Quantitative assessment of tomographic proxies for lowermost mantle composition and mineralogy

Large low velocity provinces (LLVPs) dominate the lowermost mantle, but their detailed thermochemical nature remains a topic of discussion. Particularly, it is unclear to what extent the bridgmanite to post-perovskite phase transition is able to explain their velocity characteristics. Robust constraints on the origin of these seismic structures would shed light on large-scale dynamic processes in the mantle. Here, we examine the combined effects of both chemical heterogeneity and phase transitions on lowermost mantle tomographic signatures. We calculate synthetic seismic velocities expected from a range of scenarios for the stability of post-perovskite within models of different lowermost mantle temperatures and compositions using recent thermodynamic data. These are filtered to account for the limited resolution of seismic tomography. We are able to quantitatively compare our synthetic velocities with a recent Backus-Gilbert based tomography model. Crucially, this model provides robust ratios and correlations of velocity anomalies derived from nearly identical Vp and Vs resolution, and includes uncertainty quantification that accounts for both data and theoretical errors. By rejecting synthetic models that do not fit within tomographic uncertainties, we quantitatively show the following: (i) the root mean square of velocity anomalies cannot be entirely explained by LLVPs with a primordial composition; and (ii) elevated R_{s/p} (= dlnVs/dlnVp) and negative correlation between shear-wave and bulk-sound velocity (r_{s–c}) in the lowermost mantle cannot be explained by thermochemical LLVPs alone, but require bridgmanite and post-perovskite to co-occur at depth in the mantle. These seismological observables thus do not provide useful information about chemical heterogeneity in the deep mantle.