Time scale of magma mixing between basalt and dacite estimated for the Saga–Futagoyama volcanic rocks in northwest Kyushu, southwest Japan

A composite lava extrusion of the Saga–Futagoyama volcanic rocks, consisting of a central unit of the dacite lava flanked by the andesite lava containing phenocrysts of quartz and plagioclase, is exposed in northwest Kyushu, southwest Japan. Field and microscopic observations indicate that the andesite and the dacite simultaneously emerged at the surface. Disequilibrium relationships between plagioclase and bulk rock compositions suggest that the andesite lava is a hybrid of more mafic and more silicic magmas. The whole-rock chemistry of the composite lava suggests that the contemporaneous eruption had two end-members of basalt and dacite magmas. The time scale of magma mixing between basalt and dacite is estimated using reaction coronas around quartz and dissolution textures of plagioclases in the andesite. The typical reaction duration was estimated at 1.4 h to 12 days using reaction coronas around quartz and at 2.8 h to 1 day using dissolution textures of plagioclases. These estimates indicate that the mixing essentially occurred during ascent in a conduit. The concentration of the andesite compositions at SiO2=59 wt% requires other processes to homogenize mixed magma such as phenocryst stirring.