Isotopic fractionation of U in rocks reflecting redox conditions around a groundwater flow route

Low 234U/238U activity ratios observed in rock and mineral samples were scrutinized. U isotope fractionation leading to 234U depletion (234U/238U<1) in rocks appears to be linked to changes in redox conditions. The fractionation takes place as selective chemical release dominates over direct physical μ recoil. This preferential 234U release depends on the valence contrast between the U isotopes, 238U occurring in +4 form and ingrown 234U, due to oxidizing microenvironment, in +6 form. Observed U isotopic fractionation combined with other uranium series disequilibrium measurements provides a tool for locating redox fronts formed as a result low temperature rock-groundwater interaction