The H (sub 2) O content of andesitic magmas from three volcanoes in Japan, inferred from the infrared analysis of clinopyroxene
The H (sub 2) O content of andesitic magmas from three volcanoes in Japan, inferred from the infrared analysis of clinopyroxene
European Journal of Mineralogy (October 2011) 23 (5): 771-778
- andesites
- andesitic composition
- Asama
- Asia
- chain silicates
- clinopyroxene
- Far East
- fluid inclusions
- Honshu
- igneous rocks
- inclusions
- infrared spectra
- island arcs
- Japan
- Kagoshima Japan
- Kyushu
- magma chambers
- magmas
- melt inclusions
- partitioning
- phenocrysts
- pyroxene group
- Sakura-jima
- saturation
- silicates
- spectra
- volcanic rocks
- volcanoes
- water content
- Hokkaido-Komagatake
In this study, we have determined the H (sub 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 (sub 2) O dissolved in the melt at magma chamber was estimated by analyzing the H (sub 2) O content of clinopyroxene phenocrysts and using the partition coefficient of H (sub 2) O between melt and clinopyroxene. On the basis of infrared spectroscopic analyses, the H (sub 2) O contents of clinopyroxene were 329 + or - 65, 247 + or - 72, and 213 + or - 36 ppm for Ko-a, As-A, and Sz-Ts/P1, respectively. The partition coefficient was calculated from the H (sub 2) O content of Ko-a clinopyroxene and that of rhyolitic melt inclusions included in clinopyroxene (3.1 wt% H (sub 2) O). Assuming that clinopyroxene was in equilibrium with melt inclusions, the partition coefficient is 0.011 + or - 0.003, which results in H (sub 2) O contents of the melts at the magma chamber of 2.3 + or - 0.9 and 2.0 + or - 0.6 wt% for As-A and Sz-Ts/P1, respectively. The H (sub 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 (sub 2) O or are ready to saturate before the eruptions.