Vol.:(0123456789) 1 3 International Journal of Earth Sciences https://doi.org/10.1007/s00531-018-1657-z ORIGINAL PAPER Solid sulfur spherules near fumaroles of Hakone volcano, Japan Kei Ikehata 1  · Minori Date 1  · Jun‑ichiro Ishibashi 2  · George Kikugawa 3  · Kazutaka Mannen 3 Received: 14 April 2018 / Accepted: 12 October 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract 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 feld observations and the micro-analyses of the samples, these sulfur spher- ules 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 diference 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 signifcant fuctuations of the level of the molten sulfur forming inside the fumarolic vent, and these fuctuations 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 difcult to directly observe. Keywords Sulfur spherule · Molten sulfur · Fumarole · Hakone volcano Introduction Hakone is an active volcano located approximately 80 km southwest of Tokyo (Fig. 1a). From June 29 to July 1, 2015, the volcano produced small phreatic eruptions (Mannen et al. 2018). The eruptions occurred in Owakudani, the largest fumarolic area on the volcano. Until the eruptions, the fumarolic area had formed native sulfur crystals (fuma- rolic sulfur) near the fumarolic vents, and steam temperature was less than the local boiling point (~ 97 °C). During the eruptions, several new fumaroles formed in Owakudani and they remain active at the time of writing. Fumarolic sulfur formed by condensation near fumaroles is a common and widely observed in active volcanic areas. Compared with the fumarolic sulfur, sulfur fow and pyroclastic sulfur are not as common and generally inaccessible for sampling or observation (e.g., Ohashi 1919; Watanabe 1940; Colony and Nordlie 1973; Francis et al. 1980; Theilig 1982; Naranjo 1985; White et al. 1988; Oppenheimer and Stevenson 1989; Delmelle and Bernard 1994; Grimes et al. 1999; Kargel et al. 1999; Harris et al. 2000). In particular, occurrence of solid sulfur spherules near subaerial fumaroles is rare (Oppenheimer 1992). Their mineralogical characteristics and formation mechanisms are still poorly understood. Yel- low to greenish-gray solid sulfur spherules and ejection of molten sulfur blobs have been observed near a fumarole that was formed at Owakudani during the 2015 Hakone erup- tion. We were able to directly measure the temperature of a molten sulfur pool in the fumarole during the ejection. To our knowledge, sulfur spherules and a molten sulfur pool have not been previously documented at Hakone volcano. The purpose of this paper is to describe the occurrence and * Kei Ikehata ikkei@geol.tsukuba.ac.jp 1 Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan 2 Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 3 Hot Springs Research Institute of Kanagawa Prefecture, 586 Iriuda, Odawara, Kanagawa 250-0031, Japan