Petrochemical evidence for coupled magma chambers beneath the Sakurajima volcano, Kyushu, Japan.

Abstract

We report systematic petrochemical changes of historical lava flows from the Sakurajima volcano. The lavas are all calc-alkalic in composition. They change in composition from dacitic to andesitic at an increasing rate with time, and show a straight-linear correlation in chemical composition. Plagioclase phenocrysts show a bimodal compositional distribution with peaks at about An58 and An85, regardless of variation in bulk rock composition. The intensity of An85 peak increases, while that of the An58 peak decreases with eruption stage. Orthopyroxene phenocrysts also show a bimodal compositional distribu tion. K20 content of the lavas progressively drops at almost the same rate with every gigantic eruption. These chemical and petrological aspects with other lines of evidence support binary mixing of dacitic and basaltic magmas, and a progressive increase in proportion of basaltic magma in the mixed magmas. The mixing has progressively proceeded for 475 years at least.