Low Core-Mantle Boundary Temperature Inferred from the Solidus of Pyrolite

Melting Moments The boundary between Earth's core and mantle defines where the iron-rich liquid outer core meets the more chemically heterogeneous solid lower mantle and is marked by a sharp thermal gradient of nearly 1500 kelvin. The precise relationship between temperature and melting of the lowermost mantle constrains the structure and heat flow across the core-mantle boundary. In order to identify trace amounts of liquid as melting initiates, Nomura et al. (p. 522, published online 16 January) performed x-ray microtomographic imaging of rocks of a primitive mantle composition that had been subjected to high pressures and temperatures in a diamond anvil cell. The experimentally determined maximum melting point of 3570 kelvin suggests that some phases typically thought to lose stability in the lowermost mantle, such as MgSiO3-rich post-perovskite, may be more widely distributed than expected. High-pressure experiments suggest that Earth’s mantle melts at lower temperatures than previously predicted. The melting temperature of Earth’s mantle provides key constraints on the thermal structures of both the mantle and the core. Through high-pressure experiments and three-dimensional x-ray microtomographic imaging, we showed that the solidus temperature of a primitive (pyrolitic) mantle is as low as 3570 ± 200 kelvin at pressures expected near the boundary between the mantle and the outer core. Because the lowermost mantle is not globally molten, this provides an upper bound of the temperature at the core-mantle boundary (TCMB). Such remarkably low TCMB implies that the post-perovskite phase is present in wide areas of the lowermost mantle. The low TCMB also requires that the melting temperature of the outer core is depressed largely by impurities such as hydrogen.