Geochemical and isotopic evidences of magmatic inputs in the hydrothermal reservoir feeding the fumarolic discharges of Tacora volcano (northern Chile)

Tacora volcano is a nearly unknown and understudied 5980 m high volcano located on the northernmost border between Chile and Peru. Tacora is characterized by intense fumarolic activity with extensive, white-colored, hydrothermal areas along the NW and W flanks of the volcanic structure. The chemical and isotopic features of gas emissions indicate that the hydrothermal reservoir is significantly affected by contributions from magmatic degassing, despite relatively low outlet temperatures (82-93 degrees C). Water and gas isotopic signatures stem from degassing of andesitic magmas produced from the sediment-poor subducting slab. Isotopic data suggests CH4-CO2 isotopic equilibration occurs at similar to 400 degrees C, likely in the deepest portion of the hydrothermal-magmatic fluid reservoir. The H-2-H2O, CO-CO2, H-2-Ar, CH4-CO2 and C-2-C-3 alkenes/alkanes pairs tend to approach a chemical equilibrium at temperatures between 200 and 320 degrees C when redox conditions are more oxidizing than those determined by the typical FeO/FeO1.5 rock buffer system. Boiling occurs at shallow depth at 85 degrees C within a discontinuous, thin aquifer, enough as thermal buffer but not able to scrub the uprising magmatic-related acidic gases. (C) 2011 Elsevier B.V. All rights reserved.