phase Pv-pPv coexistence region, and the thermal structure of the deep mantle. This is motivated in part by the fact that discontinuities attributed to the Pv-pPv transition are inferred to be seismically sharper than permitted by some recent estimates of the pressure increment across the two-phase co-existence region. Here it is shown that sharp gradients in phase abundance may arise even when the two-phase loop is very broad, and therefore the pressure increment determined from thermodynamic stability alone is a poor proxy for predicting the sharpness of Pv-pPv related seismic discontinuities. The change in pPv fraction over the steepest gradients in phase can also be highly variable, which would lead to potentially complex variations in the total strength of seismic discontinuities. Latent heat plays an important role in the structure of the pPv phase change and its influence upon the geotherm. For the double-crossing scenario - in which a deeper reverse transformation from pPv to Pv occurs in a steep thermal boundary layer - latent heat release from the shallower Pv-pPv transition moderates the effects of latent heat absorption at the deeper reverse transition.