Melting experiments of mantle materials under lower mantle conditions with implications for magma ocean differentiation

Liquidus phase relations and major element partitioning have been determined for fertile peridotite and Cl chondritic mantle material to 33 and 35 GPa, respectively. In peridotite, the first liquidus phase changes from ferropericlase to Mg-perovskite at 31 GPa, and at 33 GPa liquidus Mg-perovskite is successively followed down temperature by ferropericlase and Ca-perovskite within a small temperature range. In CI chondritic mantle, Mg-perovskite is the liquidus phase followed down temperature by Ca-perovskite at pressures higher than 28 GPa. Ferropericlase is absent at the liquidus. Differentiation by crystal fractionation of Mg-perovskite, ferropericlase, and Ca-perovskite in a deep magma ocean has been examined for a Cl chondritic and two peridotitic bulk silicate earth models. Mass balance indicates that subtraction of about 40% Mg-perovskite and 2% Ca-perovskite from a CI chondritic bulk silicate earth yields a residual melt with some characteristics of model fertile upper mantle composition. A crystal layer composed of Mg- and Ca-perovskites would pile up to a depth ca. 1400 km, and may be characterized as an enriched and possibly heat-producing reservoir by the high capability of Ca-perovskite to accommodate large cations such as La and alkaline elements. The observed effect of pressure on element partitioning indicates that better mass balance solutions may be obtained for higher pressure liquidus phase compositions. For peridotitic bulk silicate earth models, fractionation would be quite limited, up to 10 wt.% of Mg-perovskite in addition to trace amount of Ca-Pv. (C) 2004 Elsevier B.V. All rights reserved.