Magma plumbing system of the Aso-3 large pyroclastic eruption cycle at Aso volcano, Southwest Japan: Petrological constraint on the formation of a compositionally stratified magma chamber

Aso volcano has the largest caldera (18 × 25 km in diameter) in the southwestern Japan Island Arc, and it formed as the result of four large (VEI = 6-7) pyroclastic-eruption cycles. We study the penultimate large eruption cycle, the Aso-3 cycle, which occurred 123 ka with an ejecta volume of more than 150 km3. The processes in the pre-eruptive magma chamber and the magma genesis of the Aso-3 cycle were inferred from geological data, phenocryst chemistry, and whole-rock chemical and Sr-, Nd-, and Pb isotopic analyses of juvenile clasts. The geological and petrological data indicate that the pre-eruptive magma chamber was stratified compositionally into three layers: from top to bottom, silicic, intermediate, and mafic magma layers. The three magma layers had a uniform isotope composition, suggesting that all the magmas were generated from a single source. The silicic and intermediate magmas were not generated from the mafic magma by fractional crystallization. The silicic magma has higher Ni content (compatible element) than the mafic magma. This suggests that these magmas were produced by partial melting of the same mafic crust but with differing amounts of partial melting: the silicic magma was produced by a low degree of partial melting of the source rock without fractional crystallization, and the mafic magma was produced by a large degree of partial melting followed by fractional crystallization. The intermediate magma compositions plot on the tie line between the silicic magma and the melt of the mafic magma in variation diagrams, and the intermediate magma has phenocrysts whose compositions are identical with those in the silicic magma. This observation indicates that, before the Aso-3 eruption cycle, a two-layer stratified magma chamber of the silicic and mafic magmas was formed as a result of melting of the mafic crust, which was followed by formation of the intermediate layer as a result of interfacial mixing between the silicic magma and the melt of mafic magma. During the eruption the three-layer stratified magma chamber was tapped from the top to the bottom.