Nucleosynthetic sources and the age of the solar system: U isotope variations in meteorite components

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We propose to investigate the U isotope composition (235U/238U) of meteorites and their components, particularly that of CAIs. Uranium has two long-lived isotopes, 238U and 235U, which were produced by r-process nucleosynthesis. Its isotope composition in meteoritc materials is generally considered to be constant, which is an essential prerequisite for U-Pb and Pb-Pb dating. However, recent studies revealed significant (‰-range) U isotope variations on low-T terrestrial materials and preliminary investigations on meteoritic material have revealed that some CAIs display even more heterogeneous U isotope compositions. Such U isotope variations produce an offset on Pb-Pb ages of CAIs on the order of several Ma, if for these samples the commonly accepted U isotope composition (238U/235U = 137.88) is used for age calculations.In this study, (1) we plan to systematically investigate the U isotope composition of various groups of CAIs, mineral separates of such CAIs, and other meteoritic materials, such as leachates of bulk chondrites. (2) We plan to combine the U isotope compositions with Pb-Pb dating on the same CAIs, applying the individually analyzed U isotope composition (instead of a constant ratio of 137.88) for age calculations. (3) The measured U isotope ratios will be used to develop constraints on the possible source(s) of U isotope heterogeneities that may result from a) inherited nucleosynthetic anomalies, b) initial 247Cm excess or depletion, or c) chemical reactions that caused U isotope fractionation (similar to those observed for oceanic sediments on Earth).