4:15 PM - 4:30 PM
[SVC31-20] Thumbe-lapilli-na from Nishinoshima describes the possible new volcanic stage after 2021 September
Keywords:Nishinoshima, unmanned aerial vehicle, research cruise
The recovered camera was covered with volcanic ash, in which a few lapilli with up to several millimeters were caught in the gap between the camera parts. This occurrence strongly suggests that these lapilli and ashes originated from volcanic activity during the term between September 2021 and August 2022. We performed a petrographic study of this lapilli to decipher the most recent activity of the Nishinoshima volcano.
The lapilli grain consists of microlite-rich groundmass with phenocrysts of olivine, clinopyroxene, plagioclase, and magnetite. The olivine phenocryst had Mg-decreasing zoning from the core to the rim, with the core composition of Fo72 and rim composition of Fo65. Clinopyroxene was augite with Mg#~80. Plagioclase exhibited oscillatory zoning with the composition of An~70, whereas coarse grains (>100 μm) have calcic cores up to An96.
Groundmass glass of the lapilli exhibited rhyolitic composition of SiO2 ~76 mass%, while melt inclusions found in clinopyroxene and olivine have andesitic compositions of SiO2 ~60 mass%. The latter andesitic melt is comparable with the melt composition of the Episode-4 pyroclastic materials [1], while the former rhyolitic melt has much higher SiO2 content than Episodes 1-3 Nishinoshima volcanic activities.
A magnetite geothermometer applied to the magnetite occurring in the groundmass yields 950 °C, while clinopyroxene/olivine and corresponding melt inclusion pairs give ~1000 °C.
These observations suggest that after the explosive Episode-4 stage (2019-2020), Nishinoshima may be shifted to a new volcanic stage characterized by the accumulation of more-silicic magma and the injection of higher-T andesitic magma.
[1] https://doi.org/10.3389/feart.2021.773819