Gas geochemistry of the magmatic-hydrothermal fluid reservoir in the Copahue-Caviahue Volcanic Complex (Argentina)

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). This depression is a pull-apart basin accommodating stresses between the southern Liquine-Ofqui strike slip and the northern Copahue-Antinir compressive fault systems, in a back-arc setting with respect to the Southern Andean Volcanic Zone. In this study, we present chemical (inorganic and organic) and isotope compositions (delta C-13-CO2, delta N-15, He-3/He-4, Ar-40/Ar-36, delta C-13-CH4, delta D-CH4, and delta D-H2O and delta O-18-H2O) of fumaroles and bubbling gases of thermal springs located at the foot of Copahue volcano sampled in 2006, 2007 and 2012. Helium isotope ratios, the highest observed for a Southern American volcano (R/R-a up to 7.94), indicate a non-classic arc-like setting, but rather an extensional regime subdued to asthenospheric thinning. delta C-13-CO2 values (from -8.8 parts per thousand to -6.8 parts per thousand. vs. V-PDB), delta N-15 values (+ 5.3 parts per thousand. to + 5.5 parts per thousand. vs. Air) and CO2/He-3 ratios (from 1.4 to 8.8 x 10(9)) suggest that the magmatic source is significantly affected by contamination of subducted sediments. Gases discharged from the northern sector of the CCVC show contribution of He-3-poor fluids likely permeating through local fault systems. Despite the clear mantle isotope signature in the CCVC gases, the acidic gas species have suffered scrubbing processes by a hydrothermal system mainly recharged by meteoric water. Gas geothermometry in the H2O-CO2-CH4-CO-H-2 system suggests that CO and H-2 re-equilibrate in a separated vapor phase at 200 degrees-220 degrees 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 degrees to 300 degrees C. Fumarole gases sampled in 2006-2007 show relatively low N-2/He and N-2/Ar ratios and high R/R-a values with respect to those measured in 2012. Such compositional and isotope variations were likely related to injection of mafic magma that likely triggered the 2000 eruption. Therefore, changes affecting the magmatic system had a delayed effect on the chemistry of the CCVC gases due to the presence of the hydrothermal reservoir. However, geochemical monitoring activities mainly focused on the behavior of inert gas compounds (N-2 and He), should be increased to investigate the mechanism at the origin of the unrest started in 2011. (C) 2013 Elsevier B.V. All rights reserved.