In this study, we have determined the H2O content of arc andesitic magmas erupted from three volcanoes in Japan [the 1929 A.D. eruption of Hokkaido-Komagatake (Ko-a), Tenmei eruption of Asama (As-A), and Taisho eruption of Sakurajima volcanoes (Sz-Ts/P1)]. The amount of H2O dissolved in the melt at magma chamber was estimated by analyzing the H 2O content of clinopyroxene phenocrysts and using the partition coefficient of H2O between melt and clinopyroxene. On the basis of infrared spectroscopic analyses, the H2O contents of clinopyroxene were 329 ± 65, 247 ± 72, and 213 ± 36 ppm for Ko-a, As-A, and Sz-Ts/P1, respectively. The partition coefficient was calculated from the H2O content of Ko-a clinopyroxene and that of rhyolitic melt inclusions included in clinopyroxene (3.1 wt% H2O). Assuming that clinopyroxene was in equilibrium with melt inclusions, the partition coefficient is 0.011 ± 0.003, which results in H2O contents of the melts at the magma chamber of 2.3 ± 0.9 and 2.0 ± 0.6 wt% for As-A and Sz-Ts/P1, respectively. The H2O content of magma in the magma chamber calculated at saturation on the basis of magma-chamber depth estimated from geophysical observation shows good agreement with the measured value for As-A; those of Ko-a and Sz-Ts/P1 are slightly higher than the measured values. These results indicate that arc andesitic magmas are saturated with H2O or are ready to saturate before the eruptions.
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