26Al in chondrules from CR2 chondrites

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 A1/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.