Melting phase relations in MgO-FeO-SiO2 ternary system at high pressure

Seismological observations show that the presence of a small fraction of partial melt at the base of the mantle. The knowledge of chemical composition of such partial melt is key to understand its buoyancy and stability in the lowermost mantle. Recent melting experiments performed in the deep lower mantle conditions demonstrated that 1) MgSiO₃-rich perovskite (bridgemanite) 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 [Nomura et al., 2011 Nature; Tateno et al., 2014 JGR], suggesting that melts evolves towards a FeO-rich / SiO2-poor composition upon crystallization.

Here we carried out melting experiments under both shallow and deep lower mantle pressures using a laser-heated diamond-anvil cell (DAC), in order to determine melting phase equilibria in the MgO-FeO-SiO₂ ternary system. Several different starting materials were used. After heating at high pressure, sample was recovered from a DAC, and then examined with dual beam scanning microprobe (FIB + FE-SEM) (Versa 3D^TM, FEI) and field-emission-type electron probe microanalyzer (FE-EPMA) (JXA-8530F, JEOL). On the basis of X-ray maps and quantitative point-analyses, quenched partial melt with non-stoichiometric composition was found at the center of the sample (the hottest part) and surrounded by a liquidus phase. The partial melts were sometimes in direct contact with more than one solid phases such as bridgemanite + ferropericlase or bridgemanite + stishovite, which can tightly constrain the locations of cotectic lines. These results imply that eutectic melt is strongly enriched in FeO in the MgO-FeO-SiO₂ ternary system in a wide range of lower mantle pressures.