Earth Planets Space, Vol. 54 (No. 3), pp. 217-229, 2002
Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan
(Received January 25, 2001; Revised September 26, 2001; Accepted January 11, 2002)
Abstract: Satsuma-Iwojima is located near the northwestern margin of Kikai caldera, and active Mt. Iwodake is one of the post-caldera cones. Inside the summit crater of about 300 m diameter and 50 m depth, volcanic gas with maximum temperature >800°C is discharged. At Iwodake, the rhyolite is strongly altered by volcanic vapor and acid hydrothermal solution inside and around the summit crater. The main alteration mineral assemblages at surface include quartz-tridymite-cristobalite, tridymite-cristobalite-alunite and cristobalite-(alunite) inside the summit crater and on the flank. Molybdenum blue sublimates are forming at the surface around high-temperature fumaroles inside the summit crater. The ground surface is commonly cemented by amorphous silica or cristobalite. The original rhyolite with SiO2 = 71-72 wt% has been leached, resulting in a rock with maximum SiO2 = 97 wt%. All major elements are leached except for TiO2. Although LIL (large-ion lithophile) elements have leached, HFS (high field-strength) elements appear to be immobile. Rare earth elements are considered to have been soluble as a complex with sulfate mainly in the low pH hydrothermal fluid during the silicification of the rhyolite. There is abundant Mo, Pb, Zn, Bi, Sn and much lower As in the volcanic sublimates and altered rocks inside the summit crater. By contrast, the rocks near the acid springs at the foot of the cone are As-rich, but with lower Pb, Zn, Bi and little Mo in comparison with the summit. The fractures and fumarole locations around Iwodake summit and erosional ridges on the flanks reflects a NE-SW trend that is parallel to the trace of the outer ring of Kikai caldera. In addition, this is the same direction as the alignment of Shin-Iwojima, Iwodake and Inamuradake, all of which are post-caldera cones. Zones of structural weakness parallel to the caldera rim may exist, acting as pathways for the ascending volcanic gases and hydrothermal solutions. Fumarolic activity appears to be shifting gradually to the eastern part of the summit crater.