Kenta Yoshida , Yoshihiko Tamura, Tomoki Sato, Erika Tanaka, Noriko Tada, Morihisa Hamada, Takeshi Hanyu, Qing Chang, Shigeaki Ono

Fukutoku-Oka-no-Ba is a submarine volcano located at 24°17.1′N/141°28.9′E in the Izu–Bonin–Mariana arc, and is one of the most active volcanoes in Japan. This volcano produced an explosive eruption in August 2021 that generated a large amount of volcaniclastic material, some of which drifted westward to Japan and the coastal area of East Asia as a pumice raft. The pumice clasts that drifted for >1000 km were mostly homogeneous and identical to those produced by past historical eruptions. The clasts have trachytic compositions (SiO2 = 61-63 mass% and Na2O+K2O = 8.6-10.0 mass%) and contain augite, plagioclase, olivine (Mg# ~65), and magnetite, along with a small number of mafic enclaves containing diopside and high-Mg olivine (Mg# ~ 92).
We undertook a research cruise to investigate the proximal volcaniclastic materials by dredging. The proximal materials include pumice, weakly vesiculated lapilli, and volcanic rocks, which have trachytic composition (SiO2 contents up to 64.5 mass%). The main minerals in the proximal material are similar to those in the drift pumice, although remnants of mafic magma do not occur in the SiO2-rich samples. The compositional variations of the SiO2-rich proximal samples are due to plagioclase-dominated fractional crystallization, and the higher-SiO2 samples yielded lower temperature and pressure among the studied samples.
The petrographic and geochemical characteristics of the proximal and drift ejecta from Fukutoku-Oka-no-Ba suggest the magma reservoir was stratified into two parts. The lower part experienced magma mixing with a limited volume of mafic magma, whereas the upper part underwent plagioclase-dominated fractional crystallization. The accumulation of differentiated high-SiO2 magma in the upper part of the magma reservoir might have increased the pressure in the magma conduit, which then exceeded the critical pressure required for an explosive eruption.