Abstract
High-temperature volcanic gases were collected from a growing dacitic dome of Unzen volcano, Japan. Apparent equilibrium temperatures (AET) were calculated for the following two reactions, SO 2+3H 2=H 2S+2H 2O and CO 2+H 2=CO+H 2O. Both the AET's are higher than the outlet temperature ( T o). Two hypotheses are examined as a cause of this disagreement; one is a partial advancement in the reaction SO 2+3H 2→H 2S+2H 2O, the other is an addition of external water after a final equilibrium attainment. Based on these hypotheses the equilibrium compositions, temperatures and redox potentials were estimated resulting in similar values. The estimated temperatures range from 850 to 911°C, which is consistent with a petrological estimation made with the lava; however, the estimated redox potential is one unit less in log f o 2 scale than the petrological estimation.
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