ABSTRACT

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 δ³⁴S of SO₂ and H₂S gases absorbed by KOH solution is, respectively, 2.4 and 6.5‰ 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 δ³⁴S of the KOH solution gradually increased from 2001 to the summer of 2002, then abruptly decreased. After the summer of 2002, the δ³⁴S of KOH solution again gradually increased until 2005. The increase of δ³⁴S in both periods can be explained by open system degassing of magma. A value of -0.3 to -1.0 for 1000lnα_gas-magma produces the increase in the δ³⁴S, where α_gas-magma is the fractionation factor between gas and magma. The fractionation factor α_gas-magma is a function of oxygen fugacity (ΔNNO). The value of +0.6 to +1.2 for ΔNNO required to obtain -0.3 to -1.0 for 1000lnα_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 δ³⁴S_CDT was estimated to be +4.4 to +5.7‰ 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).