On the Role of α-Recoil in Uranium Migration - Some Findings from the Palmottu Natural Analogue Site, SW Finland

Abstract

α-Recoil induced isotopie fractionation of radioactive decay chain 4n+2 nuclides in rock material has been investigated experimentally and theoretically. The fractionation studied here manifests itself in U-234 and Th-230 surplus in U-poor mineral phase adjacent to U-rich phase, and in U-234 deficit in readily dissolved U-rich phase in rock pores. In four samples out of five our modelling concept (continuous accumulation or release of U-234 and Th-230 due to α-recoil) gave an explanation in line with independent observations, indicating potential of α-recoil for dating application in U migration studies.

The Behavior of U- and Th-series Nuclides in Groundwater

“…However, it is worth noting here that such leaching will only occur when the recoil track crosses the mineral surface or a fast pathway to the surface, even though the daughter has not been directly lost. This can occur if daughters cross mineral boundaries and are implanted into adjacent minerals (e.g., Suksi and Rasilainen 1996), or if a process such as fracturing creates a new mineral face. Nuclides within recoil tracks may also be released if weathering moves the mineral boundary inwards by bulk dissolution of the mineral or by incongruent dissolution that releases those constituents that are incompatible with the secondary phase.…”

“…These atoms will then be available for leaching subsequently (Fleischer and Raabe 1978) and this has consequences for the seasonal supply of 222 Rn from soils and at the water table. This surface accumulation can also occur in minerals within low permeability rocks (Suksi and Rasilainen 1996). This raises the possibility that there are available pools of other nuclides in minerals from such rocks that have been eroded and incorporated into aquifer rocks within time periods comparable with the half-life of some nuclides.…”

“…For example, in the Palmottu site of gneisses in Finland U, as well as Th, was found to be redistributed into fracture coatings of carbonates and Fe oxides (Suksi et al 1991). Suksi and Rasilainen (1996) have demonstrated how U concentrated in these fracture fillings can implant daughters in the surrounding phases, generating large fractionations between 234 U/ 238 U and 230 Th/ 234 U. This suggests that phases that may weather at different rates later through changing conditions, new fracturing, or transport to a sedimentary aquifer and can become sources of highly fractionated nuclides into groundwater by leaching or weathering.…”

Transfer of Natural Radionuclides in Terrestrial Food Chains—A Review of Investigations in Finland

“…Additionally, natural U deposits (natural analogue sites) and U-rich rock formations have been important research topics of radionuclide migration in Finland. In these geochemical studies, determination of U and Th concentrations, oxidation states of U, as well as sample age determination via U and Th isotope ratios, have been utilized as research methods [36][37][38][39]. The majority of ingested uranium in Finland, and possibly following health effects from uranium exposure, is due to consumption of drinking water containing substantial amounts of 238 U and its daughters.…”