Abstract
High-temperature quenching experiment was carried out by T. KATSURA in order to determine the liquidus relations of the 1970 magma. The separated groundmass and artificial mixture of the glass composition were heated in the resistance furnace in the controlled atmosphere of CO2 and H2. The liquidus temperature of magnetite (Fig. 7) is sensitive to fo2. In the groundmass composition, plagioclase appears at temperatures 50 to 70℃ higher than that at which pyroxenes appear. In the glass composition, plagioclase and pyroxenes appear at about the same temperature (1100℃), very close to the maximum temperature (1090℃) observed during the eruption. It may be argued that the magma just before the eruption and whose composition is represented by the composition of glass had adjusted its composition to low total pressure and water pressure state to crystallize out plagioclase and pyroxene (and magnetite) concomitantly. On the other hand, the magma represented by the composition of the whole groundmass may have been in equilibrium with crystals under some appreciable water pressure way down in the volcanic conduit. It is suggested that these relations may be used as an effective indicator of water pressure in the magma.
