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 99.0-99.2 and Delta O-17 of -4.2 parts per thousand to -5.3 parts per thousand have resolvable excess Mg-26. Their inferred (Al-26/Al-27)(0) values range from (3.5 +/- 1.3) x 10(-6) to (6.0 +/- 3.9) x 10(-6). 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-26/Al-27)(0) of 5.23 x 10(-5), and assuming homogeneously distributed Al-26 that yielded a uniform initial Al-26/Al-27 in the Solar System. Seven chondrules lack resolvable excess Mg-26. They have lower Mg#'s (94.2-98.7) and generally higher Delta O-17 (-0.9 parts per thousand to -4.9 parts per thousand) than chondrules with resolvable excess Mg-26. Their inferred (Al-26/Al-27)(0) upper limits range from 1.3 x 10(-6) to 3.2 x 10(-6), 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-27/Mg-24 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-26/Al-27)(0) variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr. Using relationships between chondrule (Al-26/Al-27)(0), Mg#, and Delta O-17, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# >= 99, Delta O-17 similar to -5 parts per thousand chondrules with resolvable excess Mg-26 initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of similar to 100 x . After these chondrules formed, we interpret a later influx of O-16-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100x to 300x). This led to the later formation of more oxidized Mg# 94-99 chondrules with higher Delta O-17 (-5 parts per thousand to -1 parts per thousand), with low (Al-26/Al-27)(0), and hence no resolvable excess Mg-26. We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# >= 99 chondrules generally have resolved positive (Al-26/Al-27)(0) and that Mg# < 99 chondrules generally have no resolvable excess Mg-26, implying lower (Al-26/Al-27)(0). 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. (C) 2019 Elsevier Ltd. All rights reserved.