Abstract: Volatiles play a fundamental role in linking deep mantle processes to surface geochemical expressions in active volcanic arcs. The Tatun Volcano Group (TVG), northern Taiwan, represents an active but non-eruptive volcanic system where diffuse degassing provides a unique window into deep volatile migration. In this study, we investigate the spatial distribution, isotopic characteristics, and structural controls of diffuse CO₂ emissions to constrain the pathways and sources of deep-seated volatiles. A total of 448 soil CO₂ flux measurements were conducted over a 10 km² area using the closed-chamber method. Significant diffuse degassing occurs not only within hydrothermal zones but also in structurally controlled, non-hydrothermal areas. High-flux zones are spatially associated with well-developed fracture networks, indicating that tectonic structures act as efficient conduits for deep volatile ascent. Integrated carbon and helium isotope analyses (δ¹³C–CO₂ and helium isotopes) indicate a slight deep-seated signature, with an estimated ~0.2–10% mantle-derived helium component, implying contributions from a mantle–crustal volatile mixture rather than a purely shallow biogenic source. Mixing relationships among mantle, crustal carbonate, and biogenic end-members further suggest interaction between ascending deep gases and shallow hydrothermal systems, accompanied by carbonate precipitation and associated isotopic modification in parts of the hydrothermal field. Temporal monitoring in the Dayoukeng area (2009–2024) documents a ~210% expansion of degassing zones, accompanied by elevated soil temperatures and water vapor anomalies, implying dynamic volatile redistribution within the shallow crust. These results demonstrate that fracture-controlled pathways facilitate deep-seated volatile migration from depth to the surface in the TVG. Our findings provide new constraints on diffuse CO₂ flux in a convergent margin setting and highlight the value of integrating CO₂ and helium isotope tracers to quantify deep volatile contributions and transport processes in non-eruptive volcanic systems.
Keywords: Diffuse CO₂ emissions, δ¹³C–CO₂, Helium isotopes, Mantle-derived volatiles, Fracture-controlled migration
Author Profile (first or corresponding author):
Ching-Chou Fu, Male, Assistant Research Fellow/Professor, PhD, mainly engaged in gas geochemistry research. E-mail: ccfu@earth.sinica.edu.tw
