Geochemistry of the Magmatic-Hydrothermal Fluid Reservoir of Copahue Volcano (Argentina): Insights from the Chemical and Isotopic Features of Fumarolic Discharges

In this chapter the chemical (inorganic and organic) and isotopiccompositions (?13C-CO2, ?15N, 3He/4He, 40Ar/36Ar, ?13C-CH4, ?D-CH4,and ?D-H2O and ?18O-H2O) of gas discharges, collected during 6campaings carried out from 1976 to 2012, located at the foot of Copahuevolcano are presented and discussed. Gas composition is typical ofhydrothermal fluids from volcanic areas, since it consists of dominant CO2and relatively high concentrations of H2S, H2, CH4 and N2. The heliumisotopic ratios are the highest ones (R/Ra up to 7.94) observed in wholeSouth America continent. This feature is not common for gases from aclassic arc-like setting, and is possibly related to an extensional regimesubdued to asthenospheric thinning. The CO2/3He ratios (from 1.4 to8.8 × 109), slightly exceeding that of MORB gases, and the ?15N values(+1.7 to +5.5 ? vs. air) point to the occurrence of an additional crustalsource for CO2 and N2. Gas discharges of the northern sector of thevolcanic edifice are likely produced by mixing of hydrothermal gases withfluids from a shallow source permeating through the local fault systems.Gas geothermometry based on chemical reactions characterized by slowkinetics, such as those involving the CO½log XH2=XH2O ð Þ¼ 2:8Þ2-CH4redox pair, are quenched at temperatures of *260 °C and redoxconditions consistent with those measured in the geothermal wells. On thecontrary, the C3H6-C3H8 pair, H2 and CO tend to re-adjust at decreasingtemperatures and more oxidizing conditions ½log XH2=XH2O ð Þ 3:4 inthe uprising vapor phase. The hydrothermal reservoir is mainly rechargedby meteoric water whose isotopic signature is modified by water-rockinteractions. The N2/He ratios measured in 2006-2007 were significantlylower than those of 2012, possibly due to variable input of N2-bearingspecies from sediments interacting with the magmatic source. Consideringthat the R/Ra values of the 2006-2007 period were significantly higherthan those measured in 2012, such compositional variation may also beexplained by the injection of fresh N2- and 3He-rich magma that triggeredthe 2000 eruption. This hypothesis, although speculative since nogeochemical data of fumaroles are avalaible from 1997 to 2006, impliesthat a geochemical monitoring of inert gas compounds discharged fromthe hydrothermal emissions could be used to detect the occurrence atdepth of injections of new magma batches.