A multielement isotopic study of refractory FUN and F CAIs: Mass-dependent and mass-independent isotope effects

Levke Kööp, Daisuke Nakashima, Philipp R. Heck, Noriko T. Kita, Travis J. Tenner, Alexander N. Krot, Kazuhide Nagashima, Changkun Park, Andrew M. Davis

Research output: Contribution to journalArticlepeer-review

19 Citations (Scopus)

Abstract

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated objects that formed inside the Solar System. Among these are rare, enigmatic objects with large mass-dependent fractionation effects (F CAIs), which sometimes also have large nucleosynthetic anomalies and a low initial abundance of the short-lived radionuclide 26Al (FUN CAIs). We have studied seven refractory hibonite-rich CAIs and one grossite-rich CAI from the Murchison (CM2) meteorite for their oxygen, calcium, and titanium isotopic compositions. The 26Al-26Mg system was also studied in seven of these CAIs. We found mass-dependent heavy isotope enrichment in all measured elements, but never simultaneously in the same CAI. The data are hard to reconcile with a single-stage melt evaporation origin and may require reintroduction or reequilibration for magnesium, oxygen and titanium after evaporation for some of the studied CAIs. The initial 26Al/27Al ratios inferred from model isochrons span a range from <1 × 10−6 to canonical (∼5 × 10−5). The CAIs show a mutual exclusivity relationship between inferred incorporation of live 26Al and the presence of resolvable anomalies in 48Ca and 50Ti. Furthermore, a relationship exists between 26Al incorporation and Δ17O in the hibonite-rich CAIs (i.e., 26Al-free CAIs have resolved variations in Δ17O, while CAIs with resolved 26Mg excesses have Δ17O values close to −23‰). Only the grossite-rich CAI has a relatively enhanced Δ17O value (∼−17‰) in spite of a near-canonical 26Al/27Al. We interpret these data as indicating that fractionated hibonite-rich CAIs formed over an extended time period and sampled multiple stages in the isotopic evolution of the solar nebula, including: (1) an 26Al-poor nebula with large positive and negative anomalies in 48Ca and 50Ti and variable Δ17O; (2) a stage of 26Al-admixture, during which anomalies in 48Ca and 50Ti had been largely diluted and a Δ17O value of ∼−23‰ had been achieved in the CAI formation region; and (3) a nebula with an approximately canonical level of 26Al and a Δ17O value of ∼−23‰ in the CAI formation region.

Original languageEnglish
Pages (from-to)296-317
Number of pages22
JournalGeochimica et Cosmochimica Acta
Volume221
DOIs
Publication statusPublished - 2018 Jan 15

Keywords

  • Aluminum-26
  • CAI
  • Calcium isotopes
  • Fractionated and unidentified nuclear (FUN)
  • Hibonite
  • Magnesium isotopes
  • Mass-dependent fractionation
  • Meteorites
  • Oxygen isotopes
  • Solar nebula
  • Titanium isotopes

ASJC Scopus subject areas

  • Geochemistry and Petrology

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