Irradiation origin of ¹⁰Be in the solar nebula: Evidence from Li-Be-B and Al-Mg isotope systematics, and REE abundances of CAIs from Yamato-81020 CO3.05 chondrite

We have performed in situ analyses of Li-Be-B and Al-Mg isotope systematics, and abundances of rare earth elements (REEs) in two Ca-Al-rich inclusions (CAIs) from the Ornans-like carbonaceous chondrite Yamato-81020 (CO3.05). The present CO CAIs are depleted in ultra-refractory heavy REEs (group II REE pattern), suggesting condensation of these CAIs or their precursors from the solar nebula. Initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, of these CO CAIs are found to be (4.8 ± 0.5) × 10^–5 and (4.9 ± 0.3) × 10^–5 (95% confidence), indicating their contemporaneous formation with a majority of CAIs from Vigarano-like carbonaceous (CV) chondrites. Melilite grains in the present CO CAIs show clear excesses in ¹⁰B, ranging from ∼370 to ∼4300‰ relative to the chondritic B isotopic composition, which are correlated well with ⁹Be/¹¹B ratios. The correlation indicates in situ decay of ¹⁰Be in the present CO CAIs and yields initial ¹⁰Be/⁹Be ratios, (¹⁰Be/⁹Be)₀, for the individual CAIs of (2.9 ± 0.6) × 10^–3 and (2.2 ± 1.0) × 10^–3 (95% confidence), which are significantly greater than the average (¹⁰Be/⁹Be)₀ = ∼0.7 × 10^–3 recorded in CAIs from CV chondrites. The apparent variation in (¹⁰Be/⁹Be)₀ between the CO and CV CAIs, despite having indistinguishable (²⁶Al/²⁷Al)₀ of ∼5 × 10^–5, provides evidence for heterogeneous distribution of ¹⁰Be in the CAI forming-regions at the very beginning of the Solar System. The elevated (¹⁰Be/⁹Be)₀ and group II REE patterns in the CO CAIs may reflect that compared with the CV CAIs having unfractionated REEs the present CO CAIs have formed closer to the Sun where ¹⁰Be was produced more efficiently through solar cosmic ray irradiation caused by solar flares. Alternatively, if the present CO CAIs and CV CAIs formed in the same region, and ²⁶Al was distributed homogeneously at the CAI-forming region, our results indicate that solar cosmic ray fluxes at the forming region have fluctuated by a factor of six within a short duration (∼0.2 million years) inferred from the Al-Mg chronology.