MELTING TEMPERATURE OF THE EARTH'S MANTLE AND CORE: Earth's Thermal Structure

Although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs, the temperature distribution in the lower mantle is poorly known. Models depend strongly on the assumptions of the amount of internal heating and the viscosity profile. New melting data on iron to pressures of the outer core (2 Mbar) and the observed strong decrease of eutectic melting depression in the Fe-FeO-FeS system with increasing pressure, however, tightly constrain the temperature at the inner-core boundary to slightly less than 5000 K. This estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements. The resulting temperature at the core-mantle boundary of about 4000 K then requires a large temperature gradient at the bottom of the lower mantle of about 1500 K. Recent findings of the very high melting temperatures of the major lower-mantle materials Mg-Si-perovskite and magnesiowüstite indicate that this increase in temperature does not cause melting in the lower mantle.