Abstract
Melting relations in the system SiO 2 -H 2 O were studied at high temperatures and pressures. A quadruple point for the equilibrium among quartz, tridymite, and 2 fluids was found at 1160 degrees C. and 1500 bars. A second quadruple point for tridymite, 2 fluids, and cristobalite is estimated to be near 1470 degrees C. and 400 bars. The freezing point of SiO 2 in equilibrium with 2 fluid phases is depressed from 1720 degrees C. at one bar to 1130 degrees C. at 2000 bars. This freezing point is diminished by only an additional 50 degrees C. with a further increase in pressure to 9700 bars. The compositions of coexisting fluids along the upper 3-phase boundary have been determined. A critical end point for the univariant equilibrium curve was found at 1080 degrees C. and 9700 bars with a composition of approximately 75 wt% SiO 2 to 25 wt% H 2 O.
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