Melting of Granite with Excess Water to 30 Kilobars Pressure

Abstract

A chemically analyzed biotite granite from the Dinkey Lakes, Sierra Nevada Batholith, consists of 34.8 per cent quartz, 31.5 per cent plagioclase, 29 per cent orthoclase, and 4.7 per cent mafic minerals (mainly biotite). The crushed rock, sealed with 25-35 wt. per cent H_2O in platinum capsules, was reacted in piston-cylinder apparatus. The curve for the beginning of melting passed through 620° ± 5° C. at 10 kb. and 605° ± 5° C. at 15.3 kb. At about 17 kb., the plagioclase broke down to yield jadeite and quartz, and the solidus curve then began to increase in temperature with increasing pressure, reaching 670° C. at 27 kb. At higher pressures, quartz was transformed to coesite, and the melting curve continued to rise. In subsolidus runs at 20 kb., the original orthoclase and biotite were completely dissolved in the aqueous vapor phase. These experimental results, along with similar results in the SiO_2-excess portion of the system NaAlSiO_4- SiO_2-H_2O, provide no support for the theoretical predictions that solid-liquid-vapor melting curves should pass through a temperature minimum at 10 kb. or less. The water-saturated melting curve for the granite provides physical limits for the generation of silicate magmas within the crust and mantle.

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