[S-VC34] 海域火山

[SVC34-P13] 西之島の2020年噴火からわかってきたこと

*田村 芳彦¹、佐藤 智紀¹、石塚 治²、McIntosh Iona¹、吉田 健太¹、前野 深³ (1.海洋研究開発機構　海域地震火山部門、2.産業技術総合研究所、3.東京大学地震研究所)

キーワード：安山岩、玄武岩、地殻・マントル

Nishinoshima erupted andesite magmas from 1973 on and off through effusive Strombolian eruptions. Andesite magmas of Nishinoshima have been generated by olivine fractionation of primary andesitic magmas. These primary andesitic magmas are derived from the partial melting of hydrous mantle (plagioclase peridotites) at relatively low pressures, and thus, the thin crust in the Ogasawara arc is deemed to have resulted andesite magma generation in the uppermost shallow mantle. Main submarine body of Nishinoshima consists of andesite lava flows, which are similar to the lavas from 1973 eruptions, but basalt magma had been observed in the submarine flanks of Nishinoshima and its surrounding knolls, which had been interpreted as the old events of the volcano. The sudden burst of the 2020 explosive eruption of Nishinoshima was different from the past Strombolian andesite eruptions and had been caused by injection of new basalt magmas generated in deeper mantle wedge, which intruded into the andesitic magma chamber in the crust. Basalts are comparable to the previously erupted basalts in the surrounding knolls. Thus, basalt magmas could have been generated through the entire history of Nishinoshima. The eruption centers of andesite and basalt magmas had been different in Nishinoshima volcano so far, but the eruption center of basalt is deemed to have moved to the summit of the volcano (Nishinoshima island), which have resulted in mixing between existing andesite magma and new basalt magma and have caused a violent Strombolian eruption. Basaltic andesites, which are similar to those of 2020 eruptions, had come out of the volcanic center of Nishinoshima during prehistoric (pre-1973) eruptions. This is consistent with the idea that basalt magmas could have generated through the entire history of Nishinoshima. In addition to basalt genesis, Nishinoshima andesites and Nishinoshima-Minami Knoll lavas have distinct subduction components: the former is hydrous sediment melt and the latter is hydrous carbonatite fluid. ‘Mission Immiscible’ could explain the different primary andesite magmas produced beneath Nishinoshima only ~8 km apart at the shallowest mantle.