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

Abstract

The soil CO2 degassing is affected 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 the Solfatara volcano, upon a field survey of 110 points, where the CO2 flux was measured, together with temperature, CO2 concentration and oxygen and carbon isotopes within the soil. Furthermore, in some selected sites, the measurements were made at different depths, in order to analyze vertical gradients. Oxygen isotope composition appears controlled by exchange with soil water (either meteoric or fumarolic condensate), due to the fast kinetic of the isotopic equilibrium between CO2 and water. Carbon isotope composition is reliably controlled by transport-driven fractionation, due to the differences in diffusion coefficients between 13C16O2 and 12C16O2. We model the processes affecting CO2 transport across the soil in La Solfatara volcano by means of the Dusty Gas Model applied to a multicomponent system, to evaluate the reciprocal effect on diffusion of involved gases, i.e. 12C16O2, 13C16O2, N2 and O2 in our case. Both numerical and simplified analytical solutions of the equations based on the Dusty Gas Model are given. The modeling results fit well with the experimental data and put in evidence an isotope fractionation of carbon up to about þ4:4& with respect to the source value in the soil gas. This fractionation is independent from the entity of the CO2 flux, and occurs as long as a concentration gradient exists within the soil. On these grounds, the Dusty Gas Model can be applied to whichever diffusing gas mixture to evaluate the extent of chemical and/or isotopic fractionation that can affect ascending gases upon diffusion in any geothermal, volcanic or tectonic area.