Observation and investigation for the formation SPM originates in high temperature volcanic gas

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The high temperature fumarolic gasi is the source of SPM emitted from active volcanoes. In order to investigate the control on the chemical composition of high temperature volcanic gas, a degassing model of magma was constructed based on the data observed at Mt. Unzen. The volcanic gas rich in CO_2 and SO_2was estimated to be separated from magma at depth -8 km from surface. The degassed magma again exsolved a gas phase rich in H_O and CH1 at depth shallower than -1 km.The amount of SPM would be proportional to the flux of SO_2 gas. As a case study, the mechanism of SO_2 degassing was investigated at Miyakejima volcano. A diffused SO_2 gas was sampled at the flank of volcano by using the so called chemical trap method. In the method, a KOH solution was exposed over several months under open air. The volcanic SO_2 gas was absorbed into the KOH solution. At the absorption, the occurrence of a kinetically isotopic fractionation was confirmed by an laboratory experiment. Employing the experimentally determined isotopic fractionation, the δ^<34>S of S in magma of Miyakejima volcano was estimated to be identical to the value typical to the Quarternary volcanoes in Japan. The temporal change in the observed δ^<34> S of SO_2 gas, a monotonically decreasing, δ^<34> S, could be attributed to the open system degassing of magma. The isotopic fractionation between gas phase and magma depends on oxygen fugacity. The oxygen fugacity necessary to produce the isotopic fractionation compatible to the open degassing of magma was estimated to be +0.6 to +1.2 log unit higher than Ni-NiO oxygen fugacity buffer.