Fractional Melting and Freezing in the Deep Mantle and Implications for the Formation of a Basal Magma Ocean

Processes that operated in the early Earth have largely been erased or overprinted by subsequent evolution. However, some traces may persist in the deep Earth as imaged by seismology. Large‐scale features with reduced seismic velocities are simply explained as variations of composition, and small‐scale ultra‐low velocity zones are explained by the presence of Fe‐rich material that may be partially molten. Both can originate from fractional crystallization of an originally thick basal magma ocean (BMO). Many questions are raised by this scenario regarding properties of melts with various compositions, in particular the partition coefficients between melt and crystals of various elements and their relative densities. After reviewing recent progress on both the structure of the lower mantle and the mineral physics associated with partial melting/freezing of silicates at high pressure, we discuss several ways in which a BMO can be produced. We argue that, in most cases, independently of whether a melt of composition similar to that of the bulk mantle is more or less dense than crystals in equilibrium, the compositional evolution of both the magma and the solid should lead to the formation of a dense, Fe‐rich BMO whose subsequent slow evolution would explain some features of the present lower mantle. 1 Laboratoire de geologie de Lyon, ENS de Lyon, Universite Lyon-1, CNRS, Lyon, France 2 Earth‐Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo, Japan c07.indd 123 8/24/2015 4:02:55 PM