Temporal evolution of fumarolic gas geochemistry at the Nevados de Chillán Volcanic complex (2013−2023): Signals of volcanic unrest and insights into the hydrothermal system

Gas geochemistry is key to understanding volcanic processes, offering insights into subsurface magma dynamics and aiding eruption forecasting. We present a 10-year monitoring case study from Andean Southern Volcanic Zone (SVZ), one of the world's most active regions, demonstrating its value in assessing volcanic unrest. The geochemical composition of fumarolic gas emissions from the Nevados de Chillán Volcanic Complex (Chile) strongly varied across different phases of volcanic activity, permitting the evaluation of the dynamic interplay between magmatic and hydrothermal processes. During volcanic quiescence periods, (2013 and 2023), fumarolic gases were predominantly controlled by shallow meteoric-hydrothermal circulation, as suggested by Ar (40Ar/36Ar ∼ 270–290) and water isotopic signatures, low 3He/4He (or Rc) ratios (∼3.5 times the atmospheric value Ra), moderate CO2 levels (between ∼2200 and ∼ 9800 μmol/mol), and the absence of magmatic gaseous species (SO2, HCl, and HF). In contrast, the unrest phase (2016 and 2017) was marked by a rapid and significant gas character shift. Rc/Ra values increased up to >6, CO2 concentrations exceeded 12,000 μmol/mol, and magmatic gaseous species became detectable, with SO2, HCl, and HF reaching 8.5, 4.3, and 0.21 μmol/mol, respectively. Additionally, simultaneous Ar (40Ar/36Ar ∼ 370–410) and water isotopic signature shifts from meteoric origin toward a mixing with deep components were also detected. Gas geothermometry, computed through the H2/Ar* – CH4/CO2 and H2/Ar* - CO/CO2 equilibria systems, revealed a rise from 290 ± 10 °C in quiescence to 340 ± 10 °C during the unrest phase. The long-term geochemical surveillance helped detect a substantial signature of volatile-rich magmatic fluids influx into the hydrothermal system before the eruption phase (2018–2021), offering information on the early detection of unrest.