2002 Volume 36 Issue 1 Pages 1-20
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.
