科研之友
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摘要
The U-238/U-235 ratio was precisely measured in uranium minerals from 11 hydrothermal deposits of different geologic settings and ages situated in ore regions of Asia, Europe, Africa, and North America by MC-ICP-MS using a U-233-U-236 double spike. The spike was calibrated in reference to the CRM-112A standard with U-238/U-235 = 137.837 +/- 0.015 (Richter et al, 2010). The long-term reproducibility of U-238/U-235 measurement was estimated as +/- 0.07aEuro degrees by the analysis of monitor samples and the IRMM-3184 standard. The analyses were performed using 0.02-0.04-mg microsamples of uraninite, pitchblende, and coffinite, which were locally extracted from polished sections under an optical microscope. The U-238/U-235 values obtained for 50 samples of U-bearing minerals range from 137.703 to 137.821, with a 0.86aEuro degrees difference and a mean U-238/U-235 value of 137.773 +/- 0.056 (+/- 2SD). The range of U-238/U-235 variations in seven deposits with uraninite is 0.41aEuro degrees, which is twice as low as for the deposits with pitchblende-dominated ores. Our study provided the first results for U-238/U-235 variations in minerals from individual deposits. The largest variations were found in the Oktyabr'skii (Eastern Transbaikalia), Schlema-Alberoda (Erzgebirge), and Shea Creek (Athabasca basin) deposits: 0.70, 0.33, and 0.59aEuro degrees, respectively. Uranium from the early growth zones of 4-5 mm thick pitchblende spherulitic crusts is isotopically heavier (by 0.22-0.45aEuro degrees) than uranium from the latest growth zones. A similar isotopic shift in U-238/U-235 in terms of magnitude (0.31aEuro degrees) and sense was observed between pitchblende and coffinite overgrowths. The uranium isotopic composition of late pitchblende generations, the products of dissolution and reprecipitation of early phases, is 0.46aEuro degrees lighter than that of early pitchblende phases. The character of uranium isotope distribution in pitchblende aggregates is consistent with nuclear-volume-dependent isotope fractionation accompanying U(VI) reduction to U(IV) (Bigeleisen, 1996; Schauble, 2007; Stirling et al., 2007), which causes an enrichment of the U(IV)-bearing solid phase in the heavy isotope U-238. The range of U-238/U-235 ratios for 11 hydrothermal (high-temperature) deposits (137.703-137.821) lies well within the broader (two-fold) range of values determined for the low-temperature deposits Dybryn in Transbaikalia (Golubev et al., 2013) and Pepegoona in South Australia (Murphy et al., 2014). This can be explained by the fact that the uranium isotopic fractionation associating with U(VI) -> U(IV) reduction is accompanied by isotope shifts owing to the long-term interaction of groundwater with early phases within sandstone-type deposits. At the same time, owing to the higher temperatures (by 100-300A degrees C) of formation of hydrothermal deposits compared with sandstone-type deposits, nuclear-volume-dependent uranium isotope fractionation decreases by more than a factor of 2 (Bopp et al., 2009).
- 出版日期2014-12
