Abstract
Based on characteristics of the distribution pattern of the western Eger Rift spring gases, a distribution pattern is presented for the gases of the French Massif Central. The central parts of these areas with ascending magmatic CO2 are characterised by high gas fluxes, high CO2 contents of up to 99.99 vol% and isotopially heavy CO2. In the peripheries, the decrease of δ 13 C values of CO2 and CO2 contents in the gas phase is compensated by a rise in N2 contents. It can be demonstrated that gas fractionation in contrary to mixtures with isotopically light biogenic or crustal CO2 controls the distribution pattern of gas composition and isotopic composition of CO2 in these spring gases. Dissolution of CO2 results in formation of HCO3 ‐ causing isotope fractionation of CO2 and an enrichment of N2 in the gas phase. With multiple equilibrations, values of about ‐17 ‰ or lower are obtained. The scale of gas alteration depends on the gas flux and the gas-water ratios respectively and can result in N2-rich gases. Essential for the interpretation are gas flux measurements with mass balances derived for most of the springs. Without such mass balances it is not possible to discriminate between mixture and fractionation. The processes of isotopic and chemical solubility fractionations evidently control the gas distribution pattern in other regions as well.
Highlights
In areas with both low CO2 abundance and contents in the gas phase as for example in complexes of crystalline basement or in areas with younger volcanic activity, one frequently encounters CO2 with isotopic compositions of δ13C values lesser than –10 ‰
This results in a comprehensive gas fractionation concerning both gas composition and isotopic composition of CO2 and can yield N2-rich gases
A special case of these fractionations are springs with very low Ca-Mg-HCO3 contents and with isotopic composition of CO2 being still nearly unchanged due to lack of extensive HCO–3 fractionation and where the CO2 contents are solely decreased by the solution of CO2
Summary
Due to the high solubility of CO2 in water and the HCO3 formation, fractionations of the CO2-rich gases take place (e.g., Batard et al, 1982; Capasso et al, 1997; D’Alesandro et al, 1997; Chiodini et al, 1999; Weinlich et al, 1999). It can be assumed that gases migrate in different water systems, for example along various faults where dissolution of CO2 and fractionation with concomitantly formed HCO–3 can take place several times. This results in a comprehensive gas fractionation concerning both gas composition and isotopic composition of CO2 and can yield N2-rich gases
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