Unsolved problems in the lowermost mantle

Many characteristics of D″ layer may be attributed to the recently discovered MgSiO₃ post-perovskite phase without chemical heterogeneities. They include a sharp discontinuity at the top of D″, regional variation in seismic anisotropy, and a steep Clapeyron slope. However, some features remain unexplained. The seismically inferred velocity jump is too large in comparison to first principles calculations, and the sharpness of the discontinuity may require a chemical boundary. Chemical heterogeneity may play an important role in addition to the phase transformation from perovskite to post-perovskite. Phase transformation and chemical heterogeneity and the attendant changes in physical properties, such as rheology and thermal conductivity, are likely to play competing roles in defining the dynamical stability of the D″ layer. Revealing the relative roles between phase transition and chemical anomalies is an outstanding challenge in the study of the role of D″ in thermal-chemical evolution of the Earth.