Redox change in the Nishinoshima magma system during the Ep. 4 to Ep. 5 activities.

Abstract

The intermittent volcanic activities of Nishinoshima from 2013 were characterized by the effusive eruption of the andesite magma (episodes 1-3, until 2019) and subsequent explosive eruption of basalte to basaltic-andesite magma (episode 4, 2019-2020). Furthermore, possible episode 5 activity has been observed from August 2021. This study investigated the geochemical characteristics of the episodes 4 & 5 magma based on the on-land and seafloor samples collected in and around Nishinoshima, taking special care of the redox state of the glass using micro-XANES. The episode 4 eruption was characterized by the reduced felsic magma and the injection of oxidized mafic magma, while the episode 5 magma shows the oxidized felsic glass. The observed characteristics suggest that the episode 5 eruption has different features from neither episodes 1-3 nor 4 activities.