PRELIMINAR CHARACTERIZATION OF CO2 DIFFUSE DEGASSING AT COPAHUE GEOTHERMAL FIELD

LAMBERTI, M.C.; CHIODINI, G.; CARDELLINI, C.; AGUSTO, M; CASELLI, A.

Workshop; 12º Workshop on volcanic gases; 2014

The Copahue-Caviahue Volcanic Complex (hereafter CCVC, 38°S-71°W) is located in the Neuquén Province, Argentina, on a segment of the Andes range called South Volcanic Zone (33,3° - 46°S), 30 km east of the main Pleistocene-Holocene volcanic front. This complex conforms the ?Caldera Caviahue? (38°S ? 71°O), a volcano-tectonic depression defined as an intraarc extensional pull-apart basin (Ramos and Folguera, 2000; Bermúdez et al., 2002; Melnick et al., 2006; Rojas Vera et al., 2010). The CCVC hosts an important magmatic-hidrotermal system composed of the Copahue volcano (37,9°S ? 71,2°O, 2977 m s.n.m.) and of the geothermal areas inside the caldera (Agusto, 2011). These geothermal areas consist of five sites with thermal fluid emissions, located northeast of the volcano: Copahue town, Las Máquinas, Las Maquinitas and Anfiteatro in Argentinian territory, and Chancho-Co in Chilean territory (Mas et al., 1996; Mas et al., 2000). These thermal discharges consist of boiling, bubbling and mud pools with temperatures up to 96 °C, fumaroles with outlet temperatures up to 160°C and of diffuse degassing, which has not been characterized until now. In March 2014, five hydrothermal areas: Anfiteatro, Copahue town, Las Máquinas, Las Maquinitas 1 and Las Maquinitas 2, were investigated by means of specific and detailed CO2 flux (CO2) surveys over a total area of 1.21 km2. During the ten days of campaign, 1,763 measurements were performed using two accumulation chambers developed and calibrated c/o INGV-Osservatorio Vesuviano and Perugia University laboratories (Chiodini et al., 1998). The data were elaborated using the graphical statistical approach and the sequential Gaussian simulations method (GSA and SGS respectively; Chiodini et al., 1998; Cardellini et al., 2003). Four log-normal populations were defined at Copahue geothermal field by means of the GSA: two of them are characterized by very low fluxes and they are representative of background values in non-vegetated areas; most measurements belong to a third population which is representative of the background in vegetated areas; and a fourth population, characterized by a high CO2 (about fifty times the background in vegetated areas), is representative of hydrothermal-volcanic CO2 degassing. The total amount of CO2 released from each population was computed by multiplying the mean values of each population by the total extension of the surveyed areas and by the fraction of each population. Thus, the total CO2 released from the hydrothermal ? volcanic source at the Copahue geothermal field is over 180 t d-1. The SGS method was applied in order to map the diffuse degassing structure (DDS; Chiodini et al., 1998) and to obtain estimates of the total CO2 release and of its uncertainty more accurate than those obtained by GSA (Cardellini et al., 2003; Lewicki et al., 2005). The presence of well-defined DDS?s is evident in all the investigated sites with the exception of Anfiteatro. Most areas show the presence of DDS?s that seem to be controlled by tectonic features, as suggested by well-defined alignments of the CO2 anomalies and by the good correlation found between these anomalies and the structures observed in the field.