Petrology of the Ejecta and Lavas of the 1983 Eruption of Miyake-jima

Abstract

The ejecta and lava flows of the 1983 Miyake-jima eruption are basalts with a few percentages of plagioclase and less abundant pyroxene and titanomagnetite phenocrysts. Olivine and hypersthene phenocrysts are rarely found. Variations in bulk chemistry of these ejecta and lava flows are essentially controlled by variations in modal amounts of phenocrysts. Both major and trace element concentrations in these basalts indicate that they are well on the differentiation trend of the Miyake-jima volcano and mildly differentiated. Based on the compositions of the coexisting pyroxenes, the temperature of the magma on eruption is estimated to be 1100℃ or less. Three different types of vesiculated xenoliths are recognized : Type 1 is the least vesiculated and is considered to be exotic materials, type 2 is most vesiculated and has dacitic compositions, and type 3 is composed of basaltic clots set in a well vesiculated dacite glass which is identical to type 2 and has andesite or basaltic andesite composition. Type 2 and 3 often form a composite xenolith. Type 2 xenolith is derived from dacite pumice once erupted on Miyake-jima and type 3 is the mixing product of the dacite pumice and basalt. Trace element chemistry also indicates that type 2 and type 3 xenoliths are genetically related with the Miyake-jima volcano. However, type 3 xenolith has exotic composition from the Miyake-jima volcano.