Changes in the Redox State of the Nishinoshima Magmatic System During and After the 2020 Explosive Eruption

Intermittent volcanic activity has continued at Nishinoshima volcano in the Izu–Bonin arc in the western Pacific since 2013, characterized mostly by effusive Strombolian eruptions (Episodes 1–3). Subsequently, the eruption style changed suddenly to explosive violent Strombolian in mid-June 2020 (Episode 4), after which the volcano entered a period of quiescence. In 2021, volcanic activity restarted at Nishinoshima, although the eruptions were small (Episode 5). The change in eruption style between Episodes 1–3 and 4 was controlled mainly by a change in magma composition. However, the details of the redox state with respect to magma composition remain unclear. We aimed to understand the change in magma composition during Episode 4 and the subsequent activity, focusing on the redox state. Fe–K edge X-ray absorption near edge structure (XANES) analysis and petrographic observations were performed on volcanic glass in seafloor and subaerial samples from Episodes 4 and 5. The results show that the explosive eruption of Episode 4 was characterized by the intrusion of oxidized basaltic magma into a reduced andesitic magma, which is a similar tendency to another large-scale eruption in the Izu-Bonin arc (Fukutoku-Oka-no-Ba). Episode 5 shows different redox changes, involving remnants of the basaltic andesite magma from Episode 4, whose f_O2 had been reduced. The groundmass glass in Episode 5 subaerial ejecta has oxidized and differentiated (SiO₂-rich) composition possibly due to the subaerial cooling process. The observed geochemical and petrographic characteristics suggest that the small Episode 5 eruption occurred due to the continuous feed of the magma similar to Episode 4, but the magma plumbing system in the volcano made the provided high f_O2 magma reduced, which may mitigate the explosivity of the future eruption.