Mg₂SiO₄-H₂O equilibrium phase diagram. Phases at equilibrium for the system Mg₂SiO₄-H₂O are shown as a function of chemical composition (weight percent water) and temperature at 1 GPa pressure. The orange and blue areas indicate temperatures and compositions for which a single phase, vapor (blue) or liquid (orange, magma), is present at equilibrium. Areas that have the horizontal line pattern indicate temperatures and compositions for which two phases are in equilibrium.

The magma has saturation curves for both forsterite and for a water-rich phase. Cooling of a forsterite-water magma will lead to crystallization of forsterite at a temperture that depends on the weight percentage of water in the magma. Notice that crystaliztion of forsterite from a magma containing water will increase the weight percentage of water in the magma (follow the Fo saturation curve), eventually reaching the eutectic point at 1520°C where both forsterite and a water-rich, supercritical vapor are produced at the same time. Notice also that the presence of a supercritical water phase significantly lowers the equilibrium melting temperature of forsterite. Modified from Morse (1984, Fig. 19-1) based on data from Kushiro and Yoder (1969) and Hodges (1974).    Return to Diagram List