Phase relationships of I-type granite with H_2O to 35 kilobars: The Dinkey Lakes biotite-granite from the Sierra Nevada Batholith

The Dinkey Lakes biotite-granite from the Sierra Nevada batholith was reacted (with varying percentages of H_2O in sealed platinum capsules) in a piston-cylinder apparatus between 10 and 35 kbar. The results were combined with the results from previously published experiments to provide comprehensive phase relationships for an I-type granite: a P-T diagram with excess H_2O; isobaric T-X_(H_2O) diagrams at 25, 30, and 35 kbar showing H_2O-undersaturated relations; the H_2O-undersaturated liquidus surface mapped with contours for constant H_2O contents and fields for near-liquidus minerals; and the solubility of H_2O in granite liquids to 35 kbar. Results and their implications show: (1) The solidus temperature decreases from 680°C at 2 kbar to 620°C at 10 kbar, then increases to 700°C at 35 kbar because of changes from less dense to more dense subsolidus mineral assemblages. (2) The melting interval with excess H_2O, which is only 35°C at 2 kbar, increases to 105°C at 10 kbar and 150°C at 35 kbar because the liquidus minimum in the complex rock system departs from granite composition with increasing pressure. (3) The solubility Of H_2O in granite liquid is 27 ± 2.5 weight percent at 35 kbar and 850°C, indicating that a miscibility gap persists between H_2O-saturated silicate magmas and aqueous vapor phase, at least to pressures corresponding to 120-km depth in the mantle. Dissolution of alkali feldspar (20% of rock) in the subsolidus aqueous vapor phase indicates that deep-seated aqueous fluids are concentrated solutions. (4) Quartz and coesite are the liquidus minerals at mantle pressures for all H_2O contents, indicating that granites and rhyolites cannot be primary magmas from mantle peridotite or subducted oceanic gabbroic crust. (5) The liquidus surface at crustal pressures, with plagioclase and quartz as primary minerals, indicates that primary liquids of granite composition with moderate H_2O contents can be generated in the crust at reasonable temperatures; these liquids could rise to near surface levels without vesiculation. Granite liquid together with residual crustal minerals could constitute plutonic magmas of intermediate composition