Magma degassing of Satsuma-Iwojima volcano:
Constraints from melt inclusions, petrology and volcanic gases.
Genji SAITO
Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan.
E-mail: saito-g@aist.go.jp

In their trip to the surface of the Earth, magmatic gases usually stay at “magma chamber”
hotels. Satsuma-Iwojima volcano, Japan, is one of the best fields to study volatile evolution
with mafic-felsic magma interaction in the magma chamber and current degassing process
because of its contemporaneous bimodal activity of basalt and rhyolite in a small area and its
intense and long-term passive degassing activity. The maximum temperature of the volcanic
gases discharged from the summit crater of Iwodake has been almost constant for the past 10years at about 900°C (Shinohara et al., 2002). Sulfur dioxide flux measured by COSPEC isalmost constant with an average of 550 t/d since 1975 (Kazahaya et al., 2002). Most of thevolatile components in the volcanic gases are believed to be of magmatic origin (e.g.,
Shinohara et al., 1993). Intense fumarolic activity at the summit crater has probably
continued for more than 800 years (Kamada, 1964).
Petrological studies on basalts, rhyolites and mafic inclusions in the rhyolites frompost-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 volatileconcentrations (H2O, CO2, S and Cl) of the melts (Fig. 1). (1) Water concentration ofrhyolitic melts decreases with time; 3-4.6 wt.% at the time of latest caldera-forming eruption(ca. 7300 y.B.P.) of Takeshima pyroclastic flow deposit, 3 wt.% for small pyroclastic flow (ca.
500 y. B.P.) of Iwodake, and 0.7-1.4 wt.% for Showa-Iwojima rhyolitic lava eruption in 1934.
(2) Rhyolitic melts of the Takeshima and Iwodake eruptions contained CO2 of less than 40
ppm, while the Showa-Iwojima melt has higher CO2 concentration of up to 140 ppm. (3)Water and CO2 concentrationsof basaltic to andesitic melt ofInamuradake, a post-caldera
basaltic scoria cone (ca. 3000y.B.P.), are 1.2-2.8 wt.% and290 ppm, respectively. Thissuggests the following volatile
evolution processes in themagmachamber:a)agas-saturated condition due topressure variation in therhyolitic magma chamber justbefore the caldera-formingeruption; b) low pressure degassing of the rhyolitic magma chamber by magma convection inthe conduit during the active degassing period in the post-caldera stage up to the present; andc) the addition of CO2-rich volatile from the underlying basaltic magma in the stratified
magma chamber to the upper gas-undersaturated (degassed) rhyolitic magma (Fig. 2).
Current degassing process of the magma chamber could be estimated by combining
volatile concentrations of the melt inclusions with chemical composition and flux of the hightemperature volcanic gas has been discharged from the summit crater of Iwodake. Volatilecomposition of the Showa-Iwojima rhyolitic melt agrees with that of the magmatic gas (Fig.
1), suggesting that source of the magmatic gas is a magma with similar composition with thatof Showa-Iwojima magma, and that magma-gas separation occurs at a very shallow level.
The mass rate of magma degassing is estimated at 8-13 m3/s using volatile concentration(H2O=1 wt.%, CO2=100 ppm, S=110 ppm) of the Showa-Iwojima rhyolitic melt and the fluxof the magmatic gas (H2O=16000 t/d, CO2=150 t/d, S=550 t/d; Kazahaya et al., 2002).
Magma convection in a conduit, driven by the density difference between higher density
degassed and lower density non-degassed magmas (Kazahaya et al., 1994; Stevenson andBlake, 1998), explains the high emission rate of magmatic volatiles released at shallow depth
from the magma chamber. The degassed rhyolitic magma descends to the bottom of therhyolitic magma chamber and is supplied with the CO2-rich volatile components from theunderlying basaltic magma, implying that the upper rhyolitic magma works as “transporters”
of volatile components from the underlying basaltic magma to the surface. Assuming thatsimilar degassing continued for 500 years, the total volcanic gas flux requires degassing of
50-80 km3 of basaltic magma. This estimate suggests that mafic magma underlying felsicmagma contribute excess degassing of the volcano not only during eruptions (e.g., 1991
Mount Pinatubo eruption), but also during passive degassing activity.