The origin of natural gas emissions from Sardinia island, Italy

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.

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