Origin of fumarolic fluids from Tupungatito Volcano (Central Chile): interplay between magmatic, hydrothermal, and shallow meteoric sources

Abstract

Tupungatito is a poorly known volcano located about 100 km eastward of Santiago (Chile) in the northernmost sector of the South Volcanic Zone. This 5,682 m high volcano shows intense fumarolic activity. It hosts three crater lakes within the northwestern portion of the summit area. Chemical compositions of fumarolic gases and isotopic signatures of noble gases (3He/4He and 40Ar/36Ar are up to 6.09 Ra and 461, respectively), and steam (δ18O and δD) suggest that they are produced by mixing of fluids from a magmatic source rich in acidic gas compounds (SO2, HCl, and HF), and meteoric water. The magmatic–hydrothermal fluids are affected by steam condensation that controls the outlet fumarolic temperatures (<83.6 °C), the gas chemical composition, and the steam isotopic values. The δ13C–CO2 values (ranging from 0.30 and −8.16‰ vs. V-PDB) suggest that CO2 mainly derives from (1) a mantle source likely affected by significant contamination from the subducting slab, (2) the sedimentary basement, and (3) limited contribution from crustal sediments. Gas geothermometry based on the kinetically rapid H2–CO equilibria indicates equilibrium temperatures <200 °C attained in a single vapor phase at redox conditions slightly more oxidizing than those commonly characterizing hydrothermal reservoirs. Reactions in the H2O–CO2–H2–CO–CH4 system and C2–C3 alkenes/alkanes pairs, which have relatively slow kinetics, seem to equilibrate at greater depth, where temperatures are >200 °C and redox conditions are consistent with those inferred by the presence of the SO2–H2S redox pair, typical of fluids that have attained equilibrium in magmatic environment. A comprehensive conceptual geochemical model describing the circulation pattern of the Tupungatito hydrothermal–magmatic fluids is proposed. It includes fluid source regions and reequilibration processes affecting the different gas species due to changing chemical–physical conditions as the magmatic– hydrothermal fluids rise up toward the surface.