High SiF4/HF ratio detected in Satsuma-Iwojima volcano’s plume by remote FT-IR observation

Abstract

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 SO2/HCl molar ratio in the plume was about 4, that is larger than the ratio in high-temperature gases. The high content of SO2 is suggested to be caused by sulfur combustion in the crater. An average SiF4/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 SiF4 is an important species and has similar to even larger contribution for fluorine output from Satsuma-Iwojima volcano than HF. The SiF4 flux of Satsuma-Iwojima volcano is about 13 t/d, that is the largest SiF4 flux from volcanoes in the world. The observed SiF4/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%.