Isotopic fractionation of SO2 and H2S gases during the absorption by KOH solution, with the application to volcanic gas monitoring at Miyakejima Island, Japan

The chemical trap method has been used for monitoring of volcanic activity. In this method, a strong alkaline water solution was exposed in ambient air. Acidic gases, such as SO2, H2S, HCl and HF are absorbed in KOH solution without any instrumental support. For sulfur isotopic monitoring with this method, however, a sulfur isotopic fractionation between fumarolic gas and sulfur trapped in KOH solution occurs (Sakai and Ueda, 1984). In this study, the magnitude of isotopic fractionation was experimentally determined for SO2 and H2S gases. The delta S-34 of SO2 and H2S gases absorbed by KOH solution is, respectively, 2.4 and 6.5 parts per thousand lower than the gases in air, which agreed with the theoretical prediction based on molecular diffusion of SO2 and H2S in air. The KOH Solution trap method was applied to the volcanic gas at Miyakejima. The delta S-34 of the KOH solution gradually increased from 2001 to the summer of 2002, then abruptly decreased. After the summer of 2002, the delta S-34 of KOH solution again gradually increased until 2005. The increase of delta S-34 in both periods can be explained by open system degassing of magma. A value of -0.3 to -1.0 for 10001n alpha(gas-magma) produces the increase in the delta S-34, where alpha(gas-magma) is the fractionation factor between gas and magma. The fractionation factor alpha(gas-magma) is a function of oxygen fugacity (Delta NNO). The value of +0.6 to +1.2 for Delta NNO required to obtain -0.3 to -1.0 for 10001n alpha(gas-magma) is significantly higher than the oxygen fugacity of magma estimated petrologically by Yasuda et al. (2001), suggesting a disequilibrium between gas and magma in terms of oxygen fugacity. The intrinsic delta S-34(CDT) was estimated to be +4.4 to +5.7 parts per thousand for the magma of Miyakejima, which is consistent with the value of Quaternary volcanoes in the Japanese island arc estimated by Ueda and Sakai (1984).