Phase relations and their petrological implications in the system MgO-SiO_2-H_2O-CO_2 at pressures up to 100 kbar

Abstract

A comprehensive model has been developed for phase relations in the system MgO-SiO_2-H_2O-CO_2 on the basis of Schreinemakers' rules and subsolidus thermodynamic data. The assemblage forsterite plus enstatite can 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 magnesite at intermediate pressures, and at a vapor-absent eutectic at high pressures. Forsterite and H_2O-CO_2 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.