238U-235U-234U fractionation between tetravalent and hexavalent uranium in seafloor phosphorites

Variations in the U-238/U-235 ratio are mostly observed in association with changes in the uranium oxidation state and therefore controlled by changes in the redox conditions, although evidence for this process has so far been indirect. Here, the delta U-238 and delta U-234 isotope composition of different redox species is studied for the first time within the same geological samples: the bulk, reduced (U(IV)) and oxidized (U(VI)) uranium species in seafloor phosphorites. In all cases, delta U-238(IV) is higher (-0.27 to-0.81) than corresponding delta U-238(VI) (-0.64 to -1.07), with delta U-234(VI) displaying extremely high values (similar to 500-2000) relative to delta U-234(IV) (-240 to -100). By comparison, the bulk delta U-238, delta U-234 and U concentrations are-0.42 to-0.85,-10 to +20, and 63-328 ppm, respectively. These values are mostly in the range of natural variations in previously reported samples, with the bulk delta U-238 and delta U-234 values corresponding with seawater values except for a tail in delta U-238 toward lower values. The main exception is displayed by the composition of the U(VI) fraction, which ranges toward relatively low emu values but has a very strong positive delta U-234 excursion relative to comparable samples elsewhere. Given that the studied phosphorites formed in high productivity environments, where oxygen consumption was high and hence anoxic conditions could have been favored, it is assumed that most of the initial uranium in the samples was in reduced form. Indeed, even if some of the U oxidized over time, the studied samples still consist of approximately 60-80% U(IV). The process of U-238 radioactive decay resulted in the oxidation of the decay product, U-234, and consequently, the delta U-234(VI) within the samples has very high values. The evolution of delta U-238 is related to the effect of the 'nuclear field shift', which predicts that nuclides with higher atomic masses will be reduced preferentially over those with lower atomic masses. Accordingly, it is easier to reduce U-238 than U-235 in the same environment, which would result in higher delta U-238 of U(IV). This is indeed the case observed here, although in addition, the evidence here shows that the delta U-238 fractionation occurred during U oxidation and that the final differences of delta U-238 between both oxidation states reflect the combined effect of the depositional nuclear field shift and the in-situ recoil-related oxidation. A set of quantitative models are used to evaluate the role and rate of the different processes and suggest that because of the independent evolution of U(IV) and U(VI) over time, their relative fraction and isotopic compositions can be used to evaluate the formation ages of seafloor phosphorites, which has so far not been possible using U decay series. (C) 2016 Elsevier B.V. All rights reserved.