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

LAMBERTI M C; CAPONI C; AGUSTO M; VASELLI O; PECORAINO G; VENTURI S; TASSI F; CASELLI A; SZENTIVÁNYI J

BULLETIN OF VOLCANOLOGY

SPRINGER

Lugar: Berlin; Año: 2017 p. 69 - 79

0258-8900

This study presentsthe chemical and isotopic compositionsof hydrothermalgases from fumaroles dischargingaround Copahuevolcano (Argentina). Gas samples, includingthose from twofumaroles at the active summit crater, werecollected during 13surveys carried out by different researchteams from 1976 toFebruary 2016. The time-series of H2,CO and lighthydrocarbons showed episodic increases relatedto the main eventsof the last eruptive cycle that started on 19July 2012.Concentration peaks were likely caused by enhancedinput of hotmagmatic fluids affecting the hydrothermalreservoir. Thesedata contrast with the temporal variationsshown by Rc/Ra andδ13C-CO2valuesin 2012?2014, whichindicated an increasinginput from a crustal fluid source. In2015?2016, however, theseisotopic parameters showed oppositetrends; theircomposition became closer to that of the twosummit fumaroles,which possibly corresponds to that of thedeepmagmatic-related end-member. The delayed and reducedcompositionalchanges in the peripheral hydrothermal fluiddischarge inresponse to the 2012?2016eruptive events suggestthat geochemicalsurveys of these emissions are unlikely toprovide premonitorysignals of volcanic unrest if the volcanicactivity remainscentered in the main crater. Instead, an instrumentwhich is able toprovide measurements of volcanic gasesin the air(e.g.MultiGAS) may be used to detect changes at thesummit crater.Otherwise, monitoring of seismic activity andground deformation,as well as the periodic measurement of thechemistry of thewater in the Rio Agrio, which is fed by thermaldischarge from thesummit crater, seem to represent the mostreliable means ofmonitoring at Copahue. However, the relativecompositionalstability of the hydrothermal reservoir is a greatadvantage in termsof geothermal resource exploitation andcould encourage new investments in theCopahue.