| Abstract: | CO2-rich gases and groundwaters from springs and boreholes originating within the basement of the Massif Central have variable3He/4He ratios with correspondingR/Ra values ranging from 0.8 to 5.5 and 0.3 to 2.8 respectively, indicating the presence of a significant component of mantle helium. Molar concentrations of rare gases in the CO2-rich gases are approximately 5 orders of magnitude greater than in the waters and suggest that a near-surface Henry's Law fractionation has occurred between exsolving CO2 and water.δ13C values of the CO2-rich gases are in the range −4.2 to −6.1‰, i.e. in that range normally attributed to mantle carbon, but which could also represent an average crustal composition and therefore do not discriminate between mantle and crustal sources.C/3He ratios show 4 orders of magnitude variation from 1.4 × 1012 to 5 × 108 and, compared to a mantleC/3He ratio of 109, indicate that either a complex fractionation has occurred between mantle helium and mantle CO2 or more likely that mantle rare gases have been diluted by large quantities of CO2 with an average crustal carbon isotope composition. The regional distribution of3He and C does not show any obvious relationship to age or proximity of volcanic centres or major faults, suggesting that mantle-derived C and He components decoupled from their silicate melt sources at some depth.The results from this area of active fluid circulation suggest that C-isotope data derived from metamorphic terrains should be interpreted with great caution, but that input of some mantle-derived carbon is expected to accompany crustal extension. |
