Melting relations and major element partitioning in an oxidized bulk Earth model composition at 15–26 GPa

Abstract

Melting experiments were performed on an FeO-rich bulk Earth model composition in the CMFAS system in order to investigate the partitioning of major elements between coexisting minerals and melts. The starting material (34.2% SiO 2, 3.86% Al 2O 3, 35.2% FeO, 25.0% MgO and 1.88% CaO), contained in Re-capsules, was a mixture of crystalline forsterite and fayalite, and a glass containing SiO 2, Al 2O 3, and CaO. Olivine is the first liquidus phase at 10 GPa but is replaced by majoritic garnet (ga) in the 15–26 GPa range. Magnesiowüstite (mw) crystallizes close to the liquidus and is joined by perovskite (pv) at 26 GPa. The quenched melt compositions are homogeneous throughout the melt region of the charges and are only slightly enriched in Si, Ca and Fe, and depleted in Mg, relative to the starting composition. The Fe/Mg and Ca/Al ratios in all of the minerals increase rapidly below the liquidus to become compatible with the bulk composition at the solidus. At 26 GPa, a relative density sequence of mw>pv>melt>ga is observed. This indicates that majorite floating, combined with the sinking of magnesiowüstite and perovskite can be expected during the solidification of a Hadean magma ocean and in hot mantle plumes early in the Earth's history. The mineral–melt partitioning relations indicate that fractional crystallization or partial melting in the transition zone and the upper part of the lower mantle would increase the Fe/Mg and Ca/Al ratios of the melt, even if magnesiowüstite was predominant in the solid fraction. A significant contribution of accumulated mw to the segregation of the protocore is therefore unlikely. The suggested process of perovskite fractionation to the lower mantle is not capable of increasing the Mg/Si ratio in the residual melt, and the combined fractionation of perovskite and magnesiowüstite produces a melt with elevated ratios of Si/Mg, Ca/Al and Fe/Mg.