三宅島火山 1983 年噴出物の化学組成および鉱物組成

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Petrographic, mineralogical and chemical studies were made for the lavas and pyroclastics of the 1983 eruption of Miyakejima Volcano. Essential eruptive products are phenocryst-poor (8 vol%>) olivine-bearing augite basalt to basaltic andesite. Products from A and G craters are nearly aphyric with phenocrysts less than 2.5% in volume. Phenocrysts are plagioclase (core : An_<92-82>), olivine (Fo_<68-66>), augite (core : Wo_<39>En_<46>-Wo_<35>En_<45>) and titano-magnetite (TiO_2 8.5-9.3 wt%). Groundmass consists of plagioclase, augite, sub-calcic augite, pigeonite, titano-magnitite, silica minerals and brown glass. From the compositions of groundmass pyroxenes, the eruption temperature is estimated to be about 1120℃. Chemical compositions of essential eruption products are on the differentiation trend of the post-caldera lavas of the Miyakejima Volcano, and their SiO_2 contents vary from 53.3 to 55.2 wt%. SiO_2 contents of A-E craters inside the caldera are 53.3-54.0%, and products from A craters represent their liquid compositions. Products from G-K craters on the southern slope of the precaldera edifice have higher SiO_2 contents, 53.6-55.2 wt%. Products from P-S craters around the sea shore, however, are SiO_2-poor again, 53.4-53.9 wt%, and have similar compositions to those of B-E craters. Above evidence suggests that A-E, G-K and P-S eruptive fissures erupted magmas with different chemical composition coming through different feeders, respectively.
特定非営利活動法人日本火山学会の論文
1984-12-28

三宅島火山 1983 年噴出物の化学組成および鉱物組成

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