Isotopic Fractionation of U in Rocks Reflecting Redox Conditions around a Groundwater Flow Route
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Isotopic Fractionation of U in Rocks Reflecting Redox Conditions around a Groundwater Flow Route Juhani Suksi1 and Kari Rasilainen2 1 Laboratory of Radiochemistry, University of Helsinki, Finland 2 VTT Energy, Espoo, Finland ABSTRACT Low 234U/238U activity ratios observed in rock and mineral samples were scrutinized. U isotope fractionation leading to 234U depletion (234U/238U 1), indicating U removal within the latest glaciations. This zone of congruent U removal represents the oxidized part of rock matrix. Based on the U removal depth and oxidation state distribution it can be concluded that oxidation front has advanced into the rock matrix (in sample R384 over the studied core length and 20 mm in sample 389). This is also reflected in the 234U/238U activity ratio (denoted as AR hereafter), which shows values around unity, corroborating congruent U release. In sample R389 the AR profile shows a clear 234U depletion behind the oxidized rock (Figure 6). The fracture surface samples from R302 showed exceptionally large 234U depletion, indicating considerable selective chemical release of 234U. The co-located drill core sample showed considerable 234U depletion across the whole studied core length. Interestingly, the corresponding water sample shows clear 234U enrichment (AR ~ 2) [16], indicating that bulk U is not dissolved. Namely, if bulk U had dissolved to a significant extent the water would have been labeled by the low AR observed in the rock. Here it must be noted that the drill core sample in this sampling point is intensely altered and exceptionally porous with conductive structure over the matrix, allowing water-rock interaction to occur deep within the rock.
A linkage between the defined redox-fronts and the low AR could be indicated in all samples. In sample R384 oxidizing front appears to have advanced over the studied core length and 234U depletion can not be observed, unlike in sample R389. In R302 where dissolved oxygen contents has clearly decreased (Eh ≤ 0) unusually strong 234U depletion is seen on fracture calcites and throughout rock matrix. The behavior of U inventories is at least to some extent controlled by glaciation cycles which indicates that multistage behavior would give a better concept of the evolution. For the purpose of this paper multistage modeling was not necessary, but in order to obtain a better interpretation of the measured USD profiles it will undoubtedly have to be done in the future. For this purpose, our modeling tool URSE suits exceptionally well [17]. CONCLUSIONS
Isotopic fractionation of U, leading to 234U depletion in rocks, appears to reflect changes in redox conditions. This is because 234U selective chemical release dominates over direct physical α recoil at redox fronts. The chemical selectivity is postulated to be caused by contrast in valence states between the two U isotopes 238U and 234U, the latter being mostly in the more soluble 6+ state. It appears that U isotope fractionation provides a promising tool to study the intrusion depth and its range of influen
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