Geodynamic and seismic constraints on the thermochemical structure and dynamics of convection in the deep mantle

Abstract
We revisit a recent study by Forte & Mitrovica in which global geophysical observables associated with mantle convection were inverted and the existence of a strong increase in viscosity near a depth of 2000 km was inferred. Employing mineral–physics data and theory we also showed that, although there are chemical anomalies in the lowermost mantle, they are unable to inhibit the dominant thermal buoyancy of the deep–mantle mega–plumes below Africa and the Pacific Ocean. New Monte Carlo simulations are employed to explore the impact of uncertainties in current mineral–physics constraints on inferences of deep–mantle thermochemical structure. To explore the impact of the high–viscosity peak at a depth of 2000 km on the evolution of lower–mantle structure, we carried out time–dependent convection simulations. The latter show that the stability and longevity of the dominant long–wavelength heterogeneity in the lowermost mantle are controlled by this viscosity peak.