ABSTRACT

Near-liquidus relations in one-atomsphere dry and water saturated high pressure conditions were experimentally determined on a pumice erupted from Usu volcano, Hokkaido, on August 14, 1977, at temperatures of 890°C to 1220°C, up to 1400 bar, and at a fixed oxygen fugacity (f_O2) close to the Ni-NiO buffer. Plagioclase was the liquidus phase and accompanied with the subsequent phase of orthopyroxene up to 1400 bar. The temperature difference betweeen the plagioclase and orthopyroxene liquidi decreases with increasing water pressure, and an extraporation showed a crossing at about 900°C and 1500 bar. This temperature and water pressure may approximate the physical condition of the Usu magma containing both plagioclase and orthopyroxene as a small amount of phenocrysts shortly prior to eruption if the magma was saturated with H₂O. Summarizing the experimentally-determined, H₂O-saturated coprecipitation pressures and temperatures for the liquidus minerals from felsic magmas, so far reported in literature, a clear relationship is empirically derived for magmatic water pressure, temperature and the chemical composition of magmas. Using a compositional parameter such as the normative ratio of plagioclase to the sum of plagioclase plus pyroxene, the maximum water pressure and minimum temperature of magma immediately prior to eruption can be estimated graphically. This information may be useful to infer the physical conditions for historical activities of volcanoes vased on the mineralogical and petrochemical investigations.