A new contribution to the isotopic and geochemical characterization of the gas phase ascribed to the CO2-rich mineral waters (N. Portugal)

In Portuguese mainland the most important sodium bicarbonate CO2-rich thermomineral waters are found in the Northern part of the country, linked to one major NNE-trending faults, the so-called Verin-Chaves-Regua-Penacova megalineament. Along this tectonic structure different occurrences of CO2-rich thermomineral waters are found: Chaves hot waters (78 deg C) and several cold (17 deg C) CO2- rich waters. The gas phase composition and the isotopic ratios of 3He/4He and 13C/12C point out that 10 to 30% of the gases have mantelic or magmatic source (typical MORB fluids). This primitive chemical signature decreases from S to N, representing an increase of crustal contamination in that direction. Geochemical modeling applied to the hot and cold waters composition shows that water-rock interaction reactions are mainly controlled by the amount of dissolved CO2(g) instead of the water temperature. (author)