The uranium isotopic composition of the Earth and the Solar System

Recent high-precision mass spectrometric studies of the uranium isotopic composition of terrestrial and meteoritic materials have shown significant variation in the U-238/U-235 ratio, which was previously assumed to be invariant (=137.88). In this study, we have investigated 27 bulk meteorite samples from different meteorite groups and types, including carbonaceous (CM1 and CV3), enstatite (EH4) and ordinary (H-, L-, and LL-) chondrites, as well as a variety of achondrites (angrites, eucrites, and ungrouped) to constrain the distribution of U isotopic heterogeneities and to determine the average U-238/U-235 for the Solar System. The investigated bulk meteorites show a range in U-238/U-235 between 137.711 and 137.891 (1.3 parts per thousand) with the largest variations among ordinary chondrites (OCs). However, the U isotope compositions of 20 of the 27 meteorites analyzed here overlap within analytical uncertainties with the narrow range defined by terrestrial basalts (137.778-137.803), which are likely the best representatives for the U isotope composition of the bulk silicate Earth. Furthermore, the average U-238/U-235 of all investigated meteorite groups overlaps with that of terrestrial basalts (137.795 +/- 0.013). The bulk meteorite samples studied here do not show a negative correlation of U-238/U-235 with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic U-235 was generated by the decay of extant Cm-247 in the early Solar System. Rather, ordinary chondrites show a positive correlation of U-238/U-235 with Nd/U and with 1/U. The following conclusions can be drawn from this study: (1) The Solar System has a broadly homogeneous U isotope composition, and bulk samples of only a limited number of meteorites display detectable U isotope variations; (2) Bulk planetary differentiation has no significant effect on the U-238/U-235 ratio since the Earth, achondrites, and chondrites have indistinguishable U isotope compositions in average. (3) The cause of U isotopic variation in Solar System materials remains enigmatic; however, both the decay of Cm-247 and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively. The average U-238/U-235 of the investigated meteorite groups (including data compiled from the literature) and terrestrial basalts is 137.794 +/- 0.027 (at a 95% student's t confidence level, including all propagated uncertainties) and represents the best estimate for the Uisotope composition of the Earth and the Solar System. This value may be used for U-Pb and Pb-Pb dating of Solar System materials, provided the precise U isotope composition of the sample is unknown. Compared to Pb-Pb ages that were determined with the previously assumed value for U-238/U-235 (137.88), this new value results in an age adjustment of -0.9 Ma. (C) 2014 Elsevier Ltd. All rights reserved.