Geochemistry of the magmatic-hydrothermal fluid reservoir of copahue volcano (Argentina): Insights from the chemical and isotopic features of fumarolic discharges

Abstract

In this chapter the chemical (inorganic and organic) and isotopic compositions (δ13C-CO2, δ15N, 3He/4He, 40Ar/36Ar, δ13C-CH4, δD-CH4, and δD-H2O and δ18O-H2O) of gas discharges, collected during 6 campaings carried out from 1976 to 2012, located at the foot of Copahue volcano are presented and discussed. Gas composition is typical of hydrothermal fluids from volcanic areas, since it consists of dominant CO2 and relatively high concentrations of H2S, H2, CH4 and N2. The helium isotopic ratios are the highest ones (R/Ra up to 7.94) observed in whole South America continent. This feature is not common for gases from a classic arc-like setting, and is possibly related to an extensional regime subdued to asthenospheric thinning. The CO2/3He ratios (from 1.4 to 8.8 × 109), slightly exceeding that of MORB gases, and the δ15N values (+1.7 to +5.5 ‰ vs. air) point to the occurrence of an additional crustal source for CO2 and N2. Gas discharges of the northern sector of the volcanic edifice are likely produced by mixing of hydrothermal gases with fluids from a shallow source permeating through the local fault systems. Gas geothermometry based on chemical reactions characterized by slow kinetics, such as those involving the CO [log(XH2/XH2O)=-2.8)] 2-CH4 redox pair, are quenched at temperatures of ~260 °C and redox conditions consistent with those measured in the geothermal wells. On the contrary, the C3H6-C3H8 pair, H2 and CO tend to re-adjust at decreasing temperatures and more oxidizing conditions [log(XH2/XH2O)≤-3.4] in the uprising vapor phase. The hydrothermal reservoir is mainly recharged by meteoric water whose isotopic signature is modified by water-rock interactions. The N2/He ratios measured in 2006–2007 were significantly lower than those of 2012, possibly due to variable input of N2-bearing species from sediments interacting with the magmatic source. Considering that the R/Ra values of the 2006–2007 period were significantly higher than those measured in 2012, such compositional variation may also be explained by the injection of fresh N2− and 3He-rich magma that triggered the 2000 eruption. This hypothesis, although speculative since no geochemical data of fumaroles are avalaible from 1997 to 2006, implies that a geochemical monitoring of inert gas compounds discharged from the hydrothermal emissions could be used to detect the occurrence at depth of injections of new magma batches.