Gas geochemistry of the Copahue-Caviahue Volcanic Complex (Argentina).

AGUSTO M; TASSI F; CASELLI A; VASELLI O; ROUWET D; CAPACCIONI B; CHIODINI G; DARRAH T

Congreso; IAVCEI 2013 Scientific Assembly; 2013

Copahue volcano is part of the Caviahue-Copahue Volcanic Complex (CCVC), which is located in the southwestern sector of the Caviahue volcano-tectonic depression (Argentina-Chile), a pull-apart basin between the southern Liquiñe-Ofqui strike slip and northern Copahue-Antiñir compressive fault systems, in a back-arc setting with respect to the Southern Andean Volcanic Zone. During the last 250 years, at least 12 low-magnitude phreatic and phreato-magmatic eruptions occurred. In 2000 a phreatomagmatic event (VEI 1-2) caused ash fall that affected the nearby villages. Since November-December 2011 the discharge rate of fluids from the Copahue summit has significantly increased up to December 22nd 2012, when a new phreatomagmatic eruption occurred. In this study, the chemical and isotopic (δ13C of CO2 and CH4, dD of CH4 and H2O, d18O of H2O, δ15N, 3He/4He and 40Ar/36Ar) compositions of fumaroles and bubbling gases collected in 2006, 2007 and 2012, i.e. prior and during the last volcanic crisis, from five thermal areas surrounding the Copahue volcanic edifice are presented and discussed. The main aims are to i) investigate the origin of fluids and their relationship with the peculiar tectonic setting, and ii) describe the evolution in fumarolic fluids chemistry and relate it to the changing volcanic activity. The He isotopic ratios, the highest observed for a Southern American volcano (R/Ra up to 7.94), and CO2/3He ratios (from 1.4 to 8.8×109) suggest a non-classic arc-like setting, but rather an extensional regime due to asthenospheric thinning. d15N values (+5.3 to +5.5 ? vs. ATM) point to a source of N2 mainly from subducted sediments. Although these gases show a clear mantle isotopic signature, magmatic gases, such as SO2, HCl and HF are not detected, suggesting gas scrubbing by the hydrothermal reservoir constituting the main source feeding the CCVC gas discharges. Gas geothermometry in the H2O-CO2-CH4-CO-H2 system suggests that CO and H2 re-equilibrate in a vapor phase at 200-220 °C. On the contrary, rock-fluid interactions controlling CO2, CH4 production from Sabatier reaction and C3H8 dehydrogenation seem to occur within the hydrothermal reservoir at temperatures ranging from 250 to 300 °C. Fumarole gases sampled in 2006-2007 show relatively low N2/He ratios and high R/Ra values with respect to those measured in 2012 and prior to the 2000 phreatomagmatic eruption. Such compositional and isotopic variations were likely caused by an injection of fresh He-rich magma that likely triggered the 2000 eruption. Presently it is difficult to assess the effects caused to the hydrothermal system by the current phreatomagmatic activity. A high-frequency geochemical monitoring in the thermal areas located at the foothills of Copahue is necessary to identify possible magmatic signals.