Melting relations in the system Mg 4Si 4O 12 (En)Mg 3Al 2Si 3O 12 (Py) at high pressures

Abstract

Melting experiments in the system Mg 4Si 4O 12 (En)Mg 3Al 2Si 3O 12 (Py) have been carried out at 15 and 25 GPa using a uniaxial split-sphere high-pressure apparatus. Quenched melt was identified by textural and compositional observation by scanning electron microscopy and energy dispersive electron probe microanalysis. Melting on the join clinoenstatite-pyrope at 15 GPa is eutectic, and the eutectic temperature at which clinoenstatite (En 99.7Py 0.3), liquid (En 98Py 2), and garnet (En 91Py 9) coexist is 2250°C. The melting temperatures of clinoenstatite and pyrope at 15 GPa are 2350°C and 2275°C, respectively. Consequently, garnet and liquid coexist over a very narrow temperature interval in the range of composition from the eutectic to pyrope. The melting temperature of pyrope at 25 GPa exceeds 3000°C, which is substantially higher than that of MgSiO 3 perovskite. The available data suggest that the melting temperature of perovskite increases with incorporation of Al 2O 3. This is consistent with peritectic melting on the En-Py join at 25 GPa. The wide stability of garnet both in temperature and composition suggests that garnet could be the predominant liquidus phase for ultrabasic bulk compositions up to the pressure at which the perovskite rich in Al 2O 3 is stabilized.