Heat source and reservoir of geothermal areas in Latin America as inferred from thermobarometry of amphibole-bearing extrusives and fluid geochemistry: examples from Chile and Ecuador

Geothermal energy is a renewable and sustainable energy because it comes from the huge heat stored within the earth that can be considered inexhaustible if compared to human timescale. It has many interesting quality resulting from the possibility to be exploited both for direct use (thermal energy) and for power production (electric use) as well as being available continuously without daily or seasonal variations compared to other renewables (e.g. solar or wind power). Furthermore, it is considered a base-load energy since it provides a stable and reliable power around the clock at a relatively low cost and with few operational or technological risks. Countries of South America, especially those from the western regions (Pacific Ocean side), are located along an active convergent plate margin characterized by Quaternary volcanism which ensures the presence of diffuse heat flow anomalies, and creates the ideal geological conditions for one of the major exploitation potential of geothermal energy of the entire Earth Planet. Unfortunately, to date, this huge potential of geothermal energy has not yet been exploited. This PhD thesis will try to investigate (i) the heat sources of some geothermal areas located in Chile (Altiplano Puna Volcanic Complex, APVC) and Norther Ecuador and (ii) the origin of discharged thermal waters and gases of geothermal sites linked to some Ecuadorian active to semi-dormant volcanoes (between 1°S and 1°N of latitude). The capability of amphibole thermobarometry on the youngest extrusives of the geothermal areas to characterize the heat sources of the Chachimbiro (CGA, Northern Ecuador), Apacheta (AGA) and La Torta (TGA), Geothermal Areas, these latter two located in the APVC of Chile, has been successfully tested. We used the application of Ridolfi and Renzulli (2012) allowing to estimate the intrinsic physico-chemical parameters with reasonably low uncertainties (T±24 °C, P±12%, fO2±0.4 log units) and the composition of the melt in equilibrium with Mg-rich calcic amphibole in a wide range of conditions, up to 1,130°C and 2.2 GPa. Since this thermobarometric model was obtained at “perfect” equilibrium conditions, we suggest a method to avoid as much as possible the application of amphibole thermobarometry to compositions resulting from disequilibrium crystallization that could lead to misleading interpretation of P-T estimates. Furthermore we tested the reliability of the barometric model to high pressures comparing our results to a model of seismic tomography for the AGA system (Ward et al., 2014). In the section concerning the fluid geochemistry, the PhD thesis deals with a description of the geology, thermal waters and gases of the geothermal areas located between the frontal and main arcs (including the Inter-Andean Depression) of Northern Ecuador and directly associated to the Late-Pliocene and the ongoing volcanic activity (mainly active to semi-dormant volcanoes). The studied areas are related to: Chiles-Cerro Negro and Chachimbiro volcanic complexes, Cuicocha and Chacana calderas and Quilotoa volcano. Thermal water samples and bubbling gases were collected and analysed for chemical and isotopic (δD-H2O and 18δ- H2O; δ13C-CO2 and 3He/4He) compositions. Gas and thermal water solute geothermometers have been applied to infer the temperature at the fluid reservoir conditions. The surface manifestation consist of thermal springs at low to medium temperature (up to 70°C) and the chemico-physical characteristics of the investigated geothermal areas display a high degree of heterogeneity, from low enthalpy (Vertiente Rio Lisco, Oyacachi and El Pisque) to medium-high enthalpy systems [Chachimbiro (calculated T up to 170°) and Chiles-Cerro Negro (calculated T up to 190°)]. According to helium isotopes, a prevailing crustal component of the gas seem to characterize the geothermal fluids of the northernmost investigated volcanic complexes (Chiles-Cerro Negro and Chachimbiro) whereas mantle contribution is higher in the Cuicocha and Chacana calderas and Quilotoa volcano, probably associated to slab-tears of the flat-slab geometry due to the Carnegie Ridge subduction, making easier the influx to the surface of mantle-derived fluids.