Strontium isotope compositions of anorthite and olivine phenocrysts in basaltic lavas and scorias of Miyakejima volcano, Japan.

Abstract

Sr isotope compositions were determined for anorthite megacrysts, olivine phenocrysts and host basalts of Miyakejima volcano, Izu-Ogasawara arc, Japan. The anorthite megacrysts are of An97-94 in composition. The olivine phenocrysts have almost the same composition (Fo84-82) as olivine inclusions in the anorthite megacrysts. The 87Sr/86Sr ratios of anorthites range within a narrow variation of 0.70340-0.70352, while those of olivine phenocrysts and olivine inclusions show distinctively higher values (0.70357-0.70374) than those of anorthites. Host rock samples excluding megacrysts yield intermediate 87Sr/86Sr values between anorthite and olivine. This disequilibrium in 87Sr/86Sr ratios between phenocrysts and host rocks demonstrates that anorthites and olivines did not crystallize directly from the host magmas but were probably xenocrysts carried up by the magmas. The present data further suggest that the olivine phenocrysts and olivine inclusions in anorthite megacrysts have the same origin and that the crystallization of anorthite occurred posterior to olivine crystallization. The difference in 87Sr/86Sr ratios between olivine inclusions and host anorthite megacrysts provide an evidence for magma mixing between two types of magmas (both of basaltic in composition) with different 87Sr/86Sr ratios. The basaltic magmas which formed the Miyakejima volcano may also be explained by the mixing of above two end member magmas, at least in some stage (or stages) of magma evolution.