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

Carbon isotopic composition (δ 13 C) of CO 2 , CO and CH 4 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 CO 2 and CO was calculated to be ranging from 896° to 950°C for high temperature (691° to 882°C) fumarolic gases, suggesting that CO 2 and CO were in isotopic equilibrium near the vent temperature. However, low temperature (<650°C) gases were not in isotopic equilibrium between CO 2 and CO under any realistic conditions. The correlations of δ 13 C(CO), CO/CO 2 ratio and temperature can be explained in terms of kinetic conversion of CO to CO 2 with decreasing temperature. The δ 13 C values of CH 4 and CO 2 revealed that the CH 4 was not equilibrated in carbon isotope exchange reaction with CO 2 even at fumarolic gas temperature near 900°C. Correlation of δ 13 C(CH 4 ) and CH 4 /C 2 H 6 ratio indicate that CH 4 of the low CH 4 /CO 2 ratio group (<10 −5 ) is mainly originated from thermogenic decomposition of organic matter, while the high CH 4 /CO 2 ratio group gas (7.7 × 10 −4 to 7.4 × 10 −3 ) is contaminated by the CH 4 -rich gas with δ 13 C value of about −40‰.