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

Miyake-jima volcano has erupted at least 13 times during theperiod 1469–1983. To understand the historic magmaticprocesses, we focus on the mineral assemblage and chemical compositionsof crystal-clots in single samples from each of the eruptions.Most of the historic lavas consist of nearly aphyric to weaklyporphyritic basalt to andesite, but there also exist megacryst-bearingrocks. The megacrysts are considered to be xenocrysts from adeep-seated plutonic body. Many samples of each eruption containtwo types of clots beside megacrysts, termed here B-type andA-type. The B-type clots are composed of olivine, clinopyroxeneand plagioclase, whereas the A-type clots additionally containmagnetite and orthopyroxene. Compositional relationships betweenthese mafic minerals suggest that the minerals in the same typeof clots are in equilibrium. Comparing the chemical compositionsof the minerals in the two types of clots in each sample, theyare derived from distinct magmas: the B-type clots from basalticmagma and the A-type clots from andesitic magma. During thehistoric activity, the magma plumbing system appears to haveincluded two magma storage systems: a deep-seated basaltic anda shallower andesitic one. In many cases, basaltic magma hasinjected into shallower andesitic magma to form mixed magma;however, andesitic magma has sometimes erupted alone withoutextensive injections of basaltic magma. Temporal variationsof mineral compositions in the clots and estimated whole-rockcompositions of the end-member magmas suggest that the basalticmagma has differentiated gradually since 1469, and that itsmagmatic temperature has fallen from 1220 to 1180°C. Conversely,the andesitic magma has changed in a complex fashion to becomemore mafic (the magmatic temperature rose from 1050 to 1100°C).As a result of this study, it is estimated that the basalticmagma after the 1983 eruption was the least mafic, and the andesiticmagma the most mafic, of the historic eruptions. KEY WORDS: andesite; basalt; crystal-clots; evolution of magma; Miyake-jima volcano; magma mixing