Helium isotopes at Satsuma-Iwojima volcano, Japan

抄録

We present isotopic analyses of helium in fluids collected during two field trips at Satsuma-Iwojima and Shin-Iwojima islands in November 1998 and October 2000. These are the first reported helium measurements at Shin-Iwojima. Copper tubes tightly closed by clamps at both ends were used to sample 1) gases from high and low temperature fumaroles in the summit area of Iwodake cone, 2) gases from a fumarole on Shin-Iwojima, 3) gas bubbling from the sea-floor along Shin-Iwojima where an increasing bubbling intensity was observed between 1998 and 2000, 4) waters from Sakamoto and Higashi hot springs. For gas samples, two types of correction for atmospheric contamination are discussed, using either the neon concentration or just the partial pressure of condensable gases at liquid nitrogen temperature; this latter method turned out to be very efficient for many samples. For all gas samples, we found ³He/⁴He isotopic ratios between 7.1 and 8.2 times the atmospheric ratio, pointing to a magmatic origin for fumaroles both at Iwodake crater and at Shin-Iwojima island. Our measurements show a decrease of the isotopic ratios at Iwodake with decreasing temperature of the fumarole. Comparing the results of both field trips and those published in previous studies, we suggest this volcanic system is undergoing a recent increase of activity.