Publication Date:
2024-05-09
Description:
Copahue volcano is part of the Caviahue–Copahue Volcanic Complex (CCVC),which is located in the southwestern
sector of the Caviahue volcano-tectonic depression (Argentina–Chile). This depression is a pull-apart basin accommodating
stresses between the southern Liquiñe–Ofqui strike slip and the northern Copahue–Antiñir compressive
fault systems, in a back-arc setting with respect to the Southern Andean Volcanic Zone. In this study, we present
chemical (inorganic and organic) and isotope compositions (δ13C-CO2, δ15N, 3He/4He, 40Ar/36Ar, δ13C-CH4,
δD-CH4, and δD-H2O and δ18O-H2O) of fumaroles and bubbling gases of thermal springs located at the foot of
Copahue volcano sampled in 2006, 2007 and 2012. Helium isotope ratios, the highest observed for a Southern
American volcano (R/Ra up to 7.94), indicate a non-classic arc-like setting, but rather an extensional regime
subdued to asthenospheric thinning. δ13C-CO2 values (from −8.8‰ to −6.8‰ vs. V-PDB), δ15N values
(+5.3‰ to +5.5‰ vs. Air) and CO2/3He ratios (from 1.4 to 8.8 × 109) suggest that the magmatic source
is significantly affected by contamination of subducted sediments. Gases discharged from the northern sector
of the CCVC show contribution of 3He-poor fluids likely permeating through local fault systems. Despite the
clear mantle isotope signature in the CCVC gases, the acidic gas species have suffered scrubbing processes by a
hydrothermal system mainly recharged by meteoric water. Gas geothermometry in the H2O-CO2-CH4-CO-H2
system suggests that CO and H2 re-equilibrate in a separated vapor phase at 200°–220 °C. On the contrary,
rock–fluid interactions controlling CO2, CH4 production from Sabatier reaction and C3H8 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 and N2/Ar ratios and high R/Ra values with respect to
those measured in 2012. Such compositional and isotope variations were likely related to injection of mafic
magma that likely triggered the 2000 eruption. Therefore, changes affecting the magmatic systemhad a delayed
effect on the chemistry of the CCVC gases due to the presence of the hydrothermal reservoir. However, geochemical
monitoring activities mainly focused on the behavior of inert gas compounds (N2 and He), should be increased to
investigate the mechanism at the origin of the unrest started in 2011.
Description:
Published
Description:
44–56
Description:
1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
Description:
2.4. TTC - Laboratori di geochimica dei fluidi
Description:
4.5. Studi sul degassamento naturale e sui gas petroliferi
Description:
JCR Journal
Description:
restricted
Keywords:
Fluid geochemistry
;
Copahue volcano
;
Fumarolic fluid
;
Hydrothermal reservoir
;
Volcanic unrest
;
03. Hydrosphere::03.02. Hydrology::03.02.04. Measurements and monitoring
;
04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry
;
04. Solid Earth::04.08. Volcanology::04.08.01. Gases
;
04. Solid Earth::04.08. Volcanology::04.08.07. Instruments and techniques
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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