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

摘要

Carbon isotopic composition ( d ¹³C) 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 (2 and CO under any realistic conditions. The correlations of d ¹³C(CO), CO/CO₂ ratio and temperature can be explained in terms of kinetic conversion of CO to CO₂ with decreasing temperature. The d ¹³C 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 d ¹³C(CH₄) and CH₄/C₂H₆ ratio indicate that CH₄ of the low CH₄/CO₂ ratio group (-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 d ¹³C value of about -40‰.