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

摘要: 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 with Mg#'s 99.0 to 99.2 Δ17O -4.2‰ -5.3‰ have resolvable excess 26Mg. Their inferred (26Al/27Al)0 values range (3.5 ± 1.3) × 10‒6 (6.0 3.9) 10‒6. This corresponds formation times 2.2 (-0.5/+1.1) Myr 2.8 (‒0.3/+0.5) after CAIs, using a canonical 5.23 10-5, assuming homogeneously distributed 26Al that yielded uniform initial 26Al/27Al in the Solar System. Seven lack They lower (94.2 98.7) generally higher (-0.9‰ -4.9‰) than upper limits 1.3 3.2 10‒6, corresponding >2.9 >3.7 CAIs. isochrons depend critically on chondrule plagioclase, several characteristics indicate plagioclase is unaltered: (1) SIMS 27Al/24Mg depth profile patterns match those anorthite standards, SEM/EDS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals nanometer-scale micro-inclusions alteration due thermal metamorphism; (3) oxygen isotopes coexisting olivine pyroxene, indicating low extent (4) microprobe data anorthite-rich, structural silica high MgO, consistent such other petrologic type 3.00-3.05 chondrites. We conclude variabilities among studied are robust, interval at least 1.1 Myr. Using relationships between (26Al/27Al)0, Mg#, Δ17O, we interpret spatial temporal features dust, gas, H2O ice chondrule-forming environment. Mg# ≥ 99, ~-5‰ 26Mg initially formed an environment was relatively anhydrous, dust-to-gas ratio ~100×. After these formed, later influx 16O-poor into environment, ratios expanded (100× 300×). led more oxidized 94-99 (-5‰ -1‰), hence refine mean chondrite age via balance, considering 99 resolved positive < 26Mg, implying (26Al/27Al)0. obtain 3.8 0.3 which Pb-Pb Hf-W model ages aggregates. Overall, this suggests most immediately before parent body accretion.