Structural control on shallow hydrogeochemical processes at Caviahue-Copahue Volcanic Complex (CCVC), Argentina

Abstract

The Caviahue-Copahue Volcanic Complex (CCVC) hosts one of Argentina's most 25 important geothermal systems. To provide new insights into origin, circulation, and residence time 26 of fluids, the chemical and isotopic composition ( 3 He/ 4 He, δ 2 H-δ 18 O in H2O; δ 13 C-δ 18 O in CO2; 27 87 Sr/ 86 Sr) of thermal waters was measured together with the 3 H and 14 C activities. Water samples 28 were collected from hot springs (LM, TC, LMM, CB and AF) representing the five major thermal 29 zones of the CCVC and assumed to be steam-heated meteoric waters, and a well condensate (COP-30 2). The LMM, CB, and AF chemical composition and 87 Sr/ 86 Sr ratios show that water chemistry is 31 acquired locally from exchange with volcanic rocks (Sr, SiO2, among others) and from steam (H2S). 32 Two surface geothermal manifestations (LM and TC), along with the well condensate, COP-2, 33 contain a higher contribution of deep-originating fluids, with 87 Sr/ 86 Sr indicating possible 34 contribution from deep-seated granitoids or sediments from the underlying basement. 35Radiocarbon-based residence times indicate ages ranging between 13,540 and 17,520 yrs BP, 36 representing the minimum age for the geothermal reservoir waters. Tritium is mainly absent in hot 37 spring waters except for LMM and CB where the activity is close to the detection limit. This 38 indicates a minimum age older than 70 yrs for the water circulating in the shallow circuit. This 39 result suggests that shallow meteoric water have a more complex and/or deeper circuit, resulting 40 in older residence times. Helium isotopes in the CCVC span a wide range, from a pure mantle-41 derived value, of 8.35Ra, to a more crustal radiogenic signature, of 4.6Ra. The spatial variation is 42 explained by associating the geochemical data with the geological context, which includes the 43 distribution of fault-fracture meshes and different sources of magmatic volatiles underlying the 44Copahue volcano. The first order control on helium isotope signatures observed in this study seems 45 to be dominated by the degree of crustal assimilation of the magmatic sources, which is in turn 46 controlled by the local arrays of faults.The Copahue stratovolcano, part of the Caviahue-Copahue Volcanic Complex (CCVC), is 49 an active volcano that hosts a high-enthalpy geothermal system (JICA, 1992) that represents a 50 promising resource for geothermal energy in Argentina (Barcelona et al., 2020). The CCVC is 51 located in the Southern Volcanic Zone (SVZ, 33 -46 ° S) of the Andes, at the northern termination 52 of the intra-arc, strike-slip, Liquiñe-Ofqui Fault System (LOFS). Here, a trans-tensional-NE 53 transfer zone is developed, linking the LOFS and the Antiñir-Copahue Fault System (e.g. Sielfeld 54 et al, 2017).The geothermal area of interest lies to the north and northwest of the Copahue volcanic 56 edifice, with several surface emissions including fumaroles, bubbling pools, mud pools and thermal 57 springs, which are spatially associated with local extensional faults which act as fluid p...

The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin

“…Across the CVZ, air-corrected helium isotope ratios range from 0.1 R A to 2.6 R A , and are slightly lower (more radiogenic) than recent reports of He isotopes from fluids and gases within the region (which range from 0.1 R A to 5.5 R A ; Supplementary Table S1). Across the SVZ, air-corrected helium isotope ratios range from 0.7 R A to 5.0 R A , and are also generally lower (more radiogenic) than recent reports of He isotopes from the region (Agusto et al, 2013;Benavente et al, 2013;Roulleau et al, 2016;Tardani et al, 2016;Tassi et al, 2017;Lages et al, 2021;Tardani et al, 2021) which range from 0.5 R A to 8.4 R A (Supplementary Table S1).…”

“…Here, we model CO 2 / 3 He and δ 13 C data for a total of 172 gas samples. These are mostly published data (n = 158) (Hilton et al, 1993;Hoke et al, 1994;Varekamp et al, 2006;Sepúlveda et al, 2007;Aguilera, 2008;Ray et al, 2009;Tassi et al, 2009;Tassi et al, 2010;Capaccioni et al, 2011;Tassi et al, 2011;Aguilera et al, 2012;Agusto et al, 2013;Benavente et al, 2013;Chiodi et al, 2015;Aguilera et al, 2016;Benavente et al, 2016;Roulleau et al, 2016;Tardani et al, 2016;Tassi et al, 2016;Arnold et al, 2017;Tassi et al, 2017;Lopez et al, 2018;Chiodi et al, 2019;Arnold et al, 2020;Inostroza et al, 2020;Robidoux et al, 2020;Lages et al, 2021;Tardani et al, 2021;Filipovich et al, 2022) and samples from this study (n = 14). We focus on gas samples in order to understand sources, as gases are less susceptible to fractionation processes than water phase samples.…”

“…Helium (Figure 2) and carbon isotopic and relative abundance ratios (Figure 3) are reported in Table 1. For comparison, we also compile data from a number of previous studies (Hilton et al, 1993;Hoke et al, 1994;Varekamp et al, 2006;Sepúlveda et al, 2007;Aguilera, 2008;Ray et al, 2009;Tassi et al, 2009;Tassi et al, 2010;Tassi et al, 2011;Capaccioni et al, 2011;Aguilera et al, 2012;Agusto et al, 2013;Benavente et al, 2013;Chiodi et al, 2015;Aguilera et al, 2016;Benavente et al, 2016;Roulleau et al, 2016;Tardani et al, 2016;Tassi et al, 2016;Tassi et al, 2017;Arnold et al, 2017;Lopez et al, 2018;Chiodi et al, 2019;Arnold et al, 2020;Inostroza et al, 2020;Robidoux et al, 2020;Tardani et al, 2021;Lages et al, 2021;Filipovich et al, 2022; Supplementary Table S1). We also compute and report estimates of subduction and mantle derived inputs, after Sano and Marty (1995) (Table 2).…”

“…Several prior studies have characterized He-C isotope systematics in volcanic and geothermal systems in the Andes (Figure 1; e.g., Hilton et al, 1993;Hoke et al, 1994;Varekamp et al, 2006;Sepúlveda et al, 2007;Aguilera, 2008;Ray et al, 2009;Tassi et al, 2009;Tassi et al, 2010;Capaccioni et al, 2011;Tassi et al, 2011;Aguilera et al, 2012;Agusto et al, 2013;Benavente et al, 2013;Chiodi et al, 2015;Aguilera et al, 2016;Benavente et al, 2016;Roulleau et al, 2016;Tardani et al, 2016;Tassi et al, 2016;Arnold et al, 2017;Tassi et al, 2017;Lopez et al, 2018;Chiodi et al, 2019;Arnold et al, 2020;Inostroza et al, 2020;Robidoux et al, 2020;Lages et al, 2021;Tardani et al, 2021;Filipovich et al, 2022) to understand the source of the volcanic fluid and gas emissions. These studies demonstrate the cooccurrence of mantle ( 3 He/ 4 He up to ~8 R A ) and crustal ( 3 He/ 4 He ~0.05 R A ) He sources.…”

87Sr/86Sr Ratios and Atmospheric Noble Gases in Theistareykir Geothermal Fluids: A Record of Glacial Water

“…The calculated standard errors for the concentrations ranged from 1 to 3% of the measured values. See [43] for further details.…”