Liquidus Phase Relations in Pyrolite at Pressures of the Transition Zone

According to some models of planetary accretion, the Earth may have been largely molten as a consequence of numerous impacts during its accretional history, leading to the formation of a global magma ocean. Subsequently, cooling of this putative magma ocean from the bottom up could have led to the stratification of the mantle by fractional crystallization. To constrain the geochemical evolution of mantle after crystallization of the magma ocean, knowledge of the pressure-temperature stability field of mantle minerals above solidus conditions is necessary. Recent experimental studies indicate that the liquidus and solidus of anhydrous fertile peridotite at pressures of the transition zone are at a considerably lower temperature than previously estimated. In view of these recent discrepancies, we reexamine the liquidus phase relations in pyrolite in the 10-14 GPa pressure range. Experiments were performed in a multi-anvil device, using short Re- and C-capsules and thick zirconia insulators to minimize thermal gradients. Preliminary results reveal that the liquidus is located between 2050-2100 C at 14 GPa and between 1950-2000 C at 10 GPa. These brackets are significantly lower than reported earlier at identical pressures. Furthermore, we find that Ca-poor pyroxene is stable in the melting interval at 10 GPa, i.e. about 2-3 GPa higher than previously observed. Experiments to clarify the phase relations between orthopyroxene, olivine and garnet at pressures of the transition zone are in progress.