from Part I - Observations of Chondrules
Published online by Cambridge University Press: 30 June 2018
The 26Al–26Mg systematics of chondrules from ordinary and carbonaceous chondrites and their implications are reviewed. The initial 26Al/27Al ratios [(26Al/27Al)0] based on in situ analyses of chondrules from the least metamorphosed chondrites range from unresolved from zero to ~1.2 × 10‒5 and thus no chondrules have A26l/A27l0Internal ratios corresponding to the canonical level (~5.2 × 10‒5) recorded by CAIs. Assuming homogeneous distribution of 26Al in the protoplanetary disk at the canonical level, these observations suggest chondrule formation started ~1.5 million years after CAIs and lasted over a few million years. The 26Al–26Mg systematics of bulk chondrules could have recorded A26l/A27l0Bulk
ratios of chondrule precursors and may suggest that Al–Mg fractionation recorded by chondrule precursors started contemporaneously with CAIs and lasted over ~1.5 million years. The comparisons of formation ages of different meteorites and their components have been made with 26Al–26Mg, 182Hf–182W, and 206Pb–207Pb systematics. While the ages determined by 26Al–26Mg and 182Hf–182W systematics are generally consistent, those determined by 26Al–26Mg and 206Pb–207Pb systematics are largely inconsistent. The homogeneous versus heterogeneous distribution of 26Al in the protoplanetary disk remains controversial.
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