Solid sulfur spherules near fumaroles of Hakone volcano, Japan

Occurrence of sulfur spherules near subaerial fumaroles is relatively uncommon and their mineralogical characteristics and formation mechanisms are still incompletely understood. Yellow to greenish-gray solid sulfur spherules were observed near a fumarole that was formed in the fumarolic area (Owakudani) during the 2015 eruption of Hakone volcano, Japan. The yellow sulfur spherules (up to 1 mm in diameter) are composed entirely of α-sulfur, and the greenish-gray spherules (up to 2 mm in diameter) consist mainly of matrix-forming α-sulfur with lesser amounts of pyrite, amorphous silica, pyrophyllite, and rare marcasite. Based on the results of the field observations and the micro-analyses of the samples, these sulfur spherules were formed by rapid cooling of molten sulfur blobs ejected from a low-viscosity molten sulfur pool (124.7–128.7 °C) in the bottom of the fumarole during vigorous fumarolic activity. Color difference between yellow and greenish-gray sulfur spherules is explained by the presence of xenolithic altered mineral fragments in the greenish-gray sulfur spherules. Our observations indicate significant fluctuations of the level of the molten sulfur forming inside the fumarolic vent, and these fluctuations may imply the existence of other molten sulfur reservoirs beneath the fumarolic area. Presence of solid sulfur spherules in the fumarolic area may be an indicator of a molten sulfur pool within fumaroles or volcanic vents that are often difficult to directly observe.