The H2 O content of andesitic magmas from three volcanoes in Japan, inferred from the infrared analysis of clinopyroxene

Abstract

In this study, we have determined the H 2 O 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 H 2 O dissolved in the melt at magma chamber was estimated by analyzing the H 2 O content of clinopyroxene phenocrysts and using the partition coefficient of H 2 O between melt and clinopyroxene. On the basis of infrared spectroscopic analyses, the H 2 O 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 H 2 O content of Ko-a clinopyroxene and that of rhyolitic melt inclusions included in clinopyroxene (3.1 wt% H 2 O). Assuming that clinopyroxene was in equilibrium with melt inclusions, the partition coefficient is 0.011 ± 0.003, which results in H 2 O 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 H 2 O 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 H 2 O or are ready to saturate before the eruptions.