A model of phase relations in the system MgO-SiO_2-H_2O-CO_2 and prediction of the compositions of liquids coexisting with forsterite and enstatite

A comprehensive model has been developed for the system MgO-SiO_2-H_2O-CO_2 on the basis of experimental studies, Schreinemaker's rules, and thermodynamic data. The assemblage forsterite plus enstatite is predicted to melt in the presence of vapor of any H_2O/CO_2 ratio at low pressures, in the presence of vapor whose H_2O/CO_2 ratio is buffered by the presence of magnesite at intermediate pressures, and at a vapor-absent eutectic with brucite and magnetite at high pressures. The composition of the liquid at the solidus for a bulk composition of forsterite plus enstatite plus a small amount of volatiles with a 3/1 H_2O/CO_2 ratio changes from enstatite-quartz normative at 20 kbar, to periclase- forsterite normative at 50 kbar, to forsterite-enstatite normative at 90 kbar and greater pressures. Forsterite cannot coexist with H_2O-CO_2 vapor at pressures greater than 90 kbar. Thus all melting in the earth's mantle at higher pressures must be vapor-absent