The Copahue Volcanic-Hydrothermal System and Applications for Volcanic Surveillance

Abstract

During the last two decades, Copahue and its active acidic crater lake have shown different magmatic, phreatomagmatic, and hydrothermal manifestations. Geochemical data of waters from Copahue springs, lakes and rivers provide new insights into the behavior of its volcano-hydrothermal system. Temporal variations in the chemical (major anions and cations) compositions, element fluxes, and isotopic fluctuations (δD and δ18O; stable Pb isotopes, 129I/127I, δ37Cl, δ7Li, δ34S) of surface waters from Copahue for the period 1997–2012 can be related to processes in the underlying hydrothermal system. The 2000 and 2012 eruptive periods and the 2004 thermal anomaly showed increases in SO4, Cl and F contents in the waters as a result of ascending magmatic fluids. The changing chemical compositions also showed evidence for enhanced water/rock interaction during these magmatic periods, with increased concentrations of the major rock forming elements such as Al, K and Mg. Soon after the magmatic events, the elemental fluxes from the hydrothermal system decreased strongly, especially for K and Al. The latter is related to a decrease in permeability in the system through crystallization of alunite. Stable isotopic data (δD and δ18O) indicate that the hydrothermal fluids consist of a mixture of meteoric and magmatic waters that are affected by evaporation and water/rock interaction processes. The δ7Li values, polythionate concentrations and water stable isotope ratios indicate that the hot spring on the flank of the crater does not carry simple seepage from the crater lake. The crater lake and hot spring are fed by different sections from the underlying hydrothermal system. Temperatures of ~260 °C in the deeper part of the volcano-hydrothermal reservoir were estimated based on δ34S values in sulfur and dissolved sulfate. The upper part of the system is possibly more dilute and cooler at ~175 °C, based on silica geothermometry. The presence of 129I in the hydrothermal fluids indicates that subducted sediment with organic matter is one source for the magmatic volatiles. Stable Pb isotope ratios showed that the acidic hydrothermal fluids are dissolving local host rocks from the Copahue volcano edifice. The pyroclastic material ejected during the eruptive events carried the hydrothermal alteration phases silica, jarosite, alunite, and anhydrite/gypsum; liquid and particulate native sulfur was also emitted in large quantities. The chemical composition of the Upper Rio Agrio is controlled by the emissions of the feeding volcanic hot springs and mixing with glacial melt water. The compositional evolution of the large Lake Caviahue reflects inputs from the Upper Rio Agrio and melt water dilution, as well as mineral precipitation from the lake waters. The Lower Rio Agrio had copious amounts of Fe-sulfates in the river bed since 2003, which started to disappear again during the period of the 2012 eruption. Monitoring of SO4/Cl, Mg/Cl and Al/Cl in the Upper Rio Agrio waters may constitute an efficient mode of volcanic surveillance, complementing the ongoing multidisciplinary volcano monitoring and forecasting at Copahue.