Some dynamical consequences of partial melting in Earth’s deep mantle

We use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of Earth’s lowermost mantle, termed “ultralow-velocity zones” or ULVZ, are derived from partial melting of ordinary mantle. The models span the lower 500 km of Earth’s mantle, employ both temperature and melting-related contributions to buoyancy, and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer. Partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core–mantle boundary as a consequence of its isothermal and isobaric conditions, although it naturally develops variations in thickness greater than two orders of magnitude, with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere. We find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ULVZ; however, if such a melt percolates downward a dense basal liquid layer accumulates above the core–mantle boundary. The apparent requirement of a volumetrically dense and non-percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ULVZ arise from melting of ordinary mantle, and suggests that such features likely form as a consequence of more complex processes. Furthermore, these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the CMB.