A Late Episode of Irradiation in the Early Solar System: Evidence from Extinct36Cl and26Al in Meteorites

Late-formed halogen-rich phases in a refractory inclusion and a chondrule from the Allende meteorite exhibit large S-36 excesses that linearly correlate with the chlorine concentration, providing strong evidence in support of the existence of the short-lived nuclide Cl-36 (mean life of 0.43 Myr) in the early solar system. The inferred Cl-36/Cl-35 ratios at the time when these phases formed are very high (similar to 4 x 10(-6)) and essentially the same for the inclusion and the chondrule and confirm the earlier report of S-36 excess in another meteorite. In addition, the Cl-36 is decoupled from Al-26. The observed and any possible higher levels of Cl-36 cannot be the result of a supernova or AGB stellar source but require a late episode of energetic particle bombardment by the early Sun, in support of the arguments based on the previous discovery of Be-10. It is now clear that a blend of several sources is required to explain the short-lived nuclei when the solar system formed.