IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_2G-O10

Room B2Date/Time: July 23 11:30-11:45

Gas geochemistry of the Copahue-Caviahue Volcanic Complex (Argentina).
Mariano Agusto1 , Franco Tassi2 , Alberto Caselli1 , Orlando Vaselli3 , Dmitri Rouwet4 , Bruno Capaccioni5 ,S Caliro6 , Giovanni Chiodini6 , T Darrah7
1

GESVA-IDEAN, Dpto. Cs. Geologicas, FCEN, Universidad de Buenos Aires. Buenos Aires, Argentina,
2Department of Earth Sciences, University of Florence, Florence, Italy, 3 Institute of Geosciences andEarth Resources of the National Research Council (CNR-IGG), Florence, Italy, 4 Istituto NazionaleGeofisica e Vulcanologia, Sezione di Bologna, Bologna, Italy, 5 Department of Earth andGeological-Environmental Sciences, Bologna, Italy, 6 Istituto Nazionale di Geofisica e Vulcanologia,Osservatorio Vesuviano, Naples, Italy, 7 Division of Earth and Ocean Sciences, Nicholas School of theEnvironment, Durham, USA
E-mail: magusto@gl.fcen.uba.arCopahue volcano is part of the Caviahue-Copahue Volcanic Complex (CCVC), which is located in the Caviahue
volcano-tectonic caldera (Argentina-Chile), a pull-apart basin between a southern strike slip and a northern
compressive fault system, in the Southern Andean Volcanic Zone.

During the last 250 years, at least 12

low-magnitude phreatic and phreatomagmatic eruptions occurred. In 2000 a phreatomagmatic event (VEI 1-2)
caused ash fall that affected the nearby villages. Since November-December 2011 the discharge rate of fluids
from the Copahue summit has significantly increased up to December 22nd 2012, when a new phreatomagmatic
eruption occurred. In this study, the chemical and isotopic (δ13C of CO2 and CH4 , δD of CH4 and H2 O, δ18O ofH2 O, δ15N, 3He/4He and 40Ar/36Ar) compositions of fumaroles and bubbling gases collected in 2006, 2007 and2012, i.e. prior and during the last volcanic crisis, from five thermal areas surrounding the Copahue volcanic edifice
are presented and discussed. The main aims are to i) investigate the origin of fluids and their relationship with thetectonic setting, and ii) describe the evolution in fumarolic fluids chemistry and relate it to the changing volcanicactivity.
The He isotopic ratios, the highest observed for a Southern American volcano (R/Ra up to 7.94), and CO2 /3Heratios (from 1.4 to 8.8*109 ) suggest a non-classic arc-like setting, but rather an extensional regime due toasthenospheric thinning. δ15N values (+5.3 to +5.5 ‰vs ATM) point to a source of N2 mainly from subducted
sediments. Although these gases show a clear mantle isotopic signature, magmatic gases, such as SO2 , HCl andHF are not detected, suggesting gas scrubbing by the hydrothermal reservoir that feed CCVC. Gas geothermometry
in the H2 O-CO2 -CH4 -CO-H2 system suggests that CO and H2 re-equilibrate in a vapor phase at 200-220°C. On thecontrary, rock-fluid interactions controlling CO2 , CH4 production from Sabatier reaction and C3 H8 dehydrogenation
seem to occur within the hydrothermal reservoir at temperatures ranging from 250 to 300°C. Fumarole gases
sampled in 2006-2007 show relatively low N2 /He ratios and high R/Ra values with respect to those measured in2012 and prior to the 2000 eruption. Such compositional and isotopic variations were likely caused by an injectionof fresh He-rich magma that likely triggered the 2000 eruption. Presently it is difficult to assess the effects caused
to the hydrothermal system by the current phreatomagmatic activity. A high-frequency geochemical monitoring in
the thermal areas located at the foothills of Copahue is necessary to identify possible magmatic signals.

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