Initial ²⁶Al/²⁷Al in carbonaceous-chondrite chondrules: Too little ²⁶Al to melt asteroids

We report ²⁶Mg excesses correlated with Al/Mg ratios in five chondrules from the primitive CO3.0 chondrite Yamato 81020 that yield a mean initial ²⁶Al/²⁷Al ratio of only (3.8 ± 0.7) × 10^-6, about half that of ordinary chondrite (OC) chondrules. Even if asteroids formed immediately after chondrule formation, this ratio and the mean Al content of CO chondrites is only capable of raising the temperature of a well-insulated CO asteroid to 940 K, which is more than 560 K too low to produce differentiation. The same ratio combined with the higher Al content of CV chondrites results in a CV asteroid temperature of 1100 K. We calculate that the mean initial ²⁶Al/²⁷Al ratio of about 7.4 × 10^-6 found in LL chondrules is only able to produce small amounts of melting, too little to produce differentiation. These results cast serious doubt on the viability of ²⁶Al as the heat source responsible for asteroid differentiation. Inclusion of ⁶⁰Fe raises temperatures about 160 K, but this increment is not enough to cause differentiation, even of an LL-chondrite asteroid.