The lead-lead isochron age of chondrules in the CR chondrite Acfer 059 is 4564.7 +/- 0.6 million years ago (Ma), whereas the lead isotopic age of calcium-aluminum-rich inclusions (CAIs) in the CV chondrite Efremovka is 4567.2 +/- 0.6 Ma. This gives an interval of 2.5 +/- 1.2 million years (My) between formation of the CV CAIs and the CR chondrules and indicates that CAI- and chondrule-forming events lasted for at least 1.3 My. This time interval is consistent with a 2- to 3-My age difference between CR CAIs and chondrules inferred from the differences in their initial 26Al/27Al ratios and supports the chronological significance of the 26Al-26Mg systematics.

Allende meteorite Ca-Al rich inclusions U-Pb age 26 Al/ 27 Al ratio The precise knowledge of the initial 26 Al/ 27 Al ratio [(26 Al/ 27 Al) 0 ] is crucial if we are to use the very first solid objects formed in our Solar System, calcium-aluminum-rich inclusions (CAIs) as the "time zero" age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multicollector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of 27 Al/ 24 Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three microdrilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined 26 Al-26 Mg fossil isochron with an (26 Al/ 27 Al) 0 of (5.23± 0.13)× 10 − 5. Internal mineral isochrons obtained for three of these CAIs (A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (26 Al/ 27 Al) 0 =(4.96± 0.25) × 10 − 5 , anchored to our precisely determined absolute 207 Pb-206 Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the "canonical" (26 Al/ 27 Al) 0 of 5 × 10 − 5 for the early Solar System. The uncertainty in (26 Al/ 27 Al) 0 corresponds to a maximum time span of ±20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the 26 Al-26 Mg and U-Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the "supra-canonical" 26 Al/ 27 Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supracanonical 26 Al/ 27 Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS. The radiogenic 208 Pb⁎/ 206 Pb⁎ ratio obtained as a by-product from the Pb-Pb age dating is used to estimate timeintegrated 232 Th/ 238 U ratio (κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.

Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves) in the protoplanetary disk around the early Sun. The ages of primitive chondrules in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 +/- 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars. Here we report the 207Pb-206Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 +/- 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 +/- 0.9 Myr), which formed during a single-stage, highly energetic event. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave. We conclude that chondrules and metal grains in the CB chondrites formed from a vapour-melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System.

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