Early solar nebula condensates with canonical, not supracanonical, initial 26Al/27Al ratios

G. J. MacPherson, E. S. Bullock, P. E. Janney, N. T. Kita, T. Ushikubo, A. M. Davis, Meenakshi Wadhwa, A. N. Krot

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65 Scopus citations


The short-lived radionuclide 26Al existed throughout the solar nebula 4.57 Ga ago, and the initial abundance ratio (26Al/ 27Al)0, as inferred from magnesium isotopic compositions of calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites, has become a benchmark for understanding early solar system chronology. Internal mineral isochrons in most CAIs measured by secondary ion mass spectrometry (SIMS) give (26Al/27Al)0 ∼ (4-5) × 10 -5, called "canonical." Some recent high-precision analyses of (1) bulk CAIs measured by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), (2) individual CAI minerals and their mixtures measured by laser-ablation MC-ICPMS, and (3) internal isochrons measured by multicollector (MC)-SIMS indicated a somewhat higher "supracanonical" (26Al/27Al)0 ranging from (5.85 ± 0.05) × 10-5 to >7 × 10-5. These measurements were done on coarse-grained Type B and Type A CAIs that probably formed by recrystallization and/or melting of fine-grained condensate precursors. Thus the supracanonical ratios might record an earlier event, the actual nebular condensation of the CAI precursors. We tested this idea by performing in situ high-precision magnesium isotope measurements of individual minerals in a fine-grained CAI whose structures and volatility-fractionated trace element abundances mark it as a primary solar nebula condensate. Such CAIs are ideal candidates for the fine-grained precursors to the coarse-grained CAIs, and thus should best preserve a supracanonical ratio. Yet, our measured internal isochron yields (26Al/27Al)0 = (5.27 ± 0.17) × 10-5. Thus our data do not support the existence of supracanonical (26Al/27Al)0 = (5.85-7) × 10-5. There may not have been a significant time interval between condensation of the CAI precursors and their subsequent melting into coarse-grained CAIs.

Original languageEnglish (US)
Pages (from-to)L117-L121
JournalAstrophysical Journal Letters
Issue number2 PART 2
StatePublished - 2010


  • Astrochemistry
  • Nuclear reactions, nucleosynthesis, abundances
  • Protoplanetary disks

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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