Water-Saturated and -Undersaturated Melting of Metaluminous and Peraluminous Crustal Compositions at 10 kb: Evidence for the Origin of Silicic Magmas in the Taupo Volcanic Zone, New Zealand, and Other Occurrences

The melting relations of two proposed crustal source compositions for rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand, have been studied in a piston-cylinder apparatus at 10 kb total pressure and a range of water activities generated by H2O-CO2 vapour. Starting materials were glasses of intermediate composition (65 wt.% Si02 representing a metaluminous ‘I-type’ dacite and a peraluminous ‘S-type’ greywacke. Crystallization experiments were carried out over the temperature range 675 to 975°C, with aH2O values of approximately 1·0, 0·75, 0·5, and 0·25. Talc-pyrex furnace assemblies imposed oxygen fugacities close to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartz and plagioclase through 675–700°C at high aH2O, 725–750°C at aH2O≈0·5, and 800–875°C at aH2O≈0·25, corresponding to