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

3A2_2G-O11

Room B2

Date/Time: July 23 11:45-12:00

The origin of fumarolic fluids from Tupungatito Volcano (Central Chile): new insightsfrom volatile compositions
Oscar Benavente1 , Franco Tassi2 , Francisco GutiÃl’rrez1 , Orlando Vaselli2 , Felipe Aguilera312

Centro de Excelencia en Geotermia de los Andes,Universidad de Chile, Plaza Ercilla, 803, Chile,
CNR-IGG, Istituto di Geoscienze e Georisorse Via G. La Pira, 4, Firenze, Italy, 3 Departamento deGeologia, Universidad de Atacama, Copayapu 485, Copiapo, Chile
E-mail: oscar.benavente.zolezzi@gmail.com

Tupungatito is an early Pleistocene − Holocene volcano located 100 km east of Santiago, in the northernmost
sector of the South Volcanic Zone (SVZ) in Central Chile. Permanent fumarolic activity occurs at the Tupungatito
summit (Gonz á lez − Ferr á n 1995) where three crater lakes are hosted. In this study we present and discuss thevery first chemical and isotopic (δ 13C − CO2, R/Ra, and 40Ar/36Ar) composition of fumarolic gases and steam (δ 18O and δ D) collected from this volcano during two sampling campaigns carried out in February 2011 and 2012.
The main aims are to investigate the different fluid source regions and the chemical-physical processes controlling
fluid chemistry.
The chemical and isotopic data indicate that the discharged gas species and steam are likely produced by mixing
processes of fluids between a magmatic source rich in acidic gases (SO2, HCl and HF) and a meteoric − recharged
hydrothermal reservoir. The magmatic − hydrothermal fluids at the surface are affected by steam condensationthat controls the outlet fumarolic temperatures (<84 °C) favoring the dissolution of high soluble magmatic gases.
The origin of CO2 seems to be mainly related to devolatilization of the subduction carbonates, with a limited
contribution from the overlying organic − rich sediments, suggesting that the mantle source is strongly contaminatedby the slab sediments. Gas geothermometry based on the kinetically rapid H2 − H2O − CO − CO2 system
indicates equilibrium temperatures >200 °C at redox conditions more oxidizing than those commonly characterizinghydrothermal reservoirs. Reactions involving CH4 and C2 − C3 alkenes/alkanes pairs, having a relatively slow
kinetics, seem to equilibrate at greater depth where temperatures are >200 °C and redox conditions consistent withthose inferred by the presence SO2 − H2S redox pair, typical of a magmatic fluid environment. A comprehensive
conceptual geochemical model describing the circulation pattern of the Tupungatito hydrothermal − magmatic
fluids is proposed to show fluid source regions and re − equilibration processes affecting the different gas species
occurring at changing chemical − physical conditions as the magmatic − hydrothermal fluids rise up toward thesurface.
By comparing the Tupungatito gas geochemical data with those discharging from other volcanic systems locatedin the northernmost sector of the Chilean Andes, it is possible to asses that i) different climate, ii) amount andcomposition of the subducted sediment, iii) slab thermal state, and iv) crustal thickness and lithology do notapparently play a significant influence on the gas chemistry.

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