EARLY SOLAR NEBULA CONDENSATES WITH CANONICAL, NOT SUPRACANONICAL, INITIAL 26 Al/ 27 Al RATIOS

Abstract

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.