Syn-eruptive hydrothermal degassing and CO2 fluxes at the Planchón-Peteroa Volcanic complex (Chile-Argentina): evidence from dissolved gases, isotopes, and radon tracers

The 2018–2019 eruption of the Planchón-Peteroa Volcanic Complex (Chile-Argentina) provided a rare opportunity to investigate syn -eruptive degassing in peripheral hydrothermal valleys. During February 2019, we acquired a multi-parameter dataset combining diffuse soil CO₂ fluxes, free- and dissolved-gas chemistry, carbon isotopes (δ 13 C-CO₂, δ 13 C-CH₄), and radioactive tracers ( 222 Rn, 22 ⁰Rn) in two hydrothermal valleys: Valle Baños del Azufre (VBA) and Valle del Peñón (VP). Soil CO₂ fluxes show contrasting statistical and spatial patterns. VBA ( n = 171) displays a broader flux range and higher maximum values (0.5–514.9 g m −2 d −1 ), whereas VP ( n = 63) shows a narrower and smoother distribution (2.7–60.2 g m −2 d −1 ). Free gases are CO₂-dominated (61–94 vol%) but differ in redox-sensitive components and isotope signatures. VBA is characterized by elevated 222 Rn activity (up to 5 × 10 4 Bq m −3 ), higher CH₄ contents, and isotopically light methane (δ 13 C-CH₄ down to −54‰), whereas VP shows lower 222 Rn activity (≤5 × 10 3 Bq m −3 ), stronger air-water interaction, and markedly heavier methane (up to +11‰), consistent with methane oxidation and/or mixing during lateral migration. Dissolved CO₂ reaches 11.8 mmol L −1 , while dissolved CH₄ is mainly restricted to the most reducing VBA samples, reaching up to 0.06 mmol L −1 . Overall, the dataset reveals marked differences in degassing style and redox-sensitive gas signatures between the two peripheral valleys. These results provide new constraints on the behaviour of peripheral hydrothermal systems during eruptive activity and highlight their potential relevance for integrated volcano monitoring.