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

We report on the mineralogy, petrography, and in situ measured oxygen-and magnesiumisotope compositions of eight porphyritic chondrules (seven FeO-poor and one FeO-rich) from the Renazzo-like carbonaceous (CR) chondrites Graves Nunataks 95229, Grosvenor Mountains 03116, Pecora Escarpment 91082, and Queen Alexandra Range 99177, which experienced minor aqueous alteration and very mild thermal metamorphism. We find no evidence that these processes modified the oxygen-or Al-Mg isotope systematics of chondrules in these meteorites. Olivine, low-Ca pyroxene, and plagioclase within an individual chondrule have similar Oisotope compositions, suggesting crystallization from isotopically uniform melts. The only exceptions are relict grains in two of the chondrules; these grains are 16 O-enriched relative to phenocrysts of the host chondrules. Only the FeO-rich chondrule shows a resolvable excesses of 26 Mg, corresponding to an inferred initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of (2.5±1.6)×10 6 (±2SE). Combining these results with the previously reported Al-Mg isotope systematics of CR chondrules (Nagashima et al., 2014, Geochem. J. 48, 561), 7 of 22 chondrules (32%) measured show resolvable excesses of 26 Mg; the presence of excess 26 Mg does not correlate with the FeO content of chondrule silicates. In contrast, virtually all chondrules in weakly metamorphosed (petrologic type 3.0-3.1) unequilibrated ordinary chondrites (UOCs), Ornans-like carbonaceous (CO) chondrites, and the ungrouped carbonaceous chondrite Acfer 094 show resolvable excesses of 26 Mg. The inferred ( 26 Al/ 27 Al) 0 in CR chondrules with resolvable excesses of 26 Mg range from (1.0±0.4)×10 6 to (6.3±0.9)×10 6 , which is typically lower than ( 26 Al/ 27 Al) 0 in the majority of chondrules from UOCs, COs, and Acfer 094. Based on the inferred ( 26 Al/ 27 Al) 0 , three populations of CR chondrules are recognized; the population characterized by low ( 26 Al/ 27 Al) 0 (<3×10 6 ) is dominant. There are no noticeable trends with major and minor element or Oisotope compositions between these populations. The weighted mean ( 26 Al/ 27 Al) 0 of 22 CR chondrules measured is (1.8±0.3)×10 6 . An apparent agreement between the 26 Al-26 Mg ages (using weighted mean value) and the revised (using 238 U/ 235 U ratio for bulk CR chondrites of 137.7789±0.0085) 207 Pb-206 Pb age of a set of chondrules from CR chondrites (Amelin et al., 2002, Science 297, 1678) is consistent with the initial 26 Al/ 27 Al ratio in the CR chondrite chondrule-forming region at the canonical level (~5.210 5 ), allowing the use of 26 Mg systematics as a chronometer for CR chondrules. To prove chronological significance of 26 Al for CR chondrules, measurements of Al-Mg and U-Pb isotope systematics on individual chondrules are required. The presence of several generations among CR chondrules indicates some chondrules that accreted into the CR chondrite parent asteroid avoided melting by later chondrule-forming events, suggesting chondrule-forming processes may have occurred on relativ...

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Isotope mixtures for the ab initio-calibration of Mg isotope ratio measurements were prepared with unrivalled relative expanded uncertainties as low as 0.005%.