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 Δ 17 O of -4.2‰ to -5.3‰ have resolvable excess 26 Mg. Their inferred ( 26 Al/ 27 Al) 0 values range from (3.5 ± 1.3) × 10 -6 to (6.0 ± 3.9) × 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 ( 26 Al/ 27 Al) 0 of 5.23 × 10 -5 , and assuming homogeneously distributed 26Al that yielded a uniform initial 26 Al/ 27 Al in the Solar System. Seven chondrules lack resolvable excess 26 Mg. They have lower Mg#’s (94.2 to 98.7) and generally higher Δ 17 O (-0.9‰ to -4.9‰) than chondrules with resolvable excess 26 Mg. Their inferred ( 26 Al/ 27 Al) 0 upper limits range from 1.3 × 10 -6 to 3.2 × 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 27 Al/ 24 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 ( 26 Al/ 27 Al) 0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr.