Repeated large-scale eruptions from a single compositionally stratified magma chamber: An example from Aso volcano, Southwest Japan

Aso volcano has the largest caldera (18 × 25 km) in the southwestern Japan Island Arc, which was formed as a result of four gigantic pyroclastic-eruption cycles. The latest eruption cycle (Aso-4 cycle, 90 ka) produced the most voluminous deposits (> 600 km3) among the four eruption cycles. We investigated pre-eruptive magma chamber configurations and processes in compositionally stratified Aso-4 pyroclastic eruptions based on whole-rock chemical, phenocryst mineralogical, and Sr isotope analyses of juvenile clasts. Previous stratigraphical studies have suggested that the Aso-4 cycle is divided into two subcycles, in both of which eruption of voluminous silicic is followed by eruption of mafic magma. Geochemical and mineralogical results indicate that the pre-eruptive magma chamber was composed of three layers: silicic, hybrid, and mafic magma layers from the top to the bottom. The silicic magma layer was relatively homogenous, whereas the mafic magma layer was compositionally zoned where magma became more silicic with height. The hybrid magma was generated by mixing the silicic magma and the silicic end of the zoned mafic magma layer (i.e. the uppermost part of the mafic magma layer). The first subcycle began with evacuation of the upper silicic magma and successively proceeded to tap the lower hybrid and mafic magmas. The eruption of the first subcycle stopped before the whole magma chamber evacuated. During this subcycle, the hybrid magma and the upper part of the mafic magmas layer were selectively withdrawn and mixed with the overlying silicic magma. The second subcycle eruption started with the new hybrid magma, and followed by eruption of the mafic magma.