EARLY PARTIAL MELTING OF DIOPSIDE UNDER HIGH-PRESSURE

Early partial melting (EPM) in pyroxenes begins by the precipitation of tiny droplets (0.1 mu m or less) of a molten phase highly enriched in silica. At room pressure, EPM starts at temperatures markedly lower than the pyroxene solidus temperatures approximately (250 degrees C below). High-pressure, high-temperature experiments (up to 1.9 GPa and 1309 degrees C) were carried out in a piston-cylinder apparatus on diopside single crystal bearing 1.2 wt.% FeO, to determine the influence of pressure on this phenomenon. The systematic transmission electron microscopy investigation of the samples annealed at high-T, high-P reveals that EPM occurs up to P approximate to 1.5 GPa above 1208 degrees C, at pO(2) ranging from 2.0 x 10(-17) and 2.5 x 10(-16) MPa. These T-P conditions are well below the solidus temperature of pure CaMgSi2O6 (1594 degrees C at 1.5 GPa). For pressures above 1.7 +/- 0.2 GPa, no EPM precipitates are found in sample annealed for 2 h 30 min at temperatures up to 1309 degrees C.