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

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.