Importance of Long-Term Shallow Degassing of Basaltic Magma on the Genesis of Massive Felsic Magma Reservoirs: a Case Study of Aso Caldera, Kyushu, Japan.

This article focuses on the chemical composition and volatile content (H₂O and CO₂) of melt inclusions and matrix glass in eruptive products from Aso caldera, southwest Japan, to elucidate the magma genesis and plumbing system beneath the volcano. The study identifies two basalt magma types beneath Aso: water-rich basalts evolving as calc-alkalic or medium-K series, and water-poor basalts evolving as tholeiitic or high-K series, which coexist due to differences in degassing and crystallization processes. Using melt inclusion data combined with petrological modeling and volcanic gas flux measurements, the authors propose a model where volatile-rich basalt magma degasses near the surface, sinks back to depths of approximately 12 km or more, crystallizes to produce high-K rhyolitic melt, and mixes with other magmas in the plumbing system. The estimated basalt magma supply rate required to generate the observed felsic magma volumes aligns with the present-day degassed magma production inferred from gas emissions at Nakadake crater, suggesting volcanic gas monitoring can serve as a proxy for magma supply during dormant periods. The study also emphasizes that rapidly cooled small pumice fragments from fallout tephra (0.5–1 mm) provide the most reliable melt inclusion data for volatile analysis, minimizing post-eruptive diffusion and bubble formation effects.