THE WESLEYAN COPAHUE VOLCANOLOGY PROJECT: HOW AN EXPLORATORY TRIP TO THE ANDES GREW INTO A 16-YEAR RESEARCH PROJECT THAT UNCOVERED THE HEARTBEAT OF A BEEHIVE VOLCANO

In the late 1990s a chance encounter with two Argentine geologists at a conference led to Joop, Jelle, and two undergraduate students heading to the Andes to investigate a little-known volcanic crater lake bubbling with liquid sulfur. This trip proved to be the beginning of a 16-year research project that involved over a dozen undergraduate and Master’s students, spawned multiple geologic careers, and uncovered the heartbeat of a ‘beehive’ volcano.

Copahue volcano, located in the Argentine Andes on the border with Chile, has a fascinating hydrothermal system with a highly acidic (pH<1) crater lake. Associated hot springs feed into an acid river system (pH ~1.5) flowing into an acidified glacial lake (pH 2.5), which offered the Wesleyan team a unique way to assess volcanic activity over the years. Regular geochemical monitoring of the hot springs, river, and glacial lake uncovered a pattern of element fluxes that appear to wax and wane with magma intrusion and/or eruption over two eruptive periods and one non-eruptive magmatic intrusion (failed eruption?). The flux pattern shows pre-eruption enrichment of rock-forming elements (Al, K, Mg) in the fluids followed by a post-eruptive attenuation of all element fluxes.

Prior to eruption, the deep system acidifies and heats up, followed by shallow magma intrusion and/or eruption, and reaction of the newly emplaced magmatic rock with the hot acid fluids. This leads to enhanced export rates of all elements, quickly followed by the precipitation of new minerals in the hydrothermal reservoir. Saturation modelling, as well as the presence of alunite in ashes from the 2012 eruption, support the notion that post-eruption alunite saturation “clogs up” the system, reducing overall permeability and resulting in the observed strongly reduced element fluxes (particularly K and Al). This repeating cycle of acidification / rock dissolution / mineral precipitation, followed again by mineral dissolution with restoration of permeability is the ‘heartbeat’ of the Copahue magmato-hydrothermal system, driven by the magma pulses from below.