4.5 Article

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

Journal

APPLIED GEOCHEMISTRY
Volume 34, Issue -, Pages 90-101

Publisher

PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.apgeochem.2013.02.015

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The chemistry and isotope ratios of He, C (delta C-13) and H (delta D) 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 (delta C-13-CO2-2.0 parts per thousand to -3.8 parts per thousand) and He isotope ratios (R/R-A 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 (delta C-13-CH4 -6.1 parts per thousand to -22.7 parts per thousand; delta D-CH4 -9 parts per thousand to -129 parts per thousand) 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. (c) 2013 Elsevier Ltd. All rights reserved.

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