Abstract

The Callaqui volcano (south-central Chile) currently exhibits significant fumarolic activity, and its remains indicate that it has had substantial eruptive activity in the past. However, there is no prior knowledge about the fluids discharged by this volcano, which is critical for volcanic monitoring and subsequent understanding of associated volcanic hazards. This work presents the chemical and isotopic compositions of fumarolic gases from the Callaqui volcano (period 2017-2024) to identify the fluid source(s) and evaluate the physicochemical conditions operating at depth. Gases were collected by direct sampling, using Giggenbach bottles and condensation procedures. The temperature of fumarolic gases ranged from 86.3 to 215 degrees C, with a composition dominated by water vapor, CO2, and minor amounts of acid species (SO2, HCl, HF). Helium isotope ratios (5.13 to 6.69 Ra) suggest a significant contribution from MORB-like fluids, while delta 13C-CO2 isotopes (-10.8 to-9.11 parts per thousand vs. V-PDB) suggest assimilation of carbon-bearing crustal compounds. These features suggest a magmatic-hydrothermal origin of Callaqui emissions. The magmatic source corresponds to a degassing magma chamber likely hosted in an intermediate portion of the crust, whereas the hydrothermal system is fed mainly by meteoric waters. Despite the apparent large size of the hydrothermal system, since the volcano is in a rainy and snowy area, magmatically derived species still reach the surface. As a result, partial scrubbing of magma-derived species is assumed at this volcano. Following geothermometric approaches, fumarolic gases indicate the presence of high-temperature magmatic-hydrothermal fluids, with a vapor phase equilibrated at 375-415 degrees C, as well as superheated vapors up to 350 degrees C. Callaqui gases show an increase in the magmatic signal by the end of the study period, consistent with incandescent volcanic episodes and anomalous LP earthquakes reported in late 2021 and early 2022.