Extended chondrule formation intervals in distinct physicochemical environments: Evidence from Al-Mg isotope systematics of CR chondrite chondrules with unaltered plagioclase.

Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#'s of 99.0 to 99.2 and ΔO of -4.2‰ to -5.3‰ have resolvable excess Mg. Their inferred (Al/Al) values range from (3.5 ± 1.3) × 10 to (6.0 ± 3.9) × 10. This corresponds to formation times of 2.2 (-0.5/+1.1) Myr to 2.8 (‒0.3/+0.5) Myr after CAIs, using a canonical (Al/Al) of 5.23 × 10, and assuming homogeneously distributed Al that yielded a uniform initial Al/Al in the Solar System. Seven chondrules lack resolvable excess Mg. They have lower Mg#'s (94.2 to 98.7) and generally higher ΔO (-0.9‰ to -4.9‰) than chondrules with resolvable excess Mg. Their inferred (Al/Al) upper limits range from 1.3 × 10 to 3.2 × 10, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS Al/Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00-3.05 chondrites. We conclude that the resolvable (Al/Al) variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr. Using relationships between chondrule (Al/Al), Mg#, and ΔO, we interpret spatial and temporal features of dust, gas, and HO ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, ΔO ~-5‰ chondrules with resolvable excess Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ~100×. After these chondrules formed, we interpret a later influx of O-poor HO ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94-99 chondrules with higher ΔO (-5‰ to -1‰), with low (Al/Al), and hence no resolvable excess Mg. We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (Al/Al) and that Mg# < 99 chondrules generally have no resolvable excess Mg, implying lower (Al/Al). We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.