Iodine isotope ratios and halide concentrations in fluids of the Satsuma-Iwojima volcano, Japan

Fumarolic condensates and acid spring waters from Satsuma-Iwojima were sampled between 1993 and 1998 and analyzed for 129I/I and 36Cl/Cl ratios and chloride, bromine, and iodine concentrations. Halogen concentrations suggest that the fumarolic condensates represent mostly magmatic fluids, but that the hot springs are a mixture between magmatic fluids and meteoric water with some presence of seawater. The magmatic component is best preserved in the high-temperature fumaroles collected in 1998 and has a 129I/I ratio of 756 ± 47 × 10−15. This 129I/I ratio yields an iodine age of 15.5 ± 1.5 Ma, which is compatible with derivation of iodine from subducted marine sediments in this region. Ratios of I/Cl, and Br/Cl in the fumaroles also indicate that halides in the magmatic component were mainly derived from marine sediments subducted in the Nankai Trough. These observations suggest that recycling of subducted sediments is an active source of halides in the Satsuma-Iwojima volcanic system. The comparison of 129I/I ratios obtained from fumaroles over the sampling period shows that a recent meteoric component was present in 1993, but not in 1998. This observation might reflect reorganization of the fumarolic system during this period, which expelled the more recent meteoric water from the fumarolic system after 1993. The acid hot springs on the flanks of the volcano maintain a 129I isotopic signature indicative of substantial dilution of magmatic waters by mixing with old meteoric water and seawater. Fumarolic gases and hot spring waters were also analyzed for 36Cl/Cl, but the measured ratios were at, or below, the detection limit of AMS, also indicating that recent contributions of Cl from meteoric water are small in the fumaroles and coastal hot springs.