Hydrothermal element fluxes from Copahue, Argentina: A "beehive" volcano in turmoil

Copahue volcano erupted altered rock debris, siliceous dust, pyroclastic su lfur, and rare juvenile fragments between 1992 and 1995, and magmatic erupt ions occurred in July-October 2000. Prior to 2000, the Copahue crater lake, acid hot springs, and rivers carried acid brines with compositions that re flected close to congruent rock dissolution. The ratio between rock-forming elements and chloride in the central zone of the volcano-hydrothermal syst em has diminished over the past few years, reflecting increased water/rock ratios as a result of progressive rock dissolution. Magmatic activity in 20 00 provided fresh rocks for the acid fluids, resulting in higher ratios bet ween rock-forming elements and chloride in the fluids and enhanced Mg fluxe s. The higher Mg fluxes started several weeks prior to the eruption. Model data on the crater lake and river element flux determinations indicate that Copahue volcano was hollowed out at a rate of about 20 000-25 000 m(3)/yr, but that void space was filled with about equal amounts of silica and liqu id elemental sulfur. The extensive rock dissolution has weakened the intern al volcanic structure, making flank collapse a volcanic hazard at Copahue.