Volatile evolution of satsuma-iwojima volcano: Degassing process and mafic-felsic magma interaction

Abstract

Abstract Petrological and geochemical studies of volcanic rocks and melt inclusions of Satsuma-Iwojima volcano, Japan, were carried out to investigate the volatile evolution of the magma chamber since its latest caldera-forming eruption. Petrological studies on basalts, rhyolites and mafic inclusions in the rhyolites from post-caldera eruptions suggest there is a stratified magma chamber beneath the volcano, which consists of a lower basaltic layer, upper rhyolitic layer and an episodically-present, thin middle-andesitic layer. Chemical analyses of 30 melt inclusions in plagioclase and pyroxene phenocrysts from the basaltic and rhyolitic eruptions showed large variations in the volatile concentrations (H 2 O, CO 2 , S and Cl) of the melts. This suggest the following volatile evolution processes in the magma chamber: 1) a gas-saturated condition due to pressure variation in the rhyolitic magma chamber just before the caldera-forming eruption; 2) low pressure degassing of the rhyolitic magma chamber by magma convection in the conduit during the active degassing period in the post-caldera stage up to the present; and 3) the addition of CO 2 -rich volatile from the underlying basaltic magma in the stratified magma chamber to the upper gas-undersaturated (degassed) rhyolitic magma.