Implications of post-perovskite on the density of lowermost mantle structures based on geoid and core-mantle boundary topography observations

The origin of the two large low-velocity provinces (LLVPs) remains debated today. The debate has often focused on their density, which can provide us insight into their origin. For example, if LLVPs were long-lived features, they would require a higher intrinsic density (the difference in density to the background mantle under the same temperature and pressure) than their surroundings to negate their positive thermal buoyancy and to remain physically stable at the base of the mantle for billions of years. Better constraints on the origin of LLVPs would provide further insight into dynamic processes at the lower boundary of the mantle. This has implications for how the deep mantle impacts Earth’s surface.

Long-wavelength observations of the geoid and core-mantle boundary (CMB) topography are particularly sensitive to the lowermost mantle. These observables have therefore been used to infer the density of LLVPs, often attributing a higher intrinsic density, if any, to chemical heterogeneity. Yet, many of these studies have not jointly considered the effects of chemical composition with the transition from bridgmanite to post-perovskite on lowermost mantle density. This phase transition is associated with a 1-2% increase in density, but occurs primarily in cold regions, thus impacting the amplitude and spatial patterns of the geoid and CMB topography. Therefore, the presence of post-perovskite can affect inferences of LLVP chemical composition and density from geodetic observables. It is therefore important to take the presence of post-perovskite into account when inferring LLVP density and chemical composition from geoid and CMB topography observations.      

Here, we investigate the geodetic signatures expected from a range of scenarios related to the distribution of post-perovskite within different models of lowermost mantle temperature and composition. We calculate synthetic density fields from existing temperature and compositional fields as predicted by geodynamic simulations and a recent thermodynamic database. These density fields are then convolved with kernels derived from models of instantaneous mantle flow to obtain synthetic geodetic observables. We show that the effect of a higher post-perovskite density alone produces a comparable effect to chemical heterogeneity on the geoid and CMB topography. This implies that the effects of post-perovskite need to be taken into account when modelling dynamic processes and inferring physical properties in the deep mantle.