CO2 degassing at La Solfatara volcano (Phlegrean Fields): Processes affecting d13C and d18O of soil CO2

Abstract The soil CO2 degassing is a ected by processes of isotope exchange and fractionation during transport across the soil, which can deeply modify the pristine isotope composition. This has been observed in 10 the Solfatara volcano, upon a eld survey of 110 points, where the CO2 ux was measured, together with temperature, CO2 concentra- tion and oxygen and carbon isotopes within the soil. Furthermore, in some selected sites, the measurements were made at di erent depths, in order to analyze vertical gradients. Oxygen isotope composition 15 appears controlled by exchange with soil water (either meteoric or fumarolic condensate), due to the fast kinetic of the isotopic equilib- rium between CO2 and water. Carbon isotope composition is reliably controlled by transport-driven fractionation, due to the di erences in di usion coe cients between 13C16O2 and 12C16O2. We model the 20 processes a ecting CO2 transport across the soil in La Solfatara vol- cano by means of the Dusty Gas Model applied to a multicomponent system, to evaluate the reciprocal e ect on di usion of involved gases, i.e. 12C16O2, 13C16O2, N2 and O2 in our case. Both numerical and simpli ed analytical solutions of the equations based on the Dusty 25 Gas Model are given. The modeling results t well with the experimental data and put in evidence an isotope fractionation of carbon up to about +4:4h with respect to the source value in the soil gas. This fractionation is independent from the entity of the CO2 ux, and occurs as long as a concentration gradient exists within the soil. On 30 these grounds, the Dusty Gas Model can be applied to whichever dif- fusing gas mixture to evaluate the extent of chemical and/or isotopic fractionation that can a ect ascending gases upon di usion in any geothermal, volcanic or tectonic area.