Variations in the 238U/235U 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 δ238U and δ234U 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, δ238U(IV) is higher (− 0.27 to − 0.81) than corresponding δ238U(VI) (− 0.64 to − 1.07), with δ234U(VI) displaying extremely high values (~ 500–2000) relative to δ234U(IV) (− 240 to − 100). By comparison, the bulk δ238U, δ234U 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 δ238U and δ234U values corresponding with seawater values except for a tail in δ238U toward lower values. The main exception is displayed by the composition of the U(VI) fraction, which ranges toward relatively low δ238U values but has a very strong positive δ234U 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 238U radioactive decay resulted in the oxidation of the decay product, 234U, and consequently, the δ234U(VI) within the samples has very high values. The evolution of δ238U 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 238U than 235U in the same environment, which would result in higher δ238U of U(IV). This is indeed the case observed here, although in addition, the evidence here shows that the δ238U fractionation occurred during U oxidation and that the final differences of δ238U 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.
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© 2016 Elsevier B.V.
- Isotope fractionation
- Tetravalent and hexavalent uranium