Constraints on the melting temperature of the lower mantle from high-pressure experiments on MgO and magnesioüstite

Abstract

THE melting temperatures of minerals in the Mg–Fe–Si–O-system play a fundamental role in the chemical differentiation, rheology and geodynamics of the Earth's lower mantle. We have previously shown1 that the melting curve of (Mg, Fe)SiO3-perovskite—the dominant mineral in the lower mantle—is extremely steep, implying melting temperatures at the bottom of the lower mantle in excess of 7,000 K. The large difference between actual mantle temperatures and the melting temperature inferred from our experiments suggests that the viscosity of the lower mantle is much larger than that typically used in convection models2. Theoretical estimates of the melting temperature of MgO (refs 3–5) suggest even higher melting temperatures for (Mg, Fe)O-magnesiowiistite, the second most abundant mineral in the lower mantle. We show here, however, that the melting curves of these two minerals are flat compared to the perovskite melting curve, thus lowering the upper bounds for the solidus in the lowermost mantle to about 5,000 K. This reduces the estimate of the viscosity to more realistic levels but still rules out large-scale melting in the lower mantle. Because magnesiowiistite is slightly more dense than Mg–Fe–Si-perovskite due to iron partitioning, chemical segregation in the lower mantle cannot be excluded in regions where the local temperature exceeds the solidus.