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

Abstract

Accurate determination of the absolute ages of the oldest Solar System objects - chondrules and Ca-Al-rich inclusions (CAIs), requires knowledge of their 238U/235U 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 CAIs (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±0.50Ma, calculated using the measured 238U/235U=137.876±0.043 (2σ), reported relative to 238U/235U=137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: 238U/235U=137.821±0.014.The error in the age includes uncertainties in the Pb-Pb isochron intercept and in the 238U/235U ratio. Allende bulk rock and chondrules have 238U/235U=137.747±0.017 (2σ), distinctly lower than the CAI. The difference in the 238U/235U ratio of 0.129±0.046 (2σ) between the CAI and chondrules and bulk meteorite increases the 207Pb-206Pb age difference by ~1.4Ma, and eliminates apparent disagreement between the CAI-chondrule formation time interval determinations with the U-Pb and extinct nuclide (26Al-26Mg and 182Hf-182W) data. We discuss standardisation of 238U/235U measurements for U-Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAIs in the context of 238U/235U variability, and possible causes of U isotopic variations in CAIs and meteorites.