Low core-mantle boundary temperature inferred from the solidus of pyrolite

Abstract

Melting temperature of the Earth's mantle provides key constraints on the thermal structures in both the mantle and the core. We show that the solidus temperature of a primitive (pyrolitic) mantle is as low as 3570 ± 200 kelvin at the core-mantle boundary (CMB), based on the observation of small amounts of partial melt by three-dimensional x-ray micro-tomography imaging. It gives the upper bound of the CMB temperature (TCMB) since the mantle side of the CMB is not globally molten. Such remarkably low TCMB indicates that silicate post-perovskite may be present ubiquitously above the CMB. The low TCMB also requires that the melting temperature of outer core alloy is depressed largely by the impurity effect, suggesting that hydrogen is an important alloying element.