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 Ha dean magma ocean requires a knowledge of liquidus phase relations of t he mantle at very high pressures. The system MgO-MgSiO3 is a good simp lified chemical model of the mantle and provides a foundation for stud y of more complex systems that approximate the composition of the mant le more closely. We present a determination of the pressure-temperatur e univariant curve for the reaction Mg2SiO4 + MgSiO3 = Liquid at press ures up to 16.5 GPa, new data on the change in composition of the eute ctic liquid with pressure, and a pressure-temperature projection of un ivariant and invariant equilibria in the system MgO-MgSiO3 at pressure s up to 25 GPa. With increasing pressure, the eutectic curve between M g2SiO4 and MgSiO3 encounters five invariant points as follows: orthoen statite + clinoenstatite + forsterite + liquid, 11.6 GPa, 2150 degrees C; clinoenstatite + majorite + forsterite + liquid, 16.5 GPa, 2240 de grees C; majorite + forsterite + modified spinel + liquid, 16.6 GPa, 2 245 degrees C; majorite + perovskite + modified spinel + liquid, 22.4 GPa, 2430 degrees C; and perovskite + modified spinel + periclase + li quid, 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 liq uidus temperatures, and the melting reaction changes to periclase + pe rovskite = liquid. The composition of the eutectic liquid, in wt.%, va ries with pressure in a nearly linear fashion from 21% Mg2SiO4, 79% Mg SiO3 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 comp ositions indicating that perovskite and magnesiowustite would be the m ain phases to crystallize in the deeper parts of a mantle magma ocean. Published partition coefficient data show that fractional crystalliza tion of these two phases in the lower mantle would produce an upper ma ntle 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 crystall ize 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 suppre ssed. We conclude that extensive fractional crystallization of a deep magma ocean is not at present proscribed by element partitioning argum ents. (C) 1998 Elsevier Science B.V. All rights reserved.