New constraints from 26Al-26Mg chronology of anorthite bearing chondrules in unequilibrated ordinary chondrites

Abstract

Abstract 26Al-26Mg ages were determined for 14 anorthite-bearing chondrules from five different unequilibrated ordinary chondrites (UOCs) with low petrologic subtypes (3.00–3.05). In addition, oxygen three isotopes of these chondrules were also measured. The selected chondrules are highly depleted in alkali elements, and anorthite is the only mesostasis phase, though they show a range of mafic mineral compositions (Mg# 76–97 mole%) that are representative of chondrules in UOCs. The mean Δ17O values in these chondrules range from −0.44\u202f±\u202f0.23‰ to 0.49\u202f±\u202f0.15‰, in good agreement with previous studies of plagioclase-bearing chondrules from UOCs. Anorthite in all chondrules exhibit resolvable excess 26Mg (>1.0\u202f±\u202f0.4‰). Their inferred (27Al/26Al)0 range from (6.3\u202f±\u202f0.7)\u202f×\u202f10−6 to (8.9\u202f±\u202f0.3)\u202f×\u202f10−6 corresponding to a timescale for chondrule formation of 1.8\u202f±\u202f0.04\u202fMa to 2.16 ± 0.12/0.11 Ma after CAIs using a canonical (27Al/26Al)0 value of 5.25\u202f×\u202f10−5. The ages from six chondrules in LL chondrites are restricted to between 1.8\u202fMa and 1.9\u202fMa, whereas eight chondrules in L chondrites show ages from 1.8\u202fMa to 2.2\u202fMa, including three chondrules at ∼2.0\u202fMa and two chondrules at ∼2.15\u202fMa. The inferred chondrule formation ages from this study are at the peak of those previously determined for UOC chondrules, though with much shorter durations. This is potentially due to the time difference between formation of anorthite-bearing chondrules and typical UOC chondrules with alkali-rich compositions. Alternatively, younger chondrules ages in previous studies could have been the result of disturbance to the Al-Mg system in glassy mesostasis even at the low degree of thermal metamorphism in the parent bodies. Nevertheless, the high precision ages from this study (with uncertainties from 0.04\u202fMa to 0.15\u202fMa) indicate that there was potentially more than one chondrule forming event represented in the studied population. Considering data from LL chondrites only, the restricted duration (≤0.1\u202fMa) of chondrule formation ages suggests an origin in high density environments that subsequently lead to parent body formation. However, the unusually low alkali contents of the studied chondrules compared to common alkali-rich chondrules could also represent earlier chondrule formation events under relatively lower dust densities in the disk. Major chondrule forming events for UOCs might have postdated or concurrent with the younger anorthite-bearing chondrule formation at 2.15\u202fMa after CAIs, which are very close to the timing of accretion of ordinary chondrite parent bodies that are expected from thermal evolution of ordinary chondrite parent bodies.