Cooling history of a tholeiitic basalt from Funagata Volcano deduced from microstructures of minerals.

Cooling history of a tholeiitic basalt from the Kurohana area in Funagata volcano is inferred from the microstructures of clinopyroxene and plagioclase. The sample contains pigeonite, orthopyroxene, olivine and plagioclase as phenocrysts. The phenocrysts are surrounded by the overgrown rims. From the compositional difference between phenocrysts and the overgrown rims, the crystallization sequence has been divided into two stages. At the first stage, the phenocrysts of pyroxene and plagioclase crystallized at above 1473K in the magma chamber. They were kept in the chamber less than half a year, which was determined on a basis of the coarsening kinetics of pigeonite-augite intergrowth. Then, at the second stage, the rims of phenocrysts overgrew and groundmass minerals crystallized during the rapid cooling after the eruption. The cooling duration was estimated as about 4-5 years from the size of antiphase domains in plagioclase.