抄録
In general, the apparent equilibrium temperatures calculated for several reactions among gases species in high temperature fumarolic gases does not agree each other. The disagreement has been attributed to the result of some disturbances after a hypothetical equilibrium state of gas. The equilibrium state can be disturbed due to, for example, (1) the difference in reaction rate of SO2+3H2=H2S+2H2O (reaction-1) and CO2+H2=CO+H2O (reaction-2), (2) the addition of an external water or loss of water from gas, (3) the decrease of CO by a reaction with strong alkaline solution used for gas sampling, and (4) the decrease of gas pressure. Under the assumption (1) or (2), two independent equations of mass reaction can be held corresponding to the reactions-1 and -2. Each the two equations involve one unknown parameter and the unknown equilibrium temperature and known concentration of chemical species. Two different unknown parameters corresponding to the assumption (1) and (2) can not be involved into one set of two equations. Therefore the assumption (1) or (2) must be selected prior to the estimation of equilibrium temperature. In this context, an arbitrariness has been left. If a third reaction is found, the two parameters would be evaluated simultaneously. However such a reaction has not been obtained yet. The equilibrium temperatures are calculated under the assumption of (1) and (2) for the previous results of high temperature fumarolic gases from ten volcanoes. The calculated equilibrium temperatures are similar to or higher than the fumarolic temperature in many cases. However, some gases shows the equilibrium temperature lower than the fumarolic temperature, suggesting the requirement of additional reasons for the disequilibrium state of gas other than (1) and (2). The (3) and (4) may be possible to the the additional reasons. The oxygen fugacity of high temperature gases are expected to be controlled by lava, therefore the gases would show specific values for individual volcanoes. The variation of the oxygen fugacity obtained from gas composition of each volcano can be used to infer the cause for disequilibrium state of gas. The equilibrium oxygen fugacities for gases from individual volcanoes converged well under the assumption (1) rather than (2), suggesting that (1) is the dominant factor disturbing the equilibrium state of gases. The overall trend of equilibrium state for gases is the high oxygen fugacity at low temperature and low oxygen fugacity at high temperature. The trend is consistent to the "gas buffering". A relatively oxidized low temperature gas can be made due to the cooling of a reduced high temperature gas with sufficient amount of SO2 and H2S. A gas with limited concentration of SO2 and H2S such as the gases from Showa-Shinzan does not show the oxidizing trend at low temperature. The oxygen fugacity of such a gas is thought to reflect the fugacity at the formation of the gas from magma.
