Variation of volatile concentration in a magma system of Satsuma-Iwojima volcano deduced from melt inclusion analyses

Chemical analyses of 30 melt inclusions from Satsuma-lwojima volcano, Japan , were carried out to investigate volatile evolution in a magma chamber ben eath the volcano from about 6300 yr BP to the present. Large variations in volatile concentrations of melts were observed. (1) Water concentration of rhyolitic melts decreases with time; 3-4.6 wt.% at the time of latest calde ra-forming eruption of Takeshima pyroclastic flow deposit (ca. 6300 yr BP), 3 wt.% for small pyroclastic flow (ca. 1300 yr BP) of Iwodake, post-calder a rhyolitic dome, and 0.7-1.4 wt.% for submarine lava eruption (Showa- Iwoj ima) in 1934. (2) Rhyolitic melts of the Takeshima and Iwodake eruptions co ntained CO2 of less than 40 ppm, while the Showa-Iwojima melt has higher CO 2 concentration of up to140 ppm. (3) Water and CO2 concentrations of basalt ic to andesitic melt of Inamuradake, a post-caldera basaltic scoria cone, a re 1.2-2.8 wt.% and less than or equal to 290 ppm. respectively. Volatile e volution in the magma chamber is interpreted as follows: (1) the rhyolitic magma at the time of the latest caldera-forming eruption (ca. 6300 yr BP) w as gas-saturated due to pressure variation in the magma chamber because the large variation in water concentration of the melt was attributed to exsol ution of volatile in the magma prior to the eruption. Iwodake eruption (ca. 1300 yr BP) was caused by a remnant of the caldera-forming rhyolitic magma , suggested from the similarity of major element composition between these magmas. (2) Volatile composition of the Showa-Iwojima rhyolitic melt agrees with that of magmatic gases presently discharging from a summit of Iwodake , indicating the low pressure degassing condition. (3) The degassing of the magma chamber by magma convection in a conduit of Iwodake during non-erupt ive but active degassing period for longer than 800 years decreased water c oncentration of the rhyolitic magma. (4) Geological and petrological observ ations indicate that a stratified magma chamber, which consists of a lower basaltic layer and an upper rhyolitic layer, might have existed during the post-caldera stage. Addition Of CO2 from the underlying basaltic magma to t he upper gas-undersaturated (degassed) rhyolitic magma increased CO2 concen tration of the rhyolitic magma. ((C) 2001 Elsevier Science B.V. All rights reserved.