Chemical and isotopic features of thermal fluid discharges in the volcano-hydrothermal system of Caviahue-Copahue volcanic complex (Argentina).

AGUSTO, M., TASSI F., CASELLI,A., VASELLI, O., TEDESCO, D., AND POREDA, R.,

Santiago de Chile

Congreso; GEOSUR; 2007

Asoc Geologica Chilena

The Caviahue-Copahue Volcanic Complex (CCVC; Argentina-Chile), which is composed by the Copahue polygenic fissural-stratovolcano and the Caviahue squared caldera, belongs to the Southern Andean Volcanic Zone (SAVZ: 33.3ºS-46ºS). The volcano-tectonic activity in this area started in the Pliocene and it was mainly characterized by intense eruptions producing andesitic to basaltic-andesitic pyroclastic and lava flows (Pesce, 1989). Copahue (37º45’S-71º10.2’W, 2977 m a.s.l.) is an active stratovolcano, whose summit is composed by nine NE-oriented craters, that lies in the western sector of the Caviahue caldera (Linares et al., 1999). During the last 250 years, this volcano has experienced at least 12 low-magnitude phreatic and phreato-magmatic eruptions (Martini et al., 1997; Naranjo and Polanco, 2004). The last eruptive cycle took place from the easternmost crater in July 1992 and was marked by major eruptions in August 1992, September 1995 and July-October 2000 (Delpino and Bermúdez, 1993 and 2002; GVN, 2000a,b). The active crater presently hosts a hot (up to 42 °C) acidic lake (Mange, 1978; Varekamp et al., 2001; Caselli et al., 2005). In this study the chemical and isotopic (δ13C in CO2, and 3He/4He ratio) compositional features of the thermal fluid emissions seeping out in the surroundings of the volcanic edifice are discussed in order to investigate the origin of discharged fluids and their relation with the volcano-tectonic activity of this system. Gases (14 samples) were collected in November 2006 and February 2007 from 5 different thermal areas (Chanco-Co, Anfiteatro, Las Maquinas, Las Maquinitas and Copahue village) and an 1241 m deep well of the local geothermal plant. Generally speaking the chemical and isotopic compositions of sampled gases can be regarded as relatively homogeneous. Steam is the main component of the fumarolic discharges, which are characterized by outlet temperatures up to 135 °C, while CO2 dominates the composition of gases of boiling pools, whose outlet temperatures ranges between 27.1 and 96 °C. Among acidic gas species, H2S has relatively high concentrations (up to 9,023 mol/mol), while HCl and HF show relatively low contents (up to 245.6 and 8.6 mol/mol, respectively) and SO2 is below detection limit (0.001 mol/mol). Nitrogen, H2 and CH4 are present in comparable amounts (up to 10,196, 9,494 and 5,910 mol/mol, respectively) and minor contents of He, Ar and CO (up to 58.5, 33.5 and 0.08 mol/mol) were detected. Helium isotopic ratios (up to 7.94 R/Rair) indicate helium is predominantly mantle-derived when compared to that measured in the Middle Oceanic Ridge Basalts (MORB=8±1 Ra; Farley and Neroda 1998). It is worthy of noting that these helium isotopic ratios are the highest among those measured in SAVZ gas emissions (up to 6.6 R/Rair; Hilton et al., 1993). Differently, δ13C values in CO2 (between -8.2 and -7.25 ‰ PDB) are significantly lower than those of gases from a mantle source. However, the CO2/3He ratios (between 1.5 and 3.3x109) are in the range of MORB magmatic fluids (2x109; Marty and Jambon 1987). This suggests that the measured δ13C values may be tentatively ascribed to isotopic fractionation on magmatic-derived CO2 due to interactions between gases rising from the magmatic source and the hydrothermal aquifer feeding the thermal discharges. The extended liquid-dominated system is able to i) almost completely dissolve the highly-soluble compounds, such as SO2 and HF, ii) strongly decrease the CO contents due to hydrolysis processes producing formic acid (Shock 1993), and iii) affect, although at a minor extent, HCl contents, whose solubility significantly decreases at decreasing pH values (Symonds et al., 2001). On the contrary, the hydrothermal environment favors those gas species, such as H2S, H2 and CH4, that are produced at highly reducing conditions (e.g. Giggenbach et al., 1986). Therefore, the chemical and isotopic features of Copahue thermal discharges can be explained in terms of a mixing process between gases, mainly helium and CO2, from a magmatic source, possibly rising through the complex deep NNW-oriented fault system characterizing this area (Melnick et al., 2006), and contributions from a well-developed hydrothermal system. In terms of volcanic surveillance, the gas discharges seeping out at the foothill of the Copahue volcano, being filtered by a thick hydrothermal aquifer, are less sensitive to deep-seated (magmatic) inputs. This would imply that geochemical monitoring program should be mainly focused on the summit crater lake (Copahue Lake) and the gases emitted from the top of the volcanic edifice.