Amphibole-melt disequilibrium in silicic magma of Aso-4 caldera-forming eruption at Aso volcano, SW Japan

Abstract

Major element compositions were measured for amphibole in the first and largest pyroclastic flow unit, 4I-1, of Aso4 eruption occurred at ~90ka at Aso volcano, and T-SiO₂ conditions of melt crystallizing amphiboles were estimated. Amphiboles reveal T of 900-950 and 965-980℃ and SiO₂ content of coexisting melt of 68-63 and 60-57 wt.%, which are higher-T and lower-SiO₂ content than the host 4I-1 melt, indicating disequilibrium between amphiboles and the 4I-1 melt. These amphiboles were incorporated from crystal mush into the 4I-1 melt immediately before the Aso-4 eruption.