Melting in the Mg 2 SiO 4 ‐H 2 O system at 3 to 12 GPa

The melting of forsterite (Mg 2 SiO 4 ) in the presence of H 2 O 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 vapour at the solidus at > 9 GPa. Abundant quench vapour deposits in all the run products indicate extensive solubility of silicate in the vapour; 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 vapour. Silicate melts and hydrous vapours remain immiscible phases in this system to >12 GPa. Given current estimates of the temperature distribution in the mantle, a hydrous vapour 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 vapour. This depth is a maximum, because solidus temperatures in hydrous peridotite systems would be lower than those in the endmember Mg 2 SiO 4 −H 2 O system. In cooler subducting slabs, a hydrous vapour could persist down to the transition zone.