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

Abstract

We present liquidus phase equilibrium data at 3.0 GPa for the model tholeiitic basalt tetrahedron, diopside–anorthite–fosterite–quartz, in the CaO–MgO–Al2O3–SiO2 system. This pressure coincides with the invariant point (1568°C) on the simplified model lherzolite solidus that marks the transition between spinel lherzolite and garnet lherzolite (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-poor side of the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3. A large garnet primary phase volume is bordered by primary phase volumes for forsterite, spinel, sapphirine, corundum, enstatite, diopside, quartz, and kyanite. The observed absence of enstatite at the solidus of lherzolite at pressures above ∼3.3 GPa is readily understood from the phase relations in this system. During melting at these high pressures, enstatite first forms at a temperature somewhat above the solidus and then dissolves before complete melting. As pressure increases above 3.0 GPa, the aluminous pyroxene plane, MgSiO3–CaSiO3–Al2O3, becomes increasingly effective as a thermal divide that causes picritic and komatiitic melts lying on the silica-poor side of the plane to fractionate 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 trend dominated by olivine crystallization and the thermal divide is ignored.