EARLY SOLAR NEBULA CONDENSATES WITH CANONICAL, NOT SUPRACANONICAL, INITIAL {sup 26}Al/{sup 27}Al RATIOS
- Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 (United States)
- School of Earth and Space Exploration and Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287 (United States)
- WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 (United States)
- Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637 (United States)
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.
- OSTI ID:
- 21305099
- Journal Information:
- Astrophysical Journal Letters, Vol. 711, Issue 2; Other Information: DOI: 10.1088/2041-8205/711/2/L117; Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSMOLOGY AND ASTRONOMY
99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE
ABLATION
ALUMINIUM 26
ALUMINIUM 27
CALCIUM
ELEMENT ABUNDANCE
ICP MASS SPECTROSCOPY
ION MICROPROBE ANALYSIS
ISOTOPE RATIO
MAGNESIUM
MAGNESIUM ISOTOPES
MELTING
METEORITES
MINERALS
MIXTURES
NUCLEAR REACTIONS
NUCLEOSYNTHESIS
PROTOPLANETS
RECRYSTALLIZATION
SOLAR NEBULA
SOLAR SYSTEM