Carbon isotope systematics of CO2, CO and CH4 in fumarolic gases from Satsuma-Iwojima volcanic island, Japan

Abstract

Carbon isotopic composition (δ13C) of CO2, CO and CH4 were determined for fumarolic gases from Satsuma-Iwojima, to range from −3.1 to −2.6‰, from −11.6 to −2.8‰ and from −56 to −37‰, respectively. Carbon isotope exchange equilibrium temperature between CO2 and CO was calculated to be ranging from 896° to 950°C for high temperature (691° to 882°C) fumarolic gases, suggesting that CO2 and CO were in isotopic equilibrium near the vent temperature. However, low temperature (<650°C) gases were not in isotopic equilibrium between CO2 and CO under any realistic conditions. The correlations of δ13C(CO), CO/CO2 ratio and temperature can be explained in terms of kinetic conversion of CO to CO2 with decreasing temperature. The δ13C values of CH4 and CO2 revealed that the CH4 was not equilibrated in carbon isotope exchange reaction with CO2 even at fumarolic gas temperature near 900°C. Correlation of δ13C(CH4) and CH4/C2H6 ratio indicate that CH4 of the low CH4/CO2 ratio group (<10−5) is mainly originated from thermogenic decomposition of organic matter, while the high CH4/CO2 ratio group gas (7.7 × 10−4 to 7.4 × 10−3) is contaminated by the CH4-rich gas with δ13C value of about −40‰.