Bulk and Mineral Chemistry of Lavas and Ejecta of the 1983 Eruption of Miyakejima Volcano

Abstract

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 : Wo₃₉En₄₆-Wo₃₅En₄₅) and titano-magnetite (TiO₂ 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 SiO₂ contents vary from 53.3 to 55.2 wt%. SiO₂ 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 SiO₂ contents, 53.6-55.2 wt%. Products from P-S craters around the sea shore, however, are SiO₂-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.