Magmatic carbon dioxide (CO2) degassing has been documented before the 31 March 2000 eruption of Usu volcano, Hokkaido, Japan. Six months before the eruption, an increase in CO2 flux was detected on the summit caldera, from 120 (September 1998) to 340 metric tons per day (September 1999), followed by a sudden decrease to 39 metric tons per day in June 2000, 3 months after the eruption. The change in CO2 flux and seismic observations suggests that before the eruption, advective processes controlled gas migration toward the surface. The decrease in flux after the eruption at the summit caldera could be due to a rapid release of CO2 during the eruption from ascending dacitic dikes spreading away from the magma chamber beneath the caldera.

Remote FT-IR measurements of volcanic plume of Mt. Iwodake, Satsuma-Iwojima volcano, Japan, were carried out in October 1996 to understand plume chemistry of the volcano, especially characteristics of fluorine-bearing species in the plume. The SO 2 /HCl molar ratio in the plume was about 4, that is larger than the ratio in hightemperature gases. The high content of SO 2 is suggested to be caused by sulfur combustion in the crater. An average SiF 4 /HF molar ratio of 0.57 was observed for the plume, which is about one order of magnitude higher than the previously reported ratio. The result shows that SiF 4 is an important species and has similar to even larger contribution for fluorine output from Satsuma-Iwojima volcano than HF. The SiF 4 flux of Satsuma-Iwojima volcano is about 13 t/d, that is the largest SiF 4 flux from volcanoes in the world. The observed SiF 4 /HF ratio cannot be explained only by the high-temperature fumarolic composition of the volcano according to the thermodynamic calculations. The ratio can be explained if contribution of F-rich low-temperature fumaroles to the total gas flux of the volcano is as high as 40%.

No abstract