Persistent gas emission originating from a deep basaltic magma reservoir of an active volcano: the case of Aso volcano, Japan

Abstract

Volcanic gas emission is considered to reflect the degassing of magma beneath volcanoes. The combined observations of gas measurement and petrological study are expected to constrain the volatile concentrations and storage depths of the pre-eruptive and primitive magma. Aso volcano (Japan) is a constantly-monitored, persistently-degassing volcano, and an ideal site to acquire gas and petrologic data. We analyzed the melt inclusions and phenocryst minerals of Holocene basaltic eruption products, and reported their major and volatile element concentrations. The samples showed abundant evidence of magma mixing, such as reverse mineral zoning, and highly variable mineral and glass compositions. SiO2 measured in melt inclusions varied from 46.0–65.8 wt. %. High-volatile concentration, S up to 3750 ppm, was only found in mafic melt inclusions hosted by high-Fo olivine phenocrysts (~ Fo82). The pre-eruptive storage depths were determined from volatile concentrations: 2 and 4 km depth for Strombolian eruption and sub-Plinian eruption, respectively. The volatile-rich primitive magma, one end member of the mixed-magma, originated from a deeper level (> 10 km) than these magma reservoirs. Initial volatile concentrations of the primitive magma were determined using multiple constraints: > 4.68 wt. % H2O, 400—750 ppm CO2, 3750 ppm S, 716 ppm Cl, and 324 ppm F. The observed variation of volcanic gas composition was best explained by the mixing of the gas segregated from at least a depth of 10 km, with that from the shallow reservoirs. This study illustrated the method to identify the primitive mafic magma responsible for deep volatile flux in a mature volcano with complex magmatic processes.