抄録
Several factors controlling the chemical composition of gas released from magma are briefly reviewed from a viewpoint that the compositional changes of gas released from magma may happen before volcanic eruption and that the studies of those compositional changes are important to find out the chemical precursors of eruption. The following factors are noted : (1) The compositional dependency of fugacity of an individual gas in multicomponent gaseous systems and the solubility of each gas into magma under high pressure and temperature should be fully investigated. (2) It is essential to understand the dissolution mechanism of gas into magma and also the chemical form of dissolved gas in magma. (3) It is also important to know the chemical form of chloride ion in the gaseous phase coexisting with hydrous magma, i.e. metal choride or hydrogen chloride. An example of chemical composition of gas released from magma is presented based on a quite simplified model of magma-gas system. It is implied that the chemical composition strongly depends on the pressure at which the gas released from the magma.
