PER MIL 238U/235U VARIATIONS IN URANIUM ORES: EVIDENCE FOR ISOTOPIC FRACTIONATION BY NUCLEAR FIELD SHIFT?

The ratio of uranium's two long-lived isotopes, ²³⁸U and ²³⁵U, has generally been considered invariant in nature with a value of 137.88 ±3‰ (Chen and Wasserburg; Anal. Chem., 1981). However, gas mass spectrometry results from uranium ores found small variations in ²³⁸U/²³⁵U, possibly related to the geologic setting of the deposit (Cowan and Adler; GCA, 1976). Specifically, a bimodal distribution of ²³⁸U/²³⁵U was found with magmatic deposits having higher ²³⁵U wt% mean and sandstone-type, lower-temperature deposits having lower ²³⁵U wt% mean.

We have measured the ²³⁸U/²³⁵U in three sandstone-type and three magmatic-type uranium ores based on those listed in Cowan and Adler (provided courtesy of Smithsonian Institution). We analyzed these by MC-ICPMS using a high precision double-spike technique, utilizing ²³³U/²³⁶U to control for instrumental mass bias. Reproducibility of the offset between IRMM U-A and NBS960 is 0.2‰. Repeat analyses of the ore samples show the sandstone-type ores to be consistently depleted in 235U relative to magmatic-type ores by ≈1‰.

While microbial reduction of uranium in the laboratory can fractionate these isotopes (Rademacher et al; ES&T; 2006), such a process should result in an isotopically light uranium deposit in low temperature ores, opposite to the heavy isotopic composition observed. However, work by Bigeleisen (J. Am. Chem. Soc.; 1996) and more recently by Schauble (AGU annual meeting, 2006) has shown the importance of the nuclear field shift, an unconventional isotope effect dependant on nuclear volume. This effect is of increasing magnitude with increasing atomic mass, and for uranium can potentially exceed the bond-energy isotope effect by a factor of three. We propose that the observed depletion in ²³⁵U in sandstone-type ores is the result of the nuclear field shift effect overwhelming a smaller bonding related isotopic effect. This could represent the first evidence of the nuclear field shift found in nature.