High precision 26Al-26Mg chronology of chondrules in unequilibrated ordinary chondrites: Evidence for restricted formation ages

Abstract

Chondrules in ordinary chondrites are considered to form in high density environments, likely related to the evolution of protoplanets and large planetesimals. In order to determine the timing of their formation at high time resolution (≤0.1 Ma), we conducted high precision Al-Mg chronology of 17 porphyritic chondrules from 6 different unequilibrated ordinary chondrites (UOCs) of low petrologic subtypes (3.00–3.05). Detailed petrology, mineralogy, and oxygen isotope ratios of individual chondrules were also obtained that include 10 additional chondrules without Al-Mg ages. Seventeen chondrules for Al-Mg chronology consist of 14 chondrules with plagioclase (An1-An87) and three chondrules with Na-rich glassy mesostasis, all of which have high 27Al/24Mg ratios (30–3000). The inferred initial (26Al/27Al)0 ratios range between (6.5 ± 0.6) × 10–6 to (9.5 ± 1.0) × 10–6, corresponding to chondrule formation ages of 1.74 ± 0.12/0.11 Ma to 2.13 ± 0.09 Ma after CAIs, which have a canonical (26Al/27Al)0 ratio of 5.25 × 10–5. Six albite-bearing chondrules (An<30) show a much more restricted ages range, spanning between 2.00 ± 0.11/0.10 Ma and 2.07 ± 0.11/0.10 Ma. Including 14 anorthite-bearing chondrules studied previously, chondrules in ordinary chondrites have a restricted range of formation ages from 1.8 Ma to 2.2. Ma after CAIs.Based on the newly acquired oxygen isotope data and previous high precision studies, chondrules in L and LL chondrites do not show systematic difference in their δ18O and δ17O signatures. Majority of plagioclase-bearing chondrules studied for Al-Mg chronology show similar oxygen isotope ratios to those of glass-bearing chondrules. There is no obvious difference in Al-Mg ages of chondrules between L and LL chondrites. Thus, chondrules in L and LL chondrites would have formed in common environments and processes, though they accreted to two separate parent bodies by 2.2 Ma after CAIs, which timing is consistent with the proposed thermal model for ordinary chondrite parent bodies. Onset of chondrule formation at 1.8 Ma after CAIs may be caused by the delay of Jupiter formation or the formation of protoplanets in ordinary chondrite chondrule forming regions if chondrules formed by large scale disk shock or impact jetting of protoplanets. Alternatively, early formed chondrules would not be preserved before 1.8 Ma if chondrules formed by the impacts of molten planetesimals.