Analogue studies in the alligator rivers region. In-situ measurement of uranium series nuclides with SHRIMP

摘要

The SHRIMP analyses have been conducted for rock samples from the Koongarra secondary ore deposit to obtain activity ratios of (sup 234)U/(sup 238)U and isotopic ratios of (sup 207)Pb/(sup 206)Pb and (sup 204)Pb/(sup 206)Pb. Target minerals for the analyses were iron minerals and kaolinite, which are the main weathering products in this area. The activity ratios of (sup 234)U/(sup 238)U were obtained based on counts at masses of uranium metal. The (sup 234)U/(sup 238)U activity ratios based on counts of uranium oxides were not appropriate, because count rates of (sup 234)U(sup 16)O were interfered by those of (sup 238) U(sup 12)C molecule. The activity ratios of (sup 234)U/(sup 238)U were approximately unity for crystalline iron nodules. This fact suggested that the mean residence time of uranium within the iron nodules was at least 1 million years. On the other hand, slightly higher values than unity were obtained for kaolinite. Lead isotopes were investigated and a positive relationship was recognized between (sup 207)Pb/(sup 206)Pb and (sup 204)Pb/(sup 206)Pb isotope ratios. (author)