²⁶Al in chondrules from unequilibrated L chondrites: onset and duration of chondrule formation in the early solar system

Abstract

Al-Mg isotope records in sixteen ferromagnesian chondrules from five unequilibrated ordinary chondrites (UOCs), four L chondrites with petrologic grades varying from 3.0 to 3.2 and one L/LL chondrite with petrologic grade 3.1, were studied using an ion microprobe to look for presence of the now-extinct nuclide ²⁶Al at the time of their formation. UOCs of low petrologic grades were selected to avoid possible thermal perturbation of Al-Mg isotope records in the analyzed chondrules. The presence of resolved ²⁶Mg excess (due to decay of ²⁶Al) in Al-rich phases, such as glassy mesostasis and plagioclase, was found in all the chondrules, barring a sole exception. The inferred initial ²⁶Al/²⁷Al ratios in the chondrules range from (0.6-1.9) × 10^{- 5}, and most of them have values lying between (0.6-1.1) × 10^{- 5}. Our data supports a late formation of chondrules relative to CAIs, suggested in previous studies, and indicates a time lag of at least 1.5 Ma between CAI formation and onset of an intense episode of chondrule formation in the early solar system that lasted less than a million years. The paucity of chondrules (10%) with initial ²⁶Al/²⁷Al 10^{- 5} rules out active chondrule forming events at earlier epochs. The presence of rare chondrules with initial ²⁶Al/²⁷Al 0.5 × 10^{- 5} in UOCs of low petrologic grades (3.0 to 3.2) analyzed by us and also reported previously in UOCs of petrologic grade ranging up to 3.4, most probably represent parent body processes leading to disturbance of Al-Mg isotope systematics in these cases. An extended duration of chondrule formation lasting more than a couple of million years, suggested in some earlier studies, needs reassessment. A long duration of chondrule formation is also difficult to reconcile with the problem of storage, isolation, mixing and assimilation of chondrules into their meteorite parent bodies.