Early Solar Nebula Condensates with Canonical, Not Supracanonical, Initial 26Al/27Al Ratios

Details Early Solar Nebula Condensates with Canonical, Not Supracanonical, Initial 26Al/27Al Ratios Early Solar Nebula Condensates with Canonical, Not Supracanonical, Initial 26Al/27Al Ratios

Mar 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010apj...711l.117m&link_type=abstract

The Astrophysical Journal Letters, Volume 711, Issue 2, pp. L117-L121 (2010).

Astronomy and Astrophysics

Astronomy

9

Astrochemistry, Nuclear Reactions, Nucleosynthesis, Abundances, Protoplanetary Disks

Scientific paper

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.

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