Precursory magma activities leading to Aira caldera-forming eruptions in southern Kyushu, Japan

A series of volcanic products from Aira caldera (20 × 20 km) recorded a unique history of magma reservoir underneath the caldera from 100 ka until present. The largest volume was represented by Aira pyroclastic eruptions which produced more than 400km3 of rhyolitic tephra at 29 ka consisting mainly of Ito pyroclastic flow and regional Osumi pumice fall and A-T ash fall. A trasition of magma composition was recorded in Iwato tephra formation at 60 ka, which consists of 3 fall layers, 5 flow layers and minor surge layers. Some of these layers contain scoria, banded pumice and white pumice. We made petrological descriptions and whole-rock XRF analyses for the two key Aira pyroclastic materials and products of other pre-caldera-forming events. i.e., Fukuyama pumice fall deposit (Fk; 90-86 ka), Shikine andesitic lava (Sk; ca 70 ka), Iwato tephra formation (Iwt; ca 60 ka), Shimizu ryolitic lava (Sm; 39-33 ka), Otsuka pumice fall deposit (Ot; ca 33 ka), Fukaminato tephra formation (Fm; ca 31 ka), Kenashino tephra formation (Kn; ca 30 ka). Three magma types were recognized from our study, type-A (67-70 wt. % SiO2; Fk), type-B (56-59 wt. % SiO2; Sk) and type-C (73-78wt. % SiO2; Sm, Ot, Fm and Kn). Volcanic samples from all types contain Pl, Opx, Cpx and Fe-Ti oxides phenocrysts. Type A contains abundant Hb but no Qtz phenocrysts, whereas types B and C contain abundant Qtz but no Hb phenocrsyts. Iwt samples represented a wide compositional variation (58-78 wt. % SiO2). Iwt scoria belongs to type-B, whereas Iwt pumice belongs to type C. The compositions of scoria (58-59 wt. % SiO2; type B), banded pumice (65-68 wt. % SiO2) and white pumice (71-78 wt. % SiO2; type C) make a straight line in Harker diagrams, suggesting a magma mixing event. Type-C magma emerged at 60 ka and had been active until 30 ka, then continued on to major 29 ka Aira pyroclastic eruptions. This suggests that large silicic magma reservoir which eventually led to the gigantic caldera-forming eruption possibly existed for a period of 30 ka. Mafic type-B magma was only active before and at the Iwt tephra event.