Soil carbon dioxide diffuse emission during a volcano-tectonic seismic Swarm at Laguna del Maule volcanic field

The Laguna del Maule volcanic field is an Andean system characterized by extraordinary postglacial rhyolitic volcanism and one of the highest rates of surface uplift globally. Following a sustained increase in seismic activity in April 2023, the volcanic monitoring observatories in Chile and Argentina raised the technical alert level of the Laguna del Maule volcanic field from green, its baseline activity level, to yellow. At the peak of the seismic crisis, we conducted a diffuse CO₂ flux survey at Las Nieblas fluid discharge zone, located in the Cajón Troncoso. In addition to CO₂ flux measurements, we performed gas composition analyses using a Multigas device at Las Nieblas and collected soil gas samples for carbon isotopic analysis at two fluid discharge zones: Las Nieblas and Cajón Grande. Our survey revealed that Las Nieblas emits CO₂ exclusively through cold soils, with fluxes exceeding 4500 g m⁻² d⁻¹. Statistical analysis of the flux data indicates a dual origin for the CO₂, with a larger proportion derived from an endogenous source. Carbon isotopic analysis further supports a magmatic origin for CO₂. Based on our results, the magmatic CO₂ output at Las Nieblas is estimated at 37 t d⁻¹, representing a minimum emission rate for the site. Additionally, the diffuse degassing structure mapped at Las Nieblas shows a strong linkage with the Troncoso Fault. At Cajón Grande, the carbon isotopic composition suggests an organic source for CO₂, with only a minor magmatic contribution. Overall, this preliminary study reinforces the hypothesis of a sill intrusion beneath Laguna del Maule volcanic field and suggests that the CO₂ released at Las Nieblas may be directly sourced from this silicic intrusion and emitted via the Troncoso Fault.