Supercritical fluids in rock-H2O systems have been proposed to be important agents of mass transfer in high-pressure environments such as subduction zones. For the first study, experiments were conducted on the model system NaAlSi3O8 (Ab)-H2O to investigate phase relations at pressure (P) and temperature (T) in the vicinity of critical mixing between aqueous fluid and silicate melt. Isobaric equilibrium phase relations were determined at 1.0-1.7 GPa, 600-1060 °C, and H2O mole fractions (x_(H2O)) of 0.035-0.99. A subregular solution model was used to describe the solvus curves. P- and T-dependent Margules coefficients, W_Ab and W_(H2O), and activities of H2O and Ab were formulated using compositions at the solidus and critical point at each pressure as input. The results provide a comprehensive account of the solution properties of subcritical and supercritical fluids in the Ab-H2O system at temperatures and pressures corresponding to the deep-crust regions of granite magma generation. This work may have considerable bearing on element transport, extractability of partial melt, and ultimate level of melt emplacement in the middle and upper crust.
The second study is an experimental determination at deep-crustal conditions of liquidus-H2O contents of more complex granitic systems containing albite, potassium feldspar and quartz. The liquidus water content of a granitic melt at high-P and T is important because it constrains the volume of granite that could be produced by dehydration melting of the deep crust and it strongly influences physical properties that control the ability of granitic liquids to accumulate and ascend. The high liquidus-H2O contents determined in this study present a challenge for producing voluminous amounts of metaluminous granites from lower crustal biotite-amphibole gneisses by dehydration melting. Additionally, rapid undercooling of granitic melts may lead to granophyric textures described in volcanic and impactitie surge deposits and the formation of a solid solution of coesite and feldspar.
