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An excellent 53 Mn– 53 Cr isochron for bulk CI, CM, CO, CV, CB, and ungrouped C3 chondrites seems to suggest that each carbonaceous chondrite group acquired its Mn/Cr ratio 4568 ± 1 Myr ago. This age is indistinguishable from the age of Ca–Al-rich inclusions (CAIs), which is considered to be the start of the solar system ( t 0 ). However, carbonaceous chondrites were not assembled until at least 1.5–5 Myr after t 0 , to judge by the 207 Pb– 206 Pb and 26 Al– 26 Mg ages of the chondrules within them, and by the fact that they were not melted by heat from the decay of 26 Al. Presumably, therefore, these meteorites inherited their bulk Mn–Cr isochron from precursor materials which experienced Mn–Cr fractionation at t 0 . As a possible physical mechanism for how the isochron was established initially, and later inherited by the carbonaceous chondrites, we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements, and hence had variably low Mn/Cr. The planetesimals and the undepleted (high Mn/Cr) primitive dust from which they were made shared the same initial e 53 Cr, and therefore evolved on an isochron. We suggest that later impact-disruption of the planetesimals produced dusty debris, which became mixed, in various proportions, with unprocessed (high Mn/Cr) dust before accreting to the carbonaceous chondrite parent bodies. With mixing in a closed system, the isochron was unchanged. We infer that some debris-rich material was converted to chondrules prior to accretion. The chondrules could have been formed by flash melting of the mixed dust, or could instead have been made directly by the impact splashing of molten planetesimals, or by condensation from impact-generated vapor plumes. |
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