Abstract
Abstract We have investigated the chemical and isotopic composition of recently formed gases emitted by three fumaroles on the Mt. Mihara summit peak of the Izu-Oshima volcano in order to assess the formation process of these gases. The CO2/H2O ratio of these gases varied greatly, with the σD and σ18O of the H2O being as low as a vapor phase equilibrated with the local meteoric water. These features are explained by a mixing of the vapor equilibrated with local meteoric groundwater and primary steam, which is a vapor phase generated by the mixing of magmatic gas and a cold meteoric groundwater. We concluded that there was no direct mixing between the magmatic vapor and a vapor phase of meteoric origin in the processes involved in fumarolic gas formation.
Highlights
Izu-Oshima is an insular volcano located in the PacificOcean (Fig. 1) which has shown intermittent eruptive activities
We concluded that there was no direct mixing between the magmatic vapor and a vapor phase of meteoric origin in the processes involved in fumarolic gas formation
The correlation between the δD and δ18O of H2O in fumarolic gas is shown in Fig. 4, where the local meteoric water (MW) at the Izu-Ohsima volcano (Takahashi et al, 2000), magmatic vapor (MV) characteristic of the subduction zone (Taran et al, 1990; Giggenbach, 1992), and sea water (SW) are plotted
Summary
Ocean (Fig. 1) which has shown intermittent eruptive activities. In 1986, a sequence of eruptions occurred, the magnitude of which was relatively large compared to those of previous eruptions (Fuji et al, 1988; Kawanabe, 1998). The 3He/4He ratio in the steam started to increase immediately after the eruption in 1986, reaching a maximum in later in 1986, before it decreased gradually (Sano et al, 1995) These observations suggest that the temperature of fumarolic gases and the chemical composition of volcanic fluid are intimately related to the activity of the Izu-Ohsima volcano. As all of the gases contained air, which prevented the use of normal sampling methods for fumarolic gas, such as those of Ozawa (1968) and Giggenbach (1975), we used the apparatus shown in Fig. 3(a) for the chemical analysis. A condenser (Fig. 3(b)) was used to obtain the sample for the stable isotope analysis of H2O vapor in fumarolic gas. The δ13C of the prepared CO2 gas was measured on the MAT-252
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