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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 d13C 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.
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