CHAMPAIGN, Ill. – For years, the ratio of uranium’s two long-lived isotopes, U-235 and U-238, has been considered invariant, despite measurements made in the mid-1970s that hinted otherwise. Now, with improved precision from state-of-the-art instrumentation, researchers at the University of Illinois unequivocally show this ratio actually does vary significantly in Earth materials.

The new findings are in line with recent findings in other high-mass isotope systems – such as thallium or mercury – that had been assumed to be invariant. Additionally, the new measurements “could represent the first evidence of the nuclear field shift found in nature,” said U. of I. graduate student Charles J. Bopp, who led the study…

There are two basic types of uranium ore deposits: magmatic, which develop due to hydrothermal effects; and sedimentary, which develop by chemical reduction of uranium in groundwater in subsurface aquifers.

In 1976, scientists George Cowan and Hans Adler analyzed gas mass spectrometry results of uranium hexafluoride (before artificial isotopic enrichment processes took place) derived from uranium ores around the world. This assessment revealed a slight offset in the distribution of the ratio of U-235 to U-238, with magmatic-type deposits having on average higher U-235 percentage weight and sandstone-type deposits having lower…

The observed depletion of U-235 is most likely the result of a nuclear field shift effect as isotopes partition between the water and the reduced uranium ore mineral, Bopp said. But what uranium reduction process – biotic or abiotic – is responsible is not yet clear.

“We can’t parse that apart at this stage,” Bopp said. “We observe a depletion, and we know there are microbes present in these types of deposits, but we can’t say for sure who’s doing what without a much more in-depth study of a single locality.”

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