Phase relations and their petrological implications in the system MgO-SiO2-H2O-CO2 at pressures up to lfi) kbar

A comprehensive model has been developed for phase relations in the system MgO-SiOr-HrO-CO, on the basis of Schreinemakers ' rules and subsolidus thermodynamic data. The as-semblage forsterite plus enstatite can melt in the presence of vapor of any HrO,/CO, ratio at low pressures, in the presence of vapor whose HrOlCO, ratio is bufered by magnesite at in-termediate pressures, and at a vapor-absent eutectic at high pressures. Forsterite and HrO-CO, vapor cannot coexist at pressures greater than 90 kbar. A thermal maximum is postu-lated to exist on the reaction Fo * En * V + L at pressures greater than 35 kbar. Thermal maxima on vapor-present melting reactions may be responsible for the eruption of kimber-litic magma from the upper mantle