Melting Relations of Basalt with Equilibrium Water Pressure Less Than Total Pressure

Abstract

Equilibrium H,O pressure (POH.O) was fixed at values less than total pressure (PT) in melting experiments on mixtures of 1921 Kilauea tholeiite, H,O, and CO, (58-5 mole per cent H,O, 41-5 mole per cent CO,), buffered by Ni+NiO. New determinations of the beginning of melting of mixtures of 1921 Kilauea tholeiite and H,O buffered by quartz+ fayalite+magnetite were made at 2 and 3 kb. Microprobe analyses of coexisting glass, clinopyroxene, ±olivine, ;£ amphibole were determined for several runs. Decreasing H,O fugacity (/H,O) to about six-tenths the fugacity of pure H,O ( /H,O) raises the solidus and the upper stability limit of plagioclase. Plagioclase and clinopyroxene coexist in equilibrium with liquid—a feature not observed in the pure H,O system. Amphibole is stable to about 970 °C at 2 kb, 1025 °C at 5 kb and 1060 °C at 8 kb. The Al (VI)+Ti contents of the amphibole increase with P, yielding kaersutite at 1050 °C and 8 kb. Calculated modes for the condensed phases reveal large differences in the amount of glass (liquid) present and large differences in liquid composition below and above the breakdown temperature of amphibole at 5 and 8 kb. Liquids coexisting with amphibole, clinopyroxene, olivine, and magnetite are dacitic near the solidus and silica-rich andesites around 1000 °C at 5 and 8 kb. The results of this study substantiate the model for the generation of the calcalkaline suite by partial melting of H,O-rich basalts.