Influence of phase transformations on lateral heterogeneity and dynamics in Earth's mantle

Using a self-consistent computation of phase equilibria and physical properties, we determine isobaric velocity–temperature and velocity–density scalings as a function of depth, focusing on the upper 800 km. The scalings contain an isomorphic part due to the influence of temperature on the physical properties of individual phases, and a metamorphic part due to variation of phase abundances and compositions with temperature. We show that the contribution from phase transformations is comparable in magnitude to that of temperature alone, and has important consequences for mantle structure. Both scalings are highly non-linear functions of temperature and depth even in the elastic limit due to the influence of phase transitions: near sharp phase transitions seismic velocities become much more sensitive to temperature. We expect the magnitude of lateral variations in seismic velocity to vary rapidly with depth. This result has important implications for the interpretation of smoothed tomographic models, particularly in the upper 1000 km, and possibly the bottom few hundred km, where phase transformations have a large influence on the structure. It will be important to include the metamorphic contribution of the scalings in geodynamical studies relating seismic structure to thermal structure or the gravity field. We find that the combined phase buoyancy of transitions near 520 km depth is equal in magnitude to that of the olivine to wadsleyite transition and should be included in future dynamical studies.