Uranium Micro-isotopic Analysis of Weathered Rock by a Sensitive High Resolution Ion Microprobe (SHRIMP II)

The SHRIMP II was used to measure the ²³⁴U/²³⁸U isotope ratios in weathered rock from the Koongarra uranium (U) deposit in Northern Australia. The results were compared with data obtained using sequential extraction (SE) procedures. The key attribute of the SHRIMP measurement is that sensitive isotopic data can be obtained without dissolution of the sample, and associated loss of textural relationships. The SHRIMP provides spot- by-spot isotopic ratios in individual mineral phases, and has not been previously applied to measurements of ²³⁴U/²³⁸U ratios. The SHRIMP measurements of ²³⁴U/²³⁸U activity ratios in the accumulated iron (Fe)-materials were closer to unity than in the chemically extracted amorphous Fe-oxide fraction, the primitive phase of all Fe-minerals. This indicates an approach to secular equilibrium subsequent to the deposition of U with the amorphous precursor phases. The activity ratios of ²³⁴U/²³⁸U in kaolinite were significantly lower than the ratios obtained for residual mineral phases in the SE study, although being higher than unity. This suggests that the extremely high ²³⁴U/²³⁸U ratio of the residual phase in the SE study was mainly derived from other minerals, presumably the abundant quartz rather than the kaolinite. Activity ratios exceeding unity were found in all kaolinites studied, regardless of their proximity to U-rich phases and grain boundaries. Individually, the SE procedure and SHRIMP analysis are useful techniques for measuring isotopes within mineral phases. However, in contrast to the SHRIMP, the SE procedure does not provide spatial or textural information, and quantifies isotopes in individual extractable fractions. On the other hand, the SHRIMP is not suitable for phases with very small gram size or low U content. By the parallel use of SE procedures and the SHRIMP, samples can be analyzed both quantitatively and with high spatial resolution.