Eruption mechanisms and crystal textures

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1.Viscosity measurement of subliquidus magmas : Subliquidus magmas contain crystals of various shapes and size, and their effect on bulk viscosity has not been studied previously We installed an atmosphere-controlled furnace with viscometer, and measured bulk viscosity with concominant sampling, and evaluated the melt viscosity from the glass analyses, and analyzed the crystal textures. The 1707 high-alumina basalt of Fuji volcano has plagioclase as liquidus phase, and very thin plagioclase laths caused deviation from the commonly-used Einstein-Roscoe-Marsh equation by a factor of 4. On the other hand, an alkali olivine basalt of NW Kyushu has olivine as liquidus phase and presence of 20 vol% of mildly thin plagioclase did not cause deviation from the ERM equation, which can be explained by evaluating the parameters in the Krieger-Dougherty equation.2.We evaluated the conditions of the end member magmas of magma mixing for the 1991-1995 dacite of Unzen volcano by high-pressure experiments, and also conducted experiments to determine the Cl/OH partition coefficient between hornblende and melt in the Unzen dacite, showing the strong dependence of the partition coefficient on the Mg/(Mg+Fe) ratio of hornblende. This result suggest that marked variation of Cl content of oscillatory zoned hornblende phenocryst in the Unzen dacite was caused by repeated degassing and input of fluid phase in the low-temperature magmas. Another experiments on the Unzen dacite showed solubility of sulfur is increased by the presence of Cl.3.Effect of pressure and water content on the Ca-Na partition coefficient between plagioclase and basaltic melt was evaluated experimentally, and it was shown that optimal condition for crystallization of high-An plagioclase is 2-3kb water saturated condition in arc tholeiite.4.Genetic conditions of high-magnesian volcanic rocks are examined geochemically and petrologically.