The liquidus phase relations in the MgO-FeO-SiO2 ternary system in the deep lower mantle: Implication for the solidification of a basal magma ocean

Abstract:

The large low shear velocity provinces (LLSVPs) observed in the lowermost mantle represent anomalously dense bodies, which may have been formed as a consequence of solidification of a basal magma ocean [Labrosse et al., 2007 Nature; Nomura et al., 2011 Nature]. Recent melting experiments have demonstrated that 1) MgSiO₃-rich perovskite (bridgmanite) is the first phase to crystallize from melts with a wide range of (Mg+Fe)/Si ratios in the middle to deep lower mantle and 2) iron is preferentially partitioned into melt rather than solid [Tateno et al., 2014 JGR], suggesting that melts evolve towards an FeO-rich and SiO₂-poor composition upon crystallization. However, the mineral assemblage and chemical composition of LLSVPs still remain largely uncertain. Here we carried out melting experiments at both shallow and deep lower mantle pressures using a laser-heated diamond-anvil cell (DAC), in order to examine the melting phase equilibria in the MgO-FeO-SiO₂ ternary system. A focused ion beam (FIB) and field-emission-type electron microprobe (FE-EPMA) were used for textural and chemical characterization of samples recovered from the DAC. They exhibited a melting texture with quenched partial melt at the hottest part and one or two solid phases at its outside. Our result demonstrate that a crystallizing solid assemblage changes from bridgmanite, bridgmanite + (Mg,Fe)O ferropericlase, SiO₂ seifertite + FeO-rich (Mg,Fe)O magnesiowüstite, and to seifertite + FeO upon fractional crystallization from a pyrolitic melt at the core-mantle boundary pressure. These also suggest that a residual melt left after extensive solidification is strongly enrich in FeO and thus exceedingly heavy and stable at the base of the mantle, which possibly represents an ultralow velocity zone (ULVZ).