The melting of forsterite (Mg2SiO4) in the presence of H2O was studied from 3 to 12 GPa in a multiple-anvil apparatus to constrain the maximum temperature of the hydrous solidus in peridotitic systems relevant to the Earth's mantle. The solidus has a negative slope to 12 GPa, despite the stabilization of Phase E (a hydrous magnesium silicate) coexisting with forsterite and vapor at the solidus at >9 GPa. Abundant quench vapor deposits in all the run products indicate extensive solubility of silicate in the vapor; however, there is no direct evidence for incongruent solution in the vapour to 9 GPa. At >9 GPa, the coexistence of Fo+PhE+V at the solidus requires Mg/Si≠2 in the vapor. Silicate melts and hydrous vapors remain immiscible phases in this system to >12 GPa.

Given current estimates of the temperature distribution in the mantle, a hydrous vapor can exist in the asthenospheric mantle only below ~5 GPa (~160 km depth). At greater depths, water would be present in a hydrous silicate melt rather than in a free vapor. This depth is a maximum, because solidus temperatures in hydrous peridotite systems would be lower than those in the endmember Mg2SiO4-H2O system. In cooler subducting slabs, a hydrous vapour could persist down to the transition zone. ¿ American Geophysical Union 1993