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

3W_2G-P8

Date/Time: July 23 Poster

Gas geochemistry of geothermal fluids from the Copahue Caviahue Volcanic Complex(Argentina)
Mariano Agusto1 , Franco Tassi2 , Alberto Caselli1 , Orlando Vaselli3 , Dmitri Rouwet4 , Bruno Capaccioni5 ,Stefano Caliro6 , Giovanni Chiodini6 , Tom Darrah7
1

GESVA IDEAN, Dpto. Cs. GeolÃşgicas, FCEN, Universidad de Buenos, Argentina, 2 Department ofEarth Sciences, University of Florence, Italy, 3 Institute of Geosciences and Earth Resources of theNational Research Council (CNR IGG), Italy, 4 Istituto Nazionale Geofisica e Vulcanologia, Sezione diBologna„ Italy, 5 Department of Earth and Geological Environmental Sciences, Bologna, Italy, 6 Istituto
Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Naples, Italy, 7 Division of Earth andOcean Sciences, Duke University, Durham, USA
E-mail: franco.tassi@unifi.it

Copahue volcano is part of the Caviahue Copahue Volcanic Complex (CCVC) which is located inthe southwestern sector of the Caviahue volcano tectonic depression. This depression is a basin accommodatingstresses between the southern Liquine Ofqui strike slip and northern Copahue Antinir compressive fault systems ina back arc setting with respect to the Southern Andean Volcanic Zone.
During the last 250 years at least 12 phreatic and phreatomagmatic eruptions occurred. In 2000 a phreatomagmatic
event caused ash falling that affected the nearby villages.
Since November 2011 the discharge rate of fluids from the Copahue summit has significantly increased up to December
2012 when a new phreatomagmatic eruption occurred. In this study the chemical and isotopic compositions offumaroles and bubbling gases collected in 2006 and 2007 and 2012 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 fluid source regions and their relationship with the peculiar tectonic assessmentof this volcano and ii) describe the chemical evolution of the fumarolic fluids caused by the ongoing activity.
The He isotopic ratios are the highest observed for a Southern American volcano (R/Ra up to 7.94). Nitrogen isotopevalues (+5.3 to +5.5 ‰ vs. ATM) point to a source of nitrogen mainly from subducted sediments.
Despite these gases show a clear mantle isotopic signature magmatic gases are below the instrumental detection
limit being likely affected by scrubbing processes within a hydrothermal reservoir constituting the main source
feeding the CCVC gas discharges.
Gas geothermometry suggests that CO and hydrogen equilibrate in a vapor phase at 220 celsius degrees.
Rock fluid interactions controlling carbon dioxide and methane production from Sabatier reaction and propane
dehydrogenation seem to occur within the hydrothermal reservoir at temperatures ranging from 250 to 300 celsius degrees.
Fumarole gases sampled in 2006 and 2007 show relatively low nitrogen/He ratios and high R/Ra values withrespect to those measured in 2012 and prior to the 2000 phreatomagmatic eruption. Such compositional and isotopic
variations were likely caused by an injection of fresh 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.
This can likely be evaluated by intensifing the geochemical monitoring in the thermal areas located at thefoothills of Copahue to verify the presence of possible magmatic signals.

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