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Evidence for Widespread 26Al in the Solar Nebula and Constraints for Nebula Time Scales
1S. S. Russell and G. J. MacPherson are in the Department of Mineral Sciences, MRC NHB-119, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA. G. Srinivasan, G. R. Huss, and G. J. Wasserburg are in the Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
Science (New York, N.Y.)
|August 9, 1996
Summary
Aluminum-26 (26Al) was widespread in the early solar system, found in calcium-aluminum-rich inclusions (CAIs) and chondrules. Its presence indicates a solar nebula lifetime of approximately 5 million years for CAI and chondrule formation.
Area of Science:
- Cosmochemistry
- Planetary Science
- Isotope Geochemistry
Background:
- The short-lived radionuclide Aluminum-26 (26Al) is a key tracer for understanding early solar system processes.
- Previous studies indicated widespread 26Al in calcium-aluminum-rich inclusions (CAIs), but its distribution in chondrules was less clear.
Purpose of the Study:
- To investigate the presence and distribution of 26Al in Al-rich objects from unequilibrated ordinary chondrites.
- To constrain the formation timescales of CAIs and chondrules and the lifetime of the solar nebula.
Main Methods:
- Searched for 26Mg* (from 26Al decay) in Al-rich objects (CAIs and chondrules) using mass spectrometry.
- Analyzed internal isochrons to determine initial 26Al/27Al ratios [(26Al/27Al)0].
Main Results:
- Two CAIs and two Al-rich chondrules showed evidence of initial 26Al, with CAIs yielding (26Al/27Al)0 of 5 x 10(-5).
- Two other Al-rich chondrules had lower 26Al, while five contained no resolvable 26Mg*.
- The data suggest chondrules formed over a prolonged period, with some forming ~2 million years after CAIs and others >1-3 million years later.
Conclusions:
- The widespread presence of 26Al in CAIs across different meteorite classes supports its homogeneous distribution in the early solar system.
- The varying 26Al content in chondrules, found even in the same meteorite, indicates a solar nebula lifetime of approximately 5 million years for their formation.
- Parent body metamorphism cannot explain the absence of 26Al in some chondrules, further supporting extended formation timescales.