U-Pb chronology of the Solar System's oldest solids with variable 238U/235U

Accurate determination of the absolute ages of the oldest Solar System objects - chondrules and Ca-Al-rich inclusions (CAls), requires knowledge of their U-238/U-235 ratios. This ratio was assumed to be invariant in all U-Pb dating of meteorites so far, but the recent discovery of U isotope variations in CAls (Brennecka et al., 2010a) shows that this assumption is invalid. We present the first combined high-precision U and Pb isotopic data for a CAI, and U isotopic data for chondrules and whole rock fractions of the Allende meteorite. The Pb-Pb isochron age of the CAI Sj101 is 4567.18 +/- 0.50 Ma. calculated using the measured U-238/U-235 = 137.876 +/- 0.043 (2 sigma), reported relative to U-238/U-235 = 137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: U-238/U-235 = 137.821 +/- 0.014. The error in the age includes uncertainties in the Pb-Pb isochron intercept and in the U-238/U-235 ratio. Allende bulk rock and chondrules have U-238/U-235 = 137.747 +/- 0.017 (2 sigma), distinctly lower than the CAI. The difference in the U-238/U-235 ratio of 0.129 +/- 0.046(2 sigma) between the CAI and chondrules and bulk meteorite increases the Pb-207-Pb-206 age difference by similar to 1.4 Ma, and eliminates apparent disagreement between the CAI-chondrule formation time interval determinations with the U-Pb and extinct nuclide ((26)AI-Mg-26 and Hf-182-W-182) data. We discuss standardisation of U-238/U-235 measurements for U-Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAls in the context of U-238/U-235 variability, and possible causes of U isotopic variations in CAls and meteorites. (C) 2010 Elsevier B.V. All rights reserved.