Degassing process of Satsuma-Iwojima volcano, Japan: Supply of volatile components from a deep magma chamber

Abstract

Satsuma-Iwojima volcano continuously releases magmatic volatiles from the summit of Iwodake, a rhyolitic lava dome. The temperature of fumaroles is high, between 800• and 900• C, and the water-rich composition of volcanic gases has not changed essentially over the past 10 years. Sulfur dioxide flux measured by COSPEC is almost constant with an average of 550 t/d since 1975. The present volcanic gas is likely degassed from a rhyolitic magma whose composition is similar to that erupted in 1934, 2 km east of Satsuma-Iwojima. Comparison of silicate melt inclusions and volcanic gas compositions indicates that the magma degassing pressure is very low, implying magma-gas separation at a very shallow level. The mass rate of magma degassing is estimated at 10 m 3 /s using the volatile content of the magma and the fluxes of magmatic volatiles. The rhyolitic parental magma is volatile-undersaturated in the deep magma chamber, as suggested by melt inclusion studies. Magma convection in a conduit, driven by the density difference between higher density degassed and lower density non-degassed magmas, explains the high emission rate of magmatic volatiles released at shallow depth from such a magma chamber, that is gas-undersaturated at depth. Model calculations require the conduit diameter to be greater than 50 m as a necessary condition for convection of the rhyolitic magma. Long-term convective degassing has resulted in the rhyolitic magma in the deep chamber to become depleted in volatile components. The melt-inclusion studies indicate that the rhyolitic magma responsible for discharging the present volcanic gas has been degassed during the long degassing history of the volcano and is now supplied with CO 2 -rich volatile components from an underlying basaltic magma. The total volcanic gas flux over 800 years requires degassing of 80-120 km 3 of basaltic magma.

Radionuclide behavior in high-temperature gases from Satsuma Iwojima volcano, Japan

“…Our conclusions are consistent with the degassing model proposed by Kazahaya et al (2002), provided the viscosity of the rhyolitic layer is high enough to trap isolated radionuclide atoms, but is not so high that convection of the rhyolitic magma is impeded.…”

“…Assuming that Satsuma Iwojima rhyolitic magma is undersaturated in gases (Kazahaya et al, 2002), then SO 2 exsolved from the underlying basaltic layer dissolves in the upper 0.24 km 3 rhyolite. Assuming a continuous SO 2 degassing of 550 T/day over 800 years, the ratio between the amount of SO 2 in the estimated rhyolite volume and the mean SO 2 flux allows to roughly estimate the "residence time" of SO 2 in the rhyolitic reservoir.…”

“…According to the degassing model of Kazahaya et al (2002), the shallow rhyolitic magma has a long residence time, as it was not renewed over the past 800 years. We assume that gases emitted at 3-5 km depth from the basaltic layer do not contain significant 210 Pb,210 Bi or 210 Po, due to their very low vapor pressure at 90 MPa (Pennisi and Le Cloarec, 1998).…”

“…The high temperature of the gases, as well as the permanent flux of SO 2 , averaging 550 T/day (Kazahaya et al, 2002), require that non-degassed magma convects in the shallow part of the reservoir where the degassing takes place. Kazahaya et al (2002) propose that the degassing occurs from a two-layered reservoir: the degassed magma returns at depth, owing to its higher density, and is recharged in volatiles emitted by an underlying basaltic layer. The present rhyolite is thought to be fully degassed, and acts as the transporter of dissolved gases from the underlying basalt to the surface.…”

Soil gas emission of volcanic CO2 at Satsuma-Iwojima volcano, Japan

“…• C exist inside and around the summit crater, emitting greater than 300 t day −1 of SO 2 and 100 t day −1 of CO 2 (Shinohara et al, 1993Kazahaya et al, 2002). Jogahara, a lava plateau with an elevation of about 80 m above sea level, lies outside the caldera and forms the western part of the island.…”