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 preeruptive 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 to 65.8 wt. %. High-volatile concentration, S up to 3750

Episodic transport of discrete magma batches beneath Aso volcano

“…The source of the inflation/deflation event is probably located near the inferred magma chamber 45 – 47 (Fig. 1a ), but at a shallower depth similar to the previously detected reflection void 48 , corresponding to a shallow magma storage zone (SMSZ) 43 (Fig. 3c ).…”

“…Figure 4a provides a sketch to summarize the renewed magma-plumbing system beneath Aso volcano. After the most recent Aso-4 caldera forming eruption 41 , 42 , a crystal-rich mush reservoir likely remains at a deeper depth (i.e., >10 km) during post-caldera eruptions 42 , 43 , supplying volatile-rich basaltic magma toward the magma chamber. The volatile-rich basaltic magma is thought to mix with volatile-poor silicic magma in the chamber 6 , 43 , 44 , and the mixed magma is stored at a shallower storage zone above the chamber before the eruptions.…”

Episodic transport of discrete magma batches beneath Aso volcano

“…4c). Finally, the depth of the SMSZ is very consistent with the highest gas saturation pressure of melt inclusions in scoria (~80-118 MPa, or 2-3.5 km below sea level) 23 , supporting the hypothesis that the magma (and gas) are likely transferred from a deeper reservoir 48 (i.e., a crystal-rich mush) and temporarily stalled in the SMSZ (i.e., a crystal-poor pool), which serves as a preparation zone for the storage and upward transport before upcoming eruptions.…”

Validation of Repetitive Volcanoseismic Signals in Aso Volcano, Japan With Distant Stations: Implications of Source Characterization and Remote Sensing in Uninstrumented Volcanoes

“…As suggested by Niu and Song (2021), the deformation events likely represent short‐lived, episodic transport of discrete magma batches between the roof of the magma chamber (Ohkura et al., 2009; Sudo et al., 2006; Sudo & Kong, 2001) and a preeruptive storage pool of mixed magma (Kawaguchi et al., 2020; Saito et al., 2018), the source validation using distant stations, as demonstrated in this report, further support such a scenario during the 2011–2016 eruption cycle. While similar processes occur in the past eruption cycles remain to be investigated, it is paramount to expand the detection of deformation events against past volcanic activities during which dedicated in situ stations near the edifice were unavailable.…”