Evolution of deeper basaltic and shallower andesitic magmas during the AD 1469-1983 eruptions of Miyake-Jima Volcano, Izu-Mariana Arc: Inferences from temporal variations of mineral compositions in crystal-clots

Abstract

Miyake-jima volcano has erupted at least 13 times during the period 1469-1983. To understand the historic magmatic processes, we focus on the mineral assemblage and chemical compositions of crystal-clots in single samples from each of the eruptions. Most of the historic lavas consist of nearly aphyric to weakly porphyritic basalt to andesite, but there also exist megacryst-bearing rocks. The megacrysts are considered to be xenocrysts from a deep-seated plutonic body. Many samples of each eruption contain two types of clots beside megacrysts, termed here B-type and A-type. The B-type clots are composed of olivine, clinopyroxene and plagioclase, whereas the A-type clots additionally contain magnetite and orthopyroxene. Compositional relationships between these mafic minerals suggest that the minerals in the same type of clots are in equilibrium. Comparing the chemical compositions of the minerals in the two types of clots in each sample, they are derived from distinct magmas: the B-type clots from basaltic magma and the A-type clots from andesitic magma. During the historic activity, the magma plumbing system appears to have included two magma storage systems: a deep-seated basaltic and a shallower andesitic one. In many cases, basaltic magma has injected into shallower andesitic magma to form mixed magma; however, andesitic magma has sometimes erupted alone without extensive injections of basaltic magma. Temporal variations of mineral compositions in the clots and estimated whole-rock compositions of the end-member magmas suggest that the basaltic magma has differentiated gradually since 1469, and that its magmatic temperature has fallen from 1220 to 1180°C. Conversely, the andesitic magma has changed in a complex fashion to become more mafic (the magmatic temperature rose from 1050 to 1100°C). As a result of this study, it is estimated that the basaltic magma after the 1983 eruption was the least mafic, and the andesitic magma the most mafic, of the historic eruptions.