Experimental aspects of UHP metamorphism: Granitic systems

The outstanding development of recent years in the experimental petrology o f granite systems with water is the in situ determination of critical curve s in pressure-temperature diagrams, along which the hydrous melts and silic ate-hearing vapors coexisting at low pressures merge compositionally, and f orm a single-phase supercritical fluid stable at higher pressures. Their lo cations, extending from pressures of similar to 15 kbar near 1000 degrees C to similar to 25 kbar near 700 degrees C, suggest that previous determinat ions of excess-water granite melting at higher pressures were influenced by quench problems. Under the conditions of ultrahigh-pressure metamorphism, therefore, granites in the presence of water do not melt, but are more or l ess gradually dissolved in the supercritical fluid with rising temperature, i.e., without any sharp "melting temperature." The anhydrous compositions of the supercritical fluids change from strongly incongruent, mainly potass ium silicatic at low temperatures via ultrapotassic, leucititic composition s to congruent granitic near the temperature of complete dissolution. The p resence of hydrous supercritical fluids during ultrahigh-pressure metamorph ism, possible at all temperatures whenever water is available, has a decisi ve weakening effect on the rheological properties of crustal rocks during d eep subduction.