The mineralised sediment samples display extremely variable 238U/235U ratios (herein expressed as , the per-mil deviation from the international NBL standard CRM145). The majority of mineralised sediment samples have values between and , spanning a ca. 2鈥?range. However, one sample has an unusually light isotopic composition of , which suggests a total range of U isotopic variability of up to ca. 5鈥? the largest variation found thus far in a single natural redox system. The 238U/235U isotopic signature of the mineralised sediments becomes progressively heavier (enriched in 238U) along the groundwater flow path.
The groundwaters show a greater than 2鈥?variation in their 238U/235U ratios, ranging from values of to . The majority of the groundwater data exhibit a clear systematic relationship between 238U/235U isotopic composition and U concentration; samples with the lowest U concentrations have the lowest 238U/235U ratios. The preferential incorporation of 238U during reduction of U(VI) to U(IV) and precipitation of uranium minerals leaves the groundwaters enriched in 235U, resulting in a progressive shift in 238U/235U towards lighter values in the aqueous phase as U is removed. These data can be modelled by a closed system Rayleigh fractionation model, with a fractionation factor (伪, representing the 238U/235U composition of the groundwater relative to the solid uranium minerals) ranging from 鈭?.9996 to 1.0000, with the majority of datapoints ranging from 伪 values of 0.9998 to 0.9999.
The sense and magnitude of the results of this study imply that 238U/235U fractionation is likely to be controlled by volume-dependent nuclear field shift effects during the reduction of U(VI) to U(IV) during mineralisation processes. These findings support the use of the 238U/235U isotopic system as a tracer to constrain the nature and timing of palaeoredox conditions.