Origin of uranium isotope variations in early solar nebula condensates

Creators: Tissot, François L. H.; Dauphas, Nicolas; Grossman, Lawrence

Abstract

High-temperature condensates found in meteorites display uranium isotopic variations (^(235)U/^(238)U), which complicate dating the solar system's formation and whose origin remains mysterious. It is possible that these variations are due to the decay of the short-lived radionuclide ^(247)Cm (t_(1/2) = 15.6 My) into ^(235)U, but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of ^(235)U reaching ~+6% relative to average solar system composition, which can only be due to the decay of ^(247)Cm. This allows us to constrain the ^(247)Cm/^(235)U ratio at solar system formation to (7.0 ± 1.6) × 10^(−5). This value provides new clues on the universality of the nucleosynthetic r-process of rapid neutron capture.

Additional Information

© 2016, The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited. Received for publication October 7, 2015. Accepted for publication January 12, 2016. We thank P. Heck, J. Holstein, and the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum for providing some of the CAI samples; I. Steele for help in operating the SEM at the University of Chicago; and L. Dussubieux for help in operating the LA-ICPMS at the Field Museum. We are grateful to A. M. Davis and B. S. Meyer for discussions. This work was supported by grants from NASA (Laboratory Analysis of Returned Samples, NNX14AK09G; Cosmochemistry, OJ-30381-0036A and NNX15AJ25G) and NSF (Petrology and Geochemistry, EAR144495; Cooperative Studies of the Earth's Deep Interior, EAR150259) to N.D. and a NASA grant (Cosmochemistry, NNX13AE73G) to L.G. Author contributions: N.D. and F.L.H.T. designed the research; F.L.H.T. and L.G. selected the samples; F.L.H.T. performed the research; F.L.H.T., N.D., and L.G. wrote the paper. The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data are available from the authors upon request. This is Origins Lab contribution number 91.

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