ABSTRACT

Up to 15 fumarolic gas samples were collected successively within a few hours at Satsuma-Iwojima and Kuju volcanoes, southwest Japan, with the aim to evaluate the naturally occurring short-term variations in the chemical and isotopic compositions. Variations in the concentrations of the major gas components (CO₂, total S, SO₂ and H₂S) and in the hydrogen, oxygen (H₂O) and carbon (CO₂) isotopic compositions were relatively small and were within the analytical errors. Exceptions were HCl at both volcanoes, CO₂ at Satsuma-Iwojima, and D/H and ¹⁸O/¹⁶O ratios of water at Kuju volcano. Large variations were found for the minor gas components, i.e., N₂, Ar, CH₄ and CO. The observed variations in HCl and CO concentrations probably relate to sampling and analytical procedures. Variations in HCl, N₂, Ar and CH₄ concentrations, and D/H and ¹⁸O/¹⁶O ratios of water may result from processes such as gain or loss of HCl-rich liquid, sporadic additions of air and hydrothermally-derived CH₄ and change of mixing ratio between magmatic and meteoric waters. Precision associated with the gas sampling and analytical techniques used in this study were estimated by making the hypothesis that the chemical and isotopic compositions of the fumarolic gases did not change during the sampling period. The estimated precision was 1–3% for CO₂, H₂S and SO₂, 6–11% for HCl, 1–6% for H₂, 4–6% for N₂, 5–9% for Ar, 2% for CH₄ and 7–27% for CO. The precision of the isotopic analyses was estimated to be 0.05–0.2‰ for ¹³C/¹²C ratio of CO₂. Also the precision was 0.9 and 0.05–0.11‰ for D/H and ¹⁸O/¹⁶O ratios of the condensates, respectively. The results in this study suggest that careful sampling and analysis of volcanic gases provide reliable chemical and isotopic compositions that can be used to monitor volcanic activities.