A comprehensive model has been developed for phase relations in the system MgO−SiO2−H2O−CO2 on the basis of Schreinemakers’ rules and subsolidus thermodynamic data. The assemblage forsterite plus enstatite can melt in the presence of vapor of any H2O/CO2 ratio at low pressures, in the presence of vapor whose H20/C02 ratio is buffered by magnesite at intermediate pressures, and at a vapor-absent eutectic at high pressures. Forsterite and H2O−CO2 vapor cannot coexist at pressures greater than 90 kbar. A thermal maximum is postulated 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 kimberlitic magma from the upper mantle.