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
We performed a research cruise YK22-15 using R/V Yokosuka from August 14 to 27, 2022, aiming at the geologic survey of Nishinoshima and Fukutoku-Oka-no-Ba, volcanoes in the Izu-Bonin arc. During the cruise, we dredged submarine rock samples and performed unmanned aerial vehicle (UAV) operation for on-land observation of Nishinoshima. In addition, we recovered a fixed-point camera that was set on the western part of the island by UAV. The fixed-point camera was set by NHK on September 6, 2021, to record the volcanic activity.
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
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