Fracturing-induced release of radiogenic 4 He and 234 U into groundwater during the last deglaciation: An alternative source to crustal helium fluxes in periglacial aquifers

Résumé 0

External 4He sources have been invoked to explain 4He concentrations in groundwater greater than those expected from in situ U and Th production. In a fractured aquifer of Ordovician age located in the St. Lawrence Lowlands (Quebec, Canada), 4He concentrations of up to 4.48 × 10−5 cm3 STP math formula were measured. Such concentrations are ∼1000 times higher than would be expected from in situ production. A concomitant increase in 4He concentration and 234U/238U activity ratio is shown, suggesting a common release process in groundwater for 234U and 4He. This process has tentatively been identified as glaciation-induced rock fracturing following the Laurentide Ice Sheet retreat. The resulting increase in exposed grain surface facilitates 234U release by α-recoil and that of radiogenic 4He by diffusion. Using a model of helium diffusion from a spherical grain, it is shown that rock fracturing facilitated the release of accumulated 4He at rates ranging from 4.2 × 10−10 to 1.06 × 10−8 cm3 STP math formula yr−1. These release rates are between 1000 and 30,000 times higher than the local U and Th steady state production rate, of 3.5 ± 1.4 × 10−13 cm3 STP math formula yr−1. Integration of 4He release rates over time yields a radiogenic 4He concentration of between 3.85 × 10−6 and 7.12 × 10−5 cm3 STP math formula, in the range of concentrations measured in the St. Lawrence Lowlands fractured aquifers. Results support the occurrence of a local radiogenic helium source to explain the excesses measured in groundwater without requiring a significant external He crustal flux.