Phase relationships of Itype granite with H2O to 35 kilobars The Dinkey Lakes biotitegranite from the Sierra Nevada Batholith

The Dinkey Lakes biotite-granite from the Sierra Nevada batholith was reacted (with varying percentages of H20 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 H20; isobaric T-X•2o diagrams at 25, 30, and 35 kbar showing H20-undersaturated relations; the H20-undersaturated liquidus surface mapped with contours for constant H20 contents and fields for near-liquidus minerals; and the solubility of H20 in granite liquids to 35 kbar. Results and their implications show: (1) The solidus temperature d creases from 680øC at 2 kbar to 620øC at 10 kbar, then increases to 700øC at 35 kbar because of changes from less dense to more dense subsolidus mineral assemblages. (2) The melting interval with excess H20, which is only 35øC at 2 kbar, increases to 105øC at 10 kbar and 150øC at 35 kbar because the liquidus m'mimum in the complex rock system departs from granite composition with increasing pressure. (3) The solubility of H20 in granite liquid is 27 _+ 2.5 weight percent at 35 kbar and 850øC, indicating that a miscibility gap persists between H20-saturated silicate magmas and aqueous vapor phase, at least o 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 toesite are the liquidus minerals at mantle pressures for all H20 contents, indicating that granites and rhyolites cannot be primary magmas from mantle peridotitc or subdueted oceanic gabbroic crust. (5) The liquidus urface at crustal pressures, with plagioclase and quartz as primary minerals, indicates that primary liquids of granite composition with moderate H20 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 plutonit magmas of intermediate composition.