Hydration and melting reactions in the system MgO-SiO_2-H_2O at pressures up to 100 kbar

The proposed model for the system MgO-SiO_2-H_2O ties subsolidus and melting reactions into an internally-consistent P-T net. None of the hydrous magnesium silicates considered is stable at the temperature of the solidus for forsterite + enstatite + H_2O; however, brucite is stable at solidus temperatures and at pressures greater than 50 kbar. In the presence of H_2O enstatite melts incongruently to forsterite plus a silica-saturated liquid at pressures up to 90 kbar. At 90 kbar the hydration reaction Fo + H_2O → En + Br intersects the solidus, and at higher pressures forsterite and H_2O do not coexist. The melting of forsterite + enstatite in the presence of a small amount of H_2O is vapor-present at pressures up to 90 kbar and generates a vapor-saturated, silica-saturated liquid. At higher pressures melting of the same bulk composition generates a silica-saturated, vapor-undersaturated liquid