MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE PYROCLASTIC MATERIALS FROM THE SEPTEMBER 2014 MOUNT ONTAKE ERUPTION, JAPAN

Mount Ontake (elevation 3067 m) in Japan suddenly erupted pyroclastic materials on September 27, 2014. The craters formed in the 2014 eruption occurred in a different location from the previously formed craters (e.g., 1979 phreatic eruption) of this volcano (Kaneko et al., 2016). We investigated mineral composition and sulfur isotopic composition of pyroclastic materials of the eruption by using XRD, EPMA, and CF-IR-MS. The pyroclastic materials were composed of altered volcanic fragments, alunite, anhydrite, biotite, cristobalite, gypsum, ilmenite, kaolin minerals, native sulfur, orthopyroxene, plagioclase, potassium feldspar, pyrite, pyrophyllite, quartz, rutile, and smectite, and most of which are likely to be derived from the acidic alteration zones of the Mt. Ontake. An absence of juvenile fragments in the pyroclastic materials suggests the 2014 eruption was also phreatic like 1979 eruption. The sulfur isotopic values of sulfur from water-leached sulfate (δ³⁴S: +15.53 ‰ and +15.22 ‰), hydrochloric acid-leached sulfate (δ³⁴S: +15.92 ‰ and +16.04 ‰), acetone-leached native sulfur (δ³⁴S: –3.86 ‰ and –3.77 ‰) and pyrite (δ³⁴S: –5.24 ‰ and –5.17 ‰) for the pyroclastic materials indicate that these sulfur species were originally produced by disproportionation of magmatic SO₂ gases in the hydrothermal system at temperatures of 270–281 °C. The temperature ranges are in good agreement with those (200–300 °C) inferred by the mineral assemblage (e.g., pyrophyllite and rutile). Except for sulfur isotopic values of anhydrite, which could be altered due to the incorporation of sulfate minerals (e.g., gypsum) with lower sulfur isotopic values around a fumarolic area into the underground materials (Sugiura et al., 1980; Kusakabe et al., 1982), no remarkable differences in the mineral assemblage and sulfur isotopic value of the pyroclastic materials were found between 2014 and 1979 Ontake phreatic eruptions, indicating geochemical similarities in the hydrothermal systems below the crater area before the 2014 and 1979 eruptions (Ikehata and Maruoka, in press).