Origin of the short-lived radionuclide ¹⁰be and its implications for the astronomical setting of cai formation in the solar protoplanetary disk

We report Li-Be-B and Al-Mg isotopic compositions of Ca-Al-rich inclusions (CAIs) in Sayh al Uhaymir 290 (CH) and Isheyevo (CH/CB) metal-rich carbonaceous chondrites. All CAIs studied here do not show resolvable excesses in ²⁶Mg, a decay product of the short-lived radionuclide ²⁶Al, which suggests their formation occurred prior to the injection of ²⁶Al into the solar system from a nearby stellar source. The inferred initial ¹⁰Be/⁹Be ratios obtained for these CAIs range from 0.17 × 10^-3 to 6.1 × 10^-3, which tend to be much higher and more variable than those of CAIs in CV3 chondrites. The high ¹⁰Be/⁹Be ratios suggest that ¹⁰Be was most likely synthesized through solar cosmic-ray irradiation. The lithium isotopic compositions of these CAIs are nearly chondritic, independent of their initial ¹⁰Be/⁹Be ratios. This can be explained by the irradiation targets being of chondritic composition; in other words, targets were most likely not solid CAI themselves, but their precursors in solar composition. The larger variations in ¹⁰Be/⁹Be ratios observed in CH and CH/CB CAIs than in CV CAIs may reflect more variable cosmic-ray fluxes from the earlier, more active Sun at an earlier evolutionary stage (class 0-I) for the former, and a later, less active stage of the Sun (class II) for the latter. If this is the case, our new Be-B and Al-Mg data set implies that the earliest formed CAIs tend to be transported into the outer part of the solar protoplanetary disk, where the parent bodies of metal-rich chondrites likely accreted.