Volcanic events and magma interactions before, during and after the caldera-forming eruption of Aso, Kyushu island, Japan

Abstract

Melt inclusions in minerals contained in tephra from different stages of Aso caldera volcano show distinct patterns of compositional plots. They provide information of different state of magma plumbing system. Melt inclusions in Aso-4 pumice representing caldera-forming eruption, those of Aso ABCD pumice and Omine scoria, which were precursory to caldera-forming eruptions, and those of post-caldera central cone pumice (ACP-2 and ACP-4) all show narrow regions in the compositional plots. They indicate existence of homogeneous magma reservoir, probably relatively large in scale. The compositions of melt inclusions are often the same as those of regional tephra or groundmass glass of pumice and scoria, and are quite similar irrespective of host minerals. ACP-1 melt inclusion exceptionally shows a very wide compositional range. It corresponds to the initiation of Holocene basaltic eruptions including that of currently active Nakadake. It may indicate the mixing of magma of different origin, or the intrusion of magma into an inhomogeneous crust initiating partial melting. Melt inclusions of ACP3/4 scoria show distinct compositional range reflecting difference depending on host minerals. The composition becomes SiO2-rich from Olivine-hosted inclusions, to pyroxene-hosted, and to plagioclase-hosted ones. Those melts were probably tapped by host minerals at different portions of the magma supplying system. The compositions of melt inclusions from Holocene basaltic activities of Aso volcano are similar to those of ACP3/4. When all the compositional data of melt inclusions are taken into consideration, development of relatively large homogeneous silicic magma reservoir at present seems unlikely.