Liquidus phase relations in the system MgO-MgSiO3 at pressures up to 25 GPa -: constraints on crystallization of a molten Hadean mantle

An understanding of the details of the crystallization history of a Hadean magma ocean requires a knowledge of liquidus phase relations of the mantle at very high pressures. The system MgO-MgSiO3 is a good simplified chemical model of the mantle and provides a foundation for study of more complex systems that approximate the composition of the mantle more closely. We present a determination of the pressure-temperature univariant curve for the reaction Mg2SiO4 + MgSiO3 = Liquid at pressures up to 16.5 GPa, new data on the change in composition of the eutectic liquid with pressure, and a pressure-temperature projection of univariant and invariant equilibria in the system MgO-MgSiO3 at pressures up to 25 GPa. With increasing pressure, the eutectic curve between Mg2SiO4 and MgSiO3 encounters five invariant points as follows: orthoenstatite + clinoenstatite + forsterite + liquid, 11.6 GPa, 2150 degrees C; clinoenstatite + majorite + forsterite + liquid, 16.5 GPa, 2240 degrees C; majorite + forsterite + modified spinel + liquid, 16.6 GPa, 2245 degrees C; majorite + perovskite + modified spinel + liquid, 22.4 GPa, 2430 degrees C; and perovskite + modified spinel + periclase + liquid, 22.6 GPa, 2440 degrees C (last two points from data of [Gasparik, T., 1990a. Phase relations in the transition zone. J. Geophys. Res. 95, 15751-15769]). Above 22.6 GPa, no form of Mg2SiO4 is stable at liquidus temperatures, and the melting reaction changes to periclase + perovskite = liquid. The composition of the eutectic liquid, in wt.%, varies with pressure in a nearly linear fashion from 21% Mg2SiO4, 79% MgSiO3 at 2 GPa to 32% Mg2SiO4, 68% Mg2SiO4 at 16.5 GPa, and reaches its maximum enrichment in Mg (45% Mg2SiO4, 55% MgSiO3) at 22.6 GPa. These data are consistent with experimental data on natural peridotite compositions indicating that perovskite and magnesiowustite would be the main phases to crystallize in the deeper parts of a mantle magma ocean. Published partition coefficient data show that fractional crystallization of these two phases in the lower mantle would produce an upper mantle with Cl chondrite normalized Ca/Al and Ca/Ti weight ratios of 2.0-2.3, far higher than primitive upper mantle estimates of 1.1-1.25 and 0.86-1.06, respectively. However, Ca-perovskite, which would crystallize in small amounts in the lower mantle, is such a powerful sink for Ca that Ca/Al and Ca/Ti enrichment of the upper mantle could be suppressed. We conclude that extensive fractional crystallization of a deep magma ocean is not at present proscribed by element partitioning arguments. (C) 1998 Elsevier Science B.V. All rights reserved.