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

Méjean, Pauline; Pinti, Daniele L.; Ghaleb, Bassam et Larocque, Marie (2017). « 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 ». Water Resources Research, 53(7), pp. 5677-5689.

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Résumé

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.

Type:	Article de revue scientifique
Mots-clés ou Sujets:	234U/238U activity ratio, 4He excess, groundwater age, α-recoil, diffusion, fracturing, deglaciation
Unité d'appartenance:	Centres institutionnels > Centre de recherche en géochimie et géodynamique (GEOTOP) Faculté des sciences > Département des sciences de la Terre et de l'atmosphère
Déposé par:	Marie Larocque
Date de dépôt:	04 oct. 2017 08:10
Dernière modification:	28 févr. 2019 14:07
Adresse URL :	http://archipel.uqam.ca/id/eprint/10472

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