ABSTRACT

Light hydrocarbon contents were determined for volcanic and geothermal gases from the Japanese island arc volcanoes. C₁-C₄ alkanes and C₂ alkene were detected, independent of fumarolic and downhole temperature. On the basis of the abundance ratios of alkanes and alkene relative to methane, the volcanic hydrocarbons are classified mainly into the fumarolic type of inorganic origin and the volcanic geothermal type of thermogenic origin. Thermodynamic calculations are made for the reactions of inorganic hydrocarbons under redox condition governed by a variety of mineral buffer systems. The results indicate that the amounts of alkanes and alkene depend essentially on the oxygen fugacity rather than temperature or other factors such as f_H2O and f_CO2. The concentrations of volcanic light hydrocarbons are probably controlled by redox conditions during the discharge process as well as equilibrium with magma.