LIQUIDUS PHASE-RELATIONS IN THE CAO-MGO-AL2O3-SIO2 SYSTEM AT 3.0 GPA - THE ALUMINOUS PYROXENE THERMAL DIVIDE AND HIGH-PRESSURE FRACTIONATION OF PICRITIC AND KOMATIITIC MAGMAS

We present liquidus phase equilibrium data at 3.0 GPa for the model th oleiitic basalt tetrahedron, diopside-anorthite-forsterite-quartz, in the CaO-MgO-Al2O3-SiO2 system. This pressure coincides with the invari ant point (1568 degrees) on the simplified model lherzolite solidus th at marks the transition between spinel lherzolite and garnet lherzolit e (fo + en + di + sp + gt + liq). The composition of the liquid at the invariant point (46.4 An, 16.0 Di, 33.5 Fo, 4.2 Qz, wt %) is a model olivine-rich basalt that lies slightly (0.2% excess fo) to the SiO2-po or side of the the aluminous pyroxene plane, MgSiO3-CaSiO3-Al2O3. A la rge garnet primary phase volume is bordered by primary phase volumes f or forsterite spinel, sapphirine, corundum, enstatite, diopside, quart z, and kyanite. The observed absence of enstatite at the solidus of lh erzolite at pressures above similar to 3.3 GPa is readily understood f rom the phase relations in this system. During melting at these high p ressures, enstatite first forms at a temperature somewhat above the so lidus and then dissolves before complete melting. As pressure increase s above 3.0 GPa, the aluminous pyroxene plane, MgSiO3-CaSiO3-Al2O3, be comes increasingly effective as a thermal divide that causes picritic and komatiitic melts lying on the silica-poor side of the plane to fra ctionate toward alkalic picritic compositions. However, if the rate of ascent of these melts is sufficiently rapid, expansion of the olivine primary phase volume as pressure decreases produces a fractionation t rend dominated by olivine crystallization and the thermal divide is ig nored.