New average values for the n(238U)/n(235U) isotope ratios of natural uranium standards

Reliable standard materials for the precise and accurate measurements of n(U-238)/n(U-235) isotope ratios in natural sample materials have gained an increasingly important role in modern geochemistry. Recent findings do not only show variability for the n(U-238)/n(U-235) isotope ratios in nature of up to 0.13% but also emphasize that accurate n(U-238)/n(U-235) isotope ratios are needed for reliable and consistent Pb-Pb dating of geological samples. The commonly used 'consensus value' of 137.88 for the n(U-238)/n(U-235) isotope ratio of the NBS SRM 960 (NBL CRM 112a) standard has been re-measured in a collaborative effort by several geochemistry laboratories and the Institute for Reference Materials and Measurements (IRMM), who produced the isotopic reference materials used for these measurements. The new data have been acquired using a variety of new isotopic reference materials, for example the new gravimetrically calibrated n(U-233)/n(U-236)-double spike IRMM-3636, combined with new measurement methods, resulting in a new average value of 137.837(15) for the n(U-238)/n(U-235) isotope ratio of NBS SRM 960. This new value is about 0.031% lower than the old consensus value. Moreover, it is traceable to the SI and an uncertainty is provided according to the Guide to the Expression of Uncertainty in Measurements (GUM). Additionally a summary of new measurements of the n(U-234)/n(U-238) isotope ratios of the NBS SRM 960 is given, which are of interest for geochemical applications. As an alternative to NBS SRM 960, the (close to) natural reference material IRMM-184 has been re-measured by several laboratories using the n(U-233)/n(U-236)-double spike IRMM-3636, resulting in a n(U-238)/n(U-235) value of 137.683(20), which is in agreement with the certified value of 137.697(41). (C) 2010 Elsevier B.V. All rights reserved.