The 2012-2016 eruptive cycle at Copahue volcano (Argentina): hints from the temporal evolution of the chemical and isotopic features of hydrothermal-type gas discharges

This study presents the chemical and isotopic compositions of hydrothermal gases collected from fumaroles located in the surroundings of Copahue volcano (Argentina). In this volcano, a new eruptive cycle, characterized by occasional phreatic and phreatomagmatic eruptions, started on July 19, 2012 and it is still ongoing. Gas samples, including those from two fumaroles from the summit of the active crater, were collected during surveys carried out by different researcher teams from 1976 to February 2016. The time-series of H2, CO and light hydrocarbons showed episodic increases in correspondence of the main eruptive events, likely caused by enhanced inputs of hot magmatic fluids affecting the hydrothermal reservoir. In 2012-2014, these evidences were apparently in contrast with the temporal variations of the Rc/Ra and d13C-CO2 values, which indicated increasing inputs from a crustal fluid source. In 2015-2016, the isotopic parameters showed opposite trends, consistent with the gas chemistry, since they were approaching the compositional features of the two summit fumaroles, which were possibly corresponding to those of the deep magmatic-related end-member. The delayed and smoothed compositional changes of the peripheral hydrothermal fluid discharges in response to the 2012-2016 eruptive events suggest that geochemical surveys of these emissions can unlikely provide premonitory signals of volcanic unrest if the volcanic activity will keep centered in the main crater. At Copahue volcano, seismic activity and ground deformation seem to represent the most reliable approaches for monitoring purposes. However, the relative compositional stability of the hydrothermal reservoir is a great advantage in terms of geothermal resource exploitation, encouraging new investments in the Copahue geothermal project abandoned in the 1990’s.