Magma degassing process during the eruption of Mt. Unzen, Japan in 1991 to 1995: Modeling with the chemical composition of volcanic gas

摘要

During the dome-forming eruption in 1991 to 1995 at Mt. Unzen, Japan high temperature volcanic gases were sampled from fumaroles located at the western basement of the dacitic lava dome. Comparing the composition of volcanic gas, the volatile in the pre-eruptive melt of magma and the groundmass of lava, we propose that the volatile in magma was liberated through three different gas separation steps. The first gas separation happened at depth deeper than 3.9 km. The liberated gas traveled along the conduit then discharged as the observed fumarolic gas. The second gas separation took place beneath the lava dome. The liberated gas was discharged on the top of lava dome. The third gas separation occurred near surface in a so called "open system degassing" process. The large variation in the CO_2/H_2O ratio of fumarolic gas could be explained by the variation of the bubble fraction in the pre-eruptive melt of magma chamber before the gas separations. The estimated bubble fraction in the melt was less than 1.7 vol.%. Such a low fraction of bubbles in the magma chamber of Mt. Unzen would be one of the reasons why the eruption was non-explosive. The fumarolic gas in the later stage of eruption was enriched in HCl and HF due to the effect of open system degassing of magma, although the gas contained CO_2 and H_2S with high concentrations. The CO_2 and H_2S would be derived from a vapor phase of hydrothermal system developed around the conduit.