ABSTRACT

The geochemical study carried out on seven natural gas emissions from Sardinia island (western-central Italy) has allowed to distinguish two different groups: i) CO₂-high and He-low gases in the Logudoro area (northern Sardinia) associated with low temperature Na-HCO₃ type waters and ii) N₂ and He-rich gases bordering the western limit of Paleozoic basement crystalline rocks associated with long-term Na-Cl and Na(Ca)-Cl thermal water circulation therein. The emission of CO₂ is prevalently related to outcrop areas of recent Quaternary extensional basaltic volcanism. The different origin of the two types of gas is even more evident when considering the helium (as R/Ra) and argon isotopic ratios for the Logudoro and the remaining areas, being 3.0-3.5 Ra and >320 and <0.4 Ra and 295, respectively. Both CO₂ and such isotopic ratios suggest: i) a deep source for the Logudoro samples where a contribution of 40-50% mantle gas can be assessed; ii) a prevalent atmospheric origin for the N₂-rich gases emerging in association with meteoric-derived thermal waters circulating in aquifers reaching 2, 000 m in depth and 80°-110°C maximum temperature inside the Paleozoic formations. Although both the N₂/Ar and ⁴⁰Ar/³⁶Ar ratios in the N₂-rich gases point, roughly, to an atmospheric origin, their relative Ne concentration, being this element almost exclusively of atmospheric origin, suggests that contributions of N₂ from other sources than air can be envisaged.