Evidence for Widespread 26 Al in the Solar Nebula and Constraints for Nebula Time Scales

• DOI: 10.1126/science.273.5276.757
• Publication Date: 2006-10-27T18:30:41Z
• Authors: S. S. Russell; G. Srinivasan; G. R. Huss; G. J. Wasserburg; G. J. MacPherson
• Keywords: 01 natural sciences; 0105 earth and related environmental sciences

A search was made for 26 Mg ( 26 Mg*) from the decay of 26 Al (half-life = 0.73 million years) in Al-rich objects from unequilibrated ordinary chondrites. Two Ca-Al-rich inclusions (CAIs) and two Al-rich chondrules (not CAIs) were found that contained 26 Al when they formed. Internal isochrons for the CAIs yielded an initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of 5 × 10 −5 , indistinguishable from most CAIs in carbonaceous chondrites. This result shows that CAIs with this level of 26 Al are present throughout the classes of chondrites and strengthens the notion that 26 Al was widespread in the early solar system. The two Al-rich chondrules have lower 26 Mg*, corresponding to a ( 26 Al/ 27 Al) 0 ratio of ∼9 × 10 −6 . Five other Al-rich chondrules contain no resolvable 26 Mg*. If chondrules and CAIs formed from an isotopically homogeneous reservoir, then the chondrules with 26 Al must have formed or been last altered ∼2 million years after CAIs formed; the 26 Mg*-free chondrules formed >1 to 3 million years later still. Because 26 Mg*-containing and 26 Mg*-free chondrules are both found in Chainpur, which was not heated to more than ∼400°C, it follows that parent body metamorphism cannot explain the absence of 26 Mg* in some of these chondrules. Rather, its absence indicates that the lifetime of the solar nebula over which CAIs and chondrules formed extended over ∼5 million years.