The uranium isotopic composition of the Earth and the Solar System. (1st January 2015) - Goldmann, Alexander
Abstract: Recent high-precision mass spectrometric studies of the uranium isotopic composition of terrestrial and meteoritic materials have shown significant variation in the 238 U/ 235 U 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 238 U/ 235 U for the Solar System. The investigated bulk meteorites show a range in 238 U/ 235 U 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 238 U/ 235 U 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 238 U/ 235 U with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic 235 U was generated by the decay of extant 247 Cm in the early Solar System. Rather, ordinary chondrites show a positive correlation of 238 U/ 235 U 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 238 U/ 235 U 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 247 Cm and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively. The average 238 U/ 235 U 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 238 U/ 235 U (137.88), this new value results in an age adjustment of −0.9 Ma.