Carbon isotope systematics of CO_2, CO and CH_4 in fumarolic gases from Satsuma-Iwojima volcanic island, Japan

摘要

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℃ for high temperature (691°to 882℃) fumarolic gases, suggesting that CO_2 and Co were in isotopic equilibrium near the vent temperature. However, low temperature (<650℃) 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℃. Correlation of δ~(13)C(CH_4) and CH_4/C_2H_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 mater, 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‰.