Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

The chemistry and isotope ratios of He, C (d13C) and H (dD) of free gases collected in the San Vittorino
plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing
in central Italy. The C isotopic composition of CO2 (d13C–CO2 2.0‰to 3.8‰) and He isotope ratios (R/RA
0.12–0.27) were used to calculate the fraction of CO2 originating from mantle degassing vs. sedimentary
sources. The results show that CO2 predominantly (average of 75%) derives from the thermo-metamorphic
reaction of limestone. Between 6% and 22% of the CO2 in the samples derives from organic-rich sedimentary
sources. The mantle source accounts for 0–6% of the total CO2; however, in two samples, located
in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence
of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in
the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but
also in the pre-Apennine and Apennine belts. Isotope ratios of CH4 (d13C–CH4 6.1‰ to 22.7‰; dD–CH4
9‰ to 129‰) show that CH4 does not appear to be related to mantle or magma degassing, but it is the
product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO2
(i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation
processes.

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