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Erik Oerter1 Michael Singleton1 M. Lee Davisson1

1, Lawrence Livermore National Laboratory, Livermore, California, United States

When exposed to humidity in an oxidizing atmosphere hydrated uranium oxide grows as a secondary mineral on aged UO2 and U3O8, and incorporates the oxygen isotope signature of the water vapor into the secondary uranium oxide, as well as hydrogen and oxygen into any hydration water. Because geospatial variation in δ2H and δ18O values of atmospheric humidity and precipitation is well understood, the H and O stable isotope composition of mineral hydration waters can give information on the environment of mineral formation. We present stable H and O isotope results of mineral hydration waters in uranium oxide materials analyzed by a new methodology wherein precise heating by thermogravimetric analysis is used to liberate water vapor for subsequent isotope analysis via a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i), which we abbreviate as TGA-IRIS. XRD analysis of U3O8 before and after heating to 350°C indicates that the U3O8 had a measurable metaschoepite phase ((UO3) nH2O, n < 2; where n is the number of hydration water molecules) along with alpha-U3O8. After heating, the metaschoepite is eliminated but the alpha-U3O8 is retained, meaning that the heating successfully extracted the water in the metaschoepite but did not disturb the U3O8. Precision for stable isotope values yielded by the TGA-IRIS method on U3O8 is ± 1.3 ‰ for δ2H and ± 0.60 ‰ for δ18O, which may be higher than that made on liquid water samples (± 1.2 ‰ for δ2H and ± 0.17 ‰ for δ18O), but is adequate given the large range of δ2H and δ18O values in natural waters. TGA-IRIS analysis of UO2 is more difficult because heating caused oxidation with the evolved water, creating oxygen isotope exchange with the water, and subsequently a perturbation in the stable isotope abundance. This does not appear to be as much of an issue for hydrogen in the evolved water. Further work is needed to optimize TGA-IRIS for analysis of UO2. The ability of TGA-IRIS to generate detailed water yield data and δ2H and δ18O values of water at varying temperatures allows for the differentiation of water in varying states of binding on mineral surfaces and within the mineral matrix. Thus, TGA-IRIS presents the possibility to isotopically differentiate the various oxygen reservoirs in hydrated minerals (including uranium oxides), which may allow TGA-IRIS to open new avenues and possibilities for research on hydrated mineral phases.

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