Chemistry and Mineralogy of the 1974 Scoria erupted from Mihara Crater of Oshima Volcano, Izu Islands

Abstract

A small eruption occurred at the Mihara Crater of Oshima Volcano at midnight, Feb. 28/Mar. 1, 1974. The ejecta dispersed around the crater are mostly composed of highly vesiculated basaltic scoria (Fig. 1) with accessory lithic fragments and a small amount of pele’s hair. Under the microscope, phenocrysts of the 1974 scoria are plagioclase, orthopyroxene, pigeonite and rare augite. The groundmass consists of fresh brown glass with a few crystallites. The chemical and modal compositions of the 1974 scoria are shown in Tables 1 and 2, together with those of the 1950-51 lava, 1954 lava, 1964 scoria and 1972 scoria. The amount of phenocrystic plagioclase in the 1950-51 lava is only 5.3%. but it increases from 17.7 to 25.3% in the later ejecta. Phenocrystic pyroxenes also tend to increase in volume in the later ejecta, though not to the same extent as plagioclase. The phenocrystic pyroxene assemblage consists of augite and orthopyroxene, in the 1950-51 and 1954 lavas, while it consists of orthopyroxene, pigeonite and augite in the ejecta of 1964, 1972 and 1974 eruptions. The chemical compositions of phenocrystic minerals and groundmass were determined by EPMA analysis. The results are shown in Tables 3, 4 and 5, and in Figs. 2, 3 and 4. The change of bulk chemical compositions of the lavas and scoria from 1950-51 to 1974 is mainly controlled by the volume of modal phenocrystic plagioclase, as is clearly demonstrated in an addition-subtraction diagram (Fig. 2). Hence the “groundmass compositions” obtained by subtracting the composition of phenocrystic plagioclase (An₉₀) from each bulk composition are nearly the same as the glass composition of the 1974 scoria. The temperature of magma in the 1950-51 eruption was calculated as ca. 1200℃ using Wood and Banno’s method (1973) based on opx-cpx pairs. Although this value is considerably higher than the direct measurement results (1125℃, Minakami, 1955), it is in fair agreement with the estimated temperature of the magma. Due to the paucity of clinopyroxene phenocrysts in the 1974 scoria its composition could not be determined. However, by inference of the orthopyroxene composition, the temperature of the magma in the 1974 eruption was probably not significantly different from that of the 1950-51 eruption. In view of the activity of the Mihara Crater from 1950-51 to 1974 the following features are apparent: 1) After ten years dormancy, eruption of a considerable amount of nearly aphyric lavas (0.03 km²) occurred, which was intermittently followed by small eruptions of lavas and scoria. 2) The liquid composition of the products is fairly uniform throughout the duration of this activity. 3) The temperature of the magma seem not to have changed significantly. These features suggests that a compositionally zoned magma chamber was formed by gravitational sinking of plagioclase prior to the 1950-51 activity.