Geocheminar spring-2015

Supercritical fluids in rock-H2O systems have been proposed to be important agents of mass transfer in high-pressure environments such as subduction zones. New experimental studies were conducted on the model granite system NaAlSi3O8(Ab)-H2O to investigate phase relations at pressure (P) and temperature (T) approaching those of critical mixing between aqueous fluid and silicate melt.

Our results provide a comprehensive account of the solution properties of subcritical and supercritical fluids in this model granite system at temperatures and pressures corresponding to the deep-crust regions of granite magma generation. Textures of quenched charges indicate that near-critical melts are extremely fluid, which fact may have considerable bearing on element transport, extractability of partial melt, and ultimate level of melt emplacement in the middle and upper crust.

The liquidus water content of a granitic melt at high pressure (P) and temperature (T) is important because it constrains the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (?0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700-1000?C, 0.6-1.5 GPa). To improve the high-P constraints on water concentration at the granite liquidus, we preformed experiments in piston-cylinder apparatus at 1 GPa using a range of granite compositions.

The reversed liquidus temperatures at 2.97, 4.15, 5.82, 7.92, and 12.00 wt% water are respectively 950-985, 875-910, 800-850, 750-775, and 650-675?C. Our results plot on the extreme end of the extrapolated water contents at the liquidus when compared to all other previous determinations, and, as a result, give significantly higher water contents than used by most dehydration melting models.