The 2012–2016 eruptive cycle at Copahue volcano (Argentina) versus the peripheral gas manifestations: hints from the chemical and isotopic features of fumarolic fluids

This study presents the chemical and isotopic compositions
of hydrothermal gases from fumaroles discharging
around Copahue volcano (Argentina). Gas samples, including
those from two fumaroles at the active summit crater, were
collected during 13 surveys carried out by different research
teams from 1976 to February 2016. The time-series of H2,
CO and light hydrocarbons showed episodic increases related
to the main events of the last eruptive cycle that started on 19
July 2012. Concentration peaks were likely caused by enhanced
input of hot magmatic fluids affecting the hydrothermal
reservoir. These data contrast with the temporal variations
shown by Rc/Ra and δ13C-CO2 values in 2012–2014, which
indicated an increasing input from a crustal fluid source. In
2015–2016, however, these isotopic parameters showed opposite
trends; their composition became closer to that of the two
summit fumaroles, which possibly corresponds to that of the
deep magmatic-related end-member. The delayed and reduced
compositional changes in the peripheral hydrothermal fluid discharge in response to the 2012–2016 eruptive events suggest
that geochemical surveys of these emissions are unlikely to
provide premonitory signals of volcanic unrest if the volcanic
activity remains centered in the main crater. Instead, an instrument
which is able to provide measurements of volcanic gases
in the air (e.g.MultiGAS) may be used to detect changes at the
summit crater. Otherwise, monitoring of seismic activity and
ground deformation, as well as the periodic measurement of the
chemistry of the water in the Rio Agrio, which is fed by thermal
discharge from the summit crater, seem to represent the most
reliable means of monitoring at Copahue. However, the relative
compositional stability of the hydrothermal reservoir is a great
advantage in terms of geothermal resource exploitation and
could encourage new investments in the Copahue geothermal
project which was abandoned in the 1990s.

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