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

• Published in: Astrophysical journal. Letters Vol. 711; no. 2
• Main Authors: MacPherson, G. J., Bullock, E. S., Janney, P. E., Wadhwa, M., Kita, N. T., Ushikubo, T., Davis, A. M., Krot, A. N.
• Format: Journal Article
• Language: English
• Published: United States 10.03.2010
• Subjects: ABLATION; ALUMINIUM 26; ALUMINIUM 27; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CALCIUM; ELEMENT ABUNDANCE; GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; ICP MASS SPECTROSCOPY; ION MICROPROBE ANALYSIS; ISOTOPE RATIO; MAGNESIUM; MAGNESIUM ISOTOPES; MELTING; METEORITES; MINERALS; MIXTURES; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; PROTOPLANETS; RECRYSTALLIZATION; SOLAR NEBULA; SOLAR SYSTEM
• DOI: 10.1088/2041-8205/711/2/L117;COUNTRYOFINPUT:INTERNATIONALATOMICENERGYAGENCY(IAEA)
• ISSN: 2041-8205

The short-lived radionuclide {sup 26}Al existed throughout the solar nebula 4.57 Ga ago, and the initial abundance ratio ({sup 26}Al/{sup 27}Al){sub 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 ({sup 26}Al/{sup 27}Al){sub 0} {approx} (4-5) x 10{sup -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' ({sup 26}Al/{sup 27}Al){sub 0} ranging from (5.85 {+-} 0.05) x 10{sup -5} to >7 x 10{sup -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 ({sup 26}Al/{sup 27}Al){sub 0} = (5.27 {+-} 0.17) x 10{sup -5}. Thus our data do not support the existence of supracanonical ({sup 26}Al/{sup 27}Al){sub 0} = (5.85-7) x 10{sup -5}. There may not have been a significant time interval between condensation of the CAI precursors and their subsequent melting into coarse-grained CAIs.