Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XX, Nos.2-3


Spinels in Basaltic Lavas and Ultramafic Inclusions of Oshima-Ōshima Volcano, North Japan

Yamamoto, Masatsugu

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Spinels and olivines in the alkali basalts and in the ultramafic inclusions congnate with the basalts of Oshima-Ōshima volcanic island, north Japan, were analysed. When the compositions of spinels in olivine phenocryst in the basalts plot in terms of Cr-Al-Fe3+ ratio, the crystallization trend proceeds from Cr-rich corner towards Fe3+ corner associated with an increase in Al/Cr+Al ratio. The spinels in the wehrlite and olivine-clinopyroxenite inclusions have the same trend, while those in the dunite inclusions which are the most aluminous among inclusions have a different trend. The olivine phenocrysts in the basalts and the olivines in the inclusions are clearly classified in terms of CaO contents considered to be controlled by pressure. The data obtained in this study show that the compositions of spinels in wehrlite and olivine-clinopyroxenite inclusions and in the olivine phenocryst of basalts, are mainly controlled by the chemistry of host magma. The petrography and chemistry indicate that the magma crystallizing dunite inclusions is lower in Al2O3 content than that crystallinzing other inclusions, and that the dunite inclusions are formed at a deeper place than the other inclusions. Therefore, the enrichment of aluminum in the spinels of the dunite is probably due to high pressure rather than the chemistry of host rock.