Volcanic eruptions impact global climate, geothermal energy is derived from volcanic heat, and many ore bodies are formed through volcanic processes. Active volcanism provides a direct means to study how our planet functions. Copahue volcano in Argentina (37.8 oS, 71 oW) started a new eruption on December 22, 2012 and is still active at low level today. This volcano emits compositionally extreme hydrothermal fluids that derive directly from a very acid and hot magmato-hydrothermal system at 1-2 km depth. Such fluids are thought to be responsible for epithermal ore deposits, eg. gold, silver and lead and may overly systems that form porphyry ore bodies. The composition of these fluids changes with the state of activity of the volcano: shallow intrusions lead to hotter and more concentrated fluids that are expelled into the ambient and acidify local rivers and lakes. Monitoring the composition of such streams during a period of volcanic activity aids in developing tools for eruption forecasting. Data collected on fluids during the 2000 eruption of Copahue suggested several parameters that indicate the intrusion of shallow magma prior to an eruption. The 2012-2013 eruptive period provides an ideal venue to test and refine these ideas. A large glacial lake that is stratified during the austral summer contains compositional information of the pre-eruptive fluids in the bottom waters, whereas the surface waters represent recent inputs. It is imperative to sample the lake prior to its seasonal overturn in late April, when the water column homogenizes and all detailed information is lost. A workshop with scientists from Argentina, Italy and the USA is convened for March 2013 to discuss the eruption, its hazards and monitoring efforts, and US scientists with an extensive research record on Copahue should participate and contribute to this event.

The research activities will contribute to the science of eruption monitoring and forecasting, which is beneficial for many people living on volcanoes worldwide. It also promotes collaboration between local Argentinian and US scientists. The local authorities plan to establish a small volcano observatory near Copahue, but data are needed to make the best possible choices of monitoring parameters. Besides the bilateral collaboration, two female US students (one undergraduate, one graduate) will collect samples on site and analyze those at Wesleyan University for their senior and MA theses. Training of US students in research on active volcanoes is important to keep the USA in a scientific leadership position with respect to assessments of both natural hazards and formation of natural resources.

Project Report

Copahue is a 3000m high volcano on the Argentina Chile border (37.5 oS) that had minor eruptions in the 1990s, a larger eruption in 2000 and an explosive eruption in 2012. The December 2012 eruption created a 1000-1500m high eruption plume that was blown 250 km south, depositing fine ash. The volcano has an active crater lake that is fed from an underlying geothermal system. Hot springs ‘leak’ water from this geothermal system acidifying a river (Rio Agrio) and large lake down stream (Lake Caviahue). The springs have a pH of 0-1.5 and Lake Caviahue has had pH values as low as 2.3 and up to 3 in the last decade. We documented changes in the composition of the hot springs prior to and after these eruptions, and designed monitoring techniques that help in volcanic surveillance by analyzing specific constituents in the river water. Prior to an eruption, the sulfur to chlorine ratio increases and the hot springs are very acid. Subsequently, common rock-forming elements such as Al, Si, Ca, Mg and Fe are released, indicating wholesale dissolution of new volcanic rock that intruded into the hydrothermal cell. Usually, an eruption follows and further reactions of the hot acid water with the fresh volcanic rock at depth lead to the precipitation of new minerals. These minerals are rich in K and Al (alunite), and plug up the pathways in the hydrothermal system. As a result, the fluid flow chokes and the release of all elements decreases, but especially those of Al and K. The composition of Lake Caviahue changes gradually in response to the variations in element inputs from the feeding river, and time series of chemical data from the lake are used to reconstruct the past inputs for each element. The hot spring fluids in March 2013 (time of our visit) were acid (pH=0.65) but the element fluxes were very small (the springs were a trickle). We interpret this as a result of the ‘clogging’ of the geothermal system with new minerals after the reaction of fresh magma and hot acid water. The December 23, 2013 eruption produced a vigorous eruption column that dispersed flat pumices south on the volcano, embedded in mm-sized ash, whereas the later ejecta were all dense cinders of 30-40 cm diameter. The composition of all the volcanic products was the same, suggesting that the magma chamber was zoned with respect to gases prior to the eruption. The top section consisted of a highly pressurized foam with vesicles of 0.2 mm diameter, underlain by a column of more degassed magma. The magma resided at a few km depth at temperatures of 1080-1190 oC. When ‘the lid blew’, the foam was chilled against the geothermal fluid, creating a quench rim on the pumices and a more inflated, swollen core in the center that formed during air travel. The time scale of magma emplacement and degassing/bubble transport for the 2012 eruption was studied through analyses of short-lived radio-isotopes in the Uranium decay chain. During residence of the magma in the earth crust, the radium-radon-210Pb series reaches ‘secular equilibrium’, that is, the Radium decay rate equals the Radon decay rate which again is in equilibrium with 210Pb. When degassing takes place deep in the earth, Radon (a noble gas) leaves the magma with the water-rich gas phase, creating over time a deficit in 210Pb because there is no parent nuclide left to create new 210Pb when all the Radon has escaped. The degree of 210Pb depletion is then a measure of the time elapsed since the Radon escaped. In some samples, the Radon daughter may become concentrated in the foam which then will show a 210Pb excess. These measurements on the Copahue cinders and pumices provide a time frame of degassing: for the 2012 eruption, degassing started in 2004, and the bubbles slowly traveled to the top of the magma reservoir (like a head of foam on a good glass of beer), increasing locally the pressure. When the pressure became too high, the eruption ensued releasing the foamy magma explosively, followed by mild explosions from the already more degassed deeper magma pod. Three-dimensional images of the quenched pumice rims are used to gather evidence on the sizes of the gas bubbles in the magma prior to eruption. Lake Caviahue and its outflow river had formed Fe-rich minerals between eruptions (2008-2010) which immobilized (adsorbed) arsenic that is released from the Copahue hot springs. During the re-acidification of the lake and its outflow, these minerals dissolved again, releasing their toxic load of arsenic downstream, which is an environmental hazard. Monitoring the Al/Mg and S/Cl in the river that drains the volcano will provide first order volcano surveillance.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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Wesleyan University
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