Melting relations and the equation of state of magmas at high pressure: Application to geodynamics

The melting relations of mantle minerals, together with the physical properties of silicate melts at high pressure, have been extensively studied during the previous decade. The melting relation of minerals and the equation of state of magmas are important properties for deducing the formation and differentiation of the Earth, and especially for elucidating the nature of the terrestrial magma ocean and the subsequent formation of the core, mantle, and crust of the Earth. Because these magmas are compressible, we expect that the olivine–magma density crossover played an important role in controlling the geochemical characteristics of the primitive mantle after the magma ocean stage of the primordial Earth. The crystal–magma density crossover is also expected to occur at the base of the upper mantle in the presence of volatiles, and at the base of the lower mantle in the Earth today. The existence of dense magmas is consistent with seismological observations of low-velocity regions existing at the base of the upper mantle beneath Japan, Europe, and the USA, and the ultralow-velocity zone (ULVZ) at the core–mantle boundary of the present Earth.