THE DISTRIBUTION OF AL-26 IN THE EARLY SOLAR-SYSTEM - A REAPPRAISAL

A compilation of over 1500 Mg-isotopic analyses of Al-rich material fr om primitive solar system matter (meteorites) shows clearly that Al-26 existed live in the early Solar System. Excesses of Mg-26 observed in refractory inclusions are not the result of mixing of ''fossil'' inte rstellar (26)M,Ig with normal solar system Mg. Some material was prese nt that contained little or no Al-26, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chond rites accreted and probably was a minor component in the accretion reg ions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebula r histories of CAIs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formatio n require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether Al-26 was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.