A melting model for the lowermost mantle using Clapeyron slopes derived from experimental data: Consequences for the thickness of ultralow velocity zones (ULVZs)

Ultralow velocity zones (ULVZs) are relatively thin regions directly above the core‐mantle boundary (CMB) that exhibit marked seismic P ‐ and S ‐velocity reductions. A viable explanation for the reduction is the presence of melt fractions within ULVZs. Since melt was found to be denser than solid in melting experiments at lowermost mantle pressures, partially molten ULVZs should exhibit negative buoyancy. Using published experimental data, we present a melting model based on Clapeyron slopes for the formation of ULVZs as partially molten regions above the CMB and apply the resulting melting curves and latent heat effects in fully dynamic, regionally constant viscosity convection simulations of the lowermost mantle. We find that the height of the ULVZs depends only moderately on Rayleigh number but strongly decreases with increasing excess density of melt over solid. The models predict excess density of at least 1% to explain observed heights. The combined effect of topography and latent heat of melting reduces the vigor of mantle convection only very slightly, while if combined with a decrease of the ULVZ viscosity, mantle flow velocities are significantly enhanced near the CMB, and overall mantle temperatures are notably increased. ULVZ heights are found to be insensitive to ULVZ viscosity (for the range isoviscous to 1/100 the viscosity of the ambient mantle).