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 distribution. K₂O 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. The magma chamber model proposed in this paper consists of upper and lower chambers and a set of a huge cylinder and a mobile plug between the chambers. Both the cylinder and the plug have same frustum form. The plug subsiding into the lower chamber drives the basaltic magma upward through an opening between the cylinder-wall and the plug, and mixes it with dacitic magma in the upper chamber. Continuous influx of volatile-rich basaltic magma into the upper chamber may result in repetitive volcanic eruptions, and causes a continuous change in magma composition in the upper chamber, which erupts as lavas with discrete compositions.