Partitioning of elements among two silicate perovskites, superphase B, and volatile-bearing melt at 23 GPa and 1500–1600°C

The partitioning of Si, Mg, and Ca in major concentrations, and Al, Ti, Sc and Sm in minor concentrations among MgSiO3 perovskite, CaSiO3 perovskite, superphase B -Mg10Si3O14[OH,F]4, and melt containing H2O and fluorine, was determined experimentally at 23 GPa and temperatures of 1500–1600°C with a split-sphere anvil apparatus (USSA-2000). MgSiO3 perovskite is the main repository for Sc, while Ti and Sm are primarily concentrated in CaSiO3 perovskite. Superphase B excludes all cations other than Mg and Si, except at high volatile contents of the melt when Ti and Al become compatible. Volatile-bearing melt is conspicuous by its high Mg content and low content of all other cations. This observation raises the possibility that the Mg/Si ratio of the Earthʹs upper mantle, which is elevated relative to most classes of primitive meteorites, was established by upward segregation of Mg-rich volatile-bearing melts from the lower to upper mantle. The most plausible scenario involves degassing of the lower mantle by removal of 6% of the melt produced by a low degree of partial melting. The possibility that the elevated Mg/Si ratio of the upper mantle could have resulted from upward segregation of residual Mg-rich volatile-bearing melts from a terrestrial magma ocean has been evaluated and shown to be unlikely. If the Earth ever had a magma ocean, it seems more probable that the elevated Mg/Si ratio of the observable upper mantle is either characteristic of the entire silicate portion of the planet, or that the magma ocean was limited in depth to the upper mantle.