Mantle-derived volatiles in continental crust: the Massif Central of France

CO₂-rich gases and groundwaters from springs and boreholes originating within the basement of the Massif Central have variable ³He ⁴He ratios with corresponding R R_a 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 mantle C 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 of ³He 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.