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Chronology of the early Solar System from chondrule-bearing calcium-aluminium-rich inclusions

Abstract

Chondrules and Ca-Al-rich inclusions (CAIs) are high-temperature components of meteorites that formed during transient heating events in the early Solar System. A major unresolved issue is the relative timing of CAI and chondrule formation1,2,3,4. From the presence of chondrule fragments in an igneous CAI, it was concluded that some chondrules formed before CAIs (ref. 5). This conclusion is contrary to the presence of relict CAIs inside chondrules6,7,8,9,10, as well as to the higher abundance of 26Al in CAIs11; both observations indicate that CAIs pre-date chondrules by 1–3 million years (Myr). Here we report that relict chondrule material in the Allende meteorite, composed of olivine and low-calcium pyroxene, occurs in the outer portions of two CAIs and is 16O-poor (Δ17O ≈ - 1‰ to -5‰). Spinel and diopside in the CAI cores are 16O-rich (Δ17O up to -20‰), whereas diopside in their outer zones, as well as melilite and anorthite, are 16O-depleted (Δ17O = -8‰ to 2‰). Both chondrule-bearing CAIs are 26Al-poor with initial 26Al/27Al ratios of (4.7 ± 1.4) × 10-6 and <1.2 × 10-6. We conclude that these CAIs had chondrule material added to them during a re-melting episode 2 Myr after formation of CAIs with the canonical 26Al/27Al ratio of 5 × 10-5.

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Figure 1: Type C CAI fragment ABC from Allende.
Figure 2: Oxygen isotopic compositions of chondrule-bearing CAIs from Allende.
Figure 3: Al-Mg evolution diagram for the type C CAIs ABC and TS26.
Figure 4: Type C CAI TS26 from Allende.

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Acknowledgements

Financial support for this project was provided by NASA (A.N.K., I.D.H., G.J.M.) and Monkasho (H.Y.). We thank R. H. Hewins for comments and suggestions.

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Correspondence to Alexander N. Krot.

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Supplementary information

Supplementary Figure S1

Oxide wt% vs. distance (µm) for compositional profiles AB and CD. (JPG 441 kb)

Supplementary Figure S2

Optical micrograph in transmitted light and combined X-ray elemental map of the CAI TS26. (JPG 867 kb)

Supplementary Figure S3

Bulk compositions of the chondrule-bearing Type C CAIs ABC plotted on a triangular diagram. (JPG 197 kb)

Supplementary Figure Legends

Legends to accompany the above Supplementary Figures. (DOC 20 kb)

Supplementary Table S1

Representative electron microprobe analyses of primary minerals in the chondrule-bearing CAIs ABC and TS26 from the CV carbonaceous chondrite Allende. (DOC 75 kb)

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Krot, A., Yurimoto, H., Hutcheon, I. et al. Chronology of the early Solar System from chondrule-bearing calcium-aluminium-rich inclusions. Nature 434, 998–1001 (2005). https://doi.org/10.1038/nature03470

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