Preliminary geochemical characterization of volcanic and geothermal fluids discharged from the Ecuadorian volcanic arc.

Abstract

In Ecuador, magmatism results from the subduction of the Nazca Plate beneath the North Western part of South America (Pennington, 1981; Kellogg and Vega, 1995; Witt et al., 2006). North of 2.5°S, the Ecuadorian Quaternary volcanic arc is characterized by about 60 volcanoes distributed in three different parallel chains. Many of these volcanoes are potentially active or currently in activity and display associated geothermal fields. South of this latitude, no active arc is present in Ecuador. Few geochemical studies of the discharged fluids have been attempted, mainly related to geothermal exploration. Nevertheless, a complete study of the fluids of the volcanic arc is lacking. The aim of this work is to present the first systematic geochemical characterization of discharged fluids from the entire Ecuadorian volcanic arc. In order to achieve this objective, 48 samples of thermal and cold waters, as well as bubbling gases, have been collected from North to South across the arc and analyzed for different geochemical parameters. A chemical study reflects the physical and chemical processes undergone by these cold and thermal waters during their circulation through the different host rocks. The chemistry of the dissolved gases, as characterized by He and CO2 contents, which are – 2 to 3 orders of magnitude higher than the ASW values - implies very active gas-water interaction processes. Moreover, the isotopic signature of dissolved and bubbling gases shows a wide compositional range, with Helium isotopic compositions ranging between 0.34 to 7.12 R/Ra and carbon isotopes compositions ranging from -1.75 to -10.50 13C Vs PDB standard. This clearly indicates the presence of at least three distinct end-members: mantle, crustal and geothermal. Helium, Nitrogen, Carbon, Oxygen and Deuterium isotopic signatures will help us to identify and characterize the main end-members of fluids involved in Ecuador’s arc-volcanism. Moreover, on the basis of the chemistry of thermal waters and associated bubbling gases the characteristics and the potential resources of the different geothermal fields are being investigated.