Variations in 238U/235U in uranium ore deposits: Isotopic signatures of the U reduction process?

Bopp, Charles John; Lundstrom, Craig C.; Johnson, Thomas M.; Glessner, Justin

doi:10.1130/g25550a.1

Cited by 94 publications

(90 citation statements)

References 22 publications

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“…The reproducibility of these standards was about 0.05‰ at Hannover and 0.1‰ at ASU (Table S7; all delta values there are reported relative to CRM-112A). The delta values for all standards used agree with those previously published (9,11,18,19,36) (Table S7).…”

Section: Methodssupporting

confidence: 86%

“…4). The results of this study suggest that, in many sedimentary deposits, enzymatic U reduction was a major driver for authigenic U enrichment, including marine sediments and shales (10,14), as well as sandstone tabular and roll-front U deposits (8,11,36). For example, in the modern Black Sea, U isotope fractionation (Δ 238 U) of ∼0.7‰ between the authigenic U in the sediment and that in the deep water column was observed (10,13,37).…”

Section: Discussionmentioning

confidence: 75%

“…Low-temperature ore deposits, such as sedimentary U deposits, require U(VI)-bearing fluids to react with a reduced zone, leading to the precipitation of U(IV) minerals (8,36). In addition, moderate authigenic U enrichment is found in a range of geological settings, including marine sediments that formed under anoxic conditions.…”

Section: Discussionmentioning

confidence: 99%

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Uranium isotopes fingerprint biotic reduction

Stylo

Neubert

Wang

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

138

153

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Knowledge of paleo-redox conditions in the Earth’s history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth’s crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium.

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Section: Methodssupporting

confidence: 86%

Section: Discussionmentioning

confidence: 75%

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Uranium isotopes fingerprint biotic reduction

Stylo

Neubert

Wang

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

138

153

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“…For these samples, the predominant fractionation mechanism has been generally assumed to be the NFS effect in the case of U(VI)/ U(IV) redox reactions. NFS effect leads to the enrichment of the heavy 238 U isotope into the U(IV) species (30)(31)(32). Some studies reported that these redox reactions can be mediated by sulfatereducing bacteria (33)(34)(35), but, in these cases, only extracellular uranium was analyzed, and the potential processes occurring inside the bacteria were not investigated.…”

Section: Significancementioning

confidence: 99%

Evidence of isotopic fractionation of natural uranium in cultured human cells

Paredes

Avazeri

Malard

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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The study of the isotopic fractionation of endogen elements and toxic heavy metals in living organisms for biomedical applications, and for metabolic and toxicological studies, is a cutting-edge research topic. This paper shows that human neuroblastoma cells incorporated small amounts of uranium (U) after exposure to 10 μM natural U, with preferential uptake of the 235 U isotope with regard to 238 U. Efforts were made to develop and then validate a procedure for highly accurate n( 238 U)/n( 235 U) determinations in microsamples of cells. We found that intracellular U is enriched in 235 U by 0.38 ± 0.13‰ (2σ, n = 7) relative to the exposure solutions. These in vitro experiments provide clues for the identification of biological processes responsible for uranium isotopic fractionation and link them to potential U incorporation pathways into neuronal cells. Suggested incorporation processes are a kinetically controlled process, such as facilitated transmembrane diffusion, and the uptake through a highaffinity uranium transport protein involving the modification of the uranyl (UO 2 2+ ) coordination sphere. These findings open perspectives on the use of isotopic fractionation of metals in cellular models, offering a probe to track uptake/transport pathways and to help decipher associated cellular metabolic processes.isotopic fractionation | uranium | neuronal cells | analytical procedure development | microsamples

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“…More recently, the IUPAC (13) recommend a value of 137.80 from the analysis of six natural ore deposits (14), confirmed by high-precision isotope ratio analyses using the IRMM 3636 233 U-236 U double spike (15,16) whose isotopic composition is traceable to SI units. The invariance of the present-day 238 U/ 235 U ratio has been brought into question by studies that have demonstrated U isotopic fractionation in terrestrial materials (17)(18)(19)(20). Such fractionation occurs during oxidation-reduction reactions (U VI to/from U IV ), coordination change during adsorption, or leaching, and is due to thermodynamic or nuclear field shift effects (21)(22)(23).…”

mentioning

confidence: 99%

238U/235U Systematics in Terrestrial Uranium-Bearing Minerals

Hiess¹,

Condon²,

McLean³

2017

Preprint

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One sentence summary:238 U/ 235 U ratios for U-bearing accessory minerals from a diverse suite of terrestrial rocks indicate a >5‰ range with an average zircon value of 238 U/ 235 U = 137.818 ± 0.045 (2σ) which is consistent with the composition of other terrestrial and meteoritic reservoirs. 2, 3). However, a robust λ 235 U can only be determined with U-Pb analyses using a tracer calibration that is traceable to SI units and free of other potential sources of bias, so we refrain from suggesting this value be adopted at present and urge caution in abandoning the Jaffey et al (1) λ 235 U determination until such a dataset has been generated and evaluated.An emerging 238 U/ 235 U dataset for a wide range of rocks, minerals, and meteorites is now available (17,18,25,(30)(31)(32)(33) and compiled here (Fig. 3)

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Variations in 238U/235U in uranium ore deposits: Isotopic signatures of the U reduction process?

Cited by 94 publications

References 22 publications

Uranium isotopes fingerprint biotic reduction

Uranium isotopes fingerprint biotic reduction

Evidence of isotopic fractionation of natural uranium in cultured human cells

238U/235U Systematics in Terrestrial Uranium-Bearing Minerals

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