ABSTRACT

We report Al-Mg isotope systematics in fourteen mineralogically pristine chondrules from six CR2 (Renazzo-type) carbonaceous chondrites measured with secondary ion mass spectrometry. Six chondrules show resolvable excesses of ²⁶Mg that correlate with Al/Mg ratios, indicating in situ decay of ²⁶Al. The inferred initial ²⁶Al/²⁷Al ratios [(²⁶Al/²⁷Al)₀] in these chondrules range from ∼1 × 10^-6 to ∼6 × 10^-6. The other eight chondrules show no detectable ²⁶Mg excesses and have upper limits of (²⁶Al/²⁷Al)₀ ∼(2-3) × 10^-6. Most CR chondrules have (²⁶Al/²⁷Al)₀ significantly lower than those in the other least metamorphosed primitive chondrites, LL3.0, CO3.0, and Acfer 094 (ungrouped). Assuming uniform distribution of ²⁶Al in the protoplanetary disk, these observations suggest that majority of CR chondrules formed >1 Myr later than those in LL3.0, CO3.0, and Acfer 094.