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 ²³⁸U/²³⁵U 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 ²³⁸U/²³⁵U for the Solar System.The investigated bulk meteorites show a range in ²³⁸U/²³⁵U between 137.711 and 137.891 (1.3‰) 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 ²³⁸U/²³⁵U 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 ²³⁸U/²³⁵U with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic ²³⁵U was generated by the decay of extant ²⁴⁷Cm in the early Solar System. Rather, ordinary chondrites show a positive correlation of ²³⁸U/²³⁵U 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 ²³⁸U/²³⁵U 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 ²⁴⁷Cm and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively.The average ²³⁸U/²³⁵U 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 U isotope 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 ²³⁸U/²³⁵U (137.88), this new value results in an age adjustment of -0.9Ma.