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

Abstract

The Laguna del Maule volcanic field is an Andean system characterized byextraordinary postglacial rhyolitic volcanism and one of the highest rates of surfaceuplift globally. Following a sustained increase in seismic activity in April 2023, thevolcanic monitoring observatories in Chile and Argentina raised the technical alert levelof 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 LasNieblas fluid discharge zone, located in the Cajón Troncoso. In addition to CO2 fluxmeasurements, we performed gas composition analyses using a Multigas device atLas Nieblas and collected soil gas samples for carbon isotopic analysis at two fluiddischarge zones: Las Nieblas and Cajón Grande. Our survey revealed that Las Nieblasemits CO2 exclusively through cold soils, with fluxes exceeding 4,500 g m⁻² d⁻¹.Statistical analysis of the flux data indicates a dual origin for the CO2, with a largerproportion derived from an endogenous source. Carbon isotopic analysis furthersupports a magmatic origin for CO2. Based on our results, the magmatic CO2 output atLas 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 stronglinkage with the Troncoso Fault. At Cajón Grande, the carbon isotopic composition alsosuggests a magmatic contribution for CO2, but in lower proportions. Overall, thispreliminary study reinforces the hypothesis of a sill intrusion beneath Laguna del Maulevolcanic field and suggests that the CO2 released at Las Nieblas may be directlysourced from this silicic intrusion and emitted via the Troncoso Fault.