Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (central Italy)

We are presenting here both chemistry and isotope ratios of helium (He), carbon (13C) and hydrogen (D) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, in order to provide new constraints to better understand mantle degassing in central Italy. Carbon isotopic composition of the carbon dioxide (δ13C-CO2 -2.0‰ to -3.8‰) and He isotope ratios (R/RA 0.12 to 0.27) are used to calculate the fraction of CO2 originating from mantle degassing versus sedimentary sources. Results show that CO2 predominantly (average of 80%) derives from thermo-metamorphic reaction of limestone. The remaining 20% are attributed to a mantle source. The presence of tectonic dislocations allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, but also in the more eastern part of central Italy. Isotope ratios of methane (13C-CH4 -6.1‰ to -22.7‰; D-CH4 -9‰ to -129‰) shows that the gas vent releases gas of thermogenic origin, involving CO2 and/or organic matter. Methane seems not to be related to mantle or magma degassing, but seems to be affected by microbial oxidation process