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 36 S excesses that linearly correlate with the chlorine concentration, providing strong evidence in support of the existence of the short-lived nuclide 36 Cl (mean life of 0.43 Myr) in the early solar system. The inferred 36 Cl/ 35 Cl ratios at the time when these phases formed are very high (~4 × 10 -6 ) and essentially the same for the inclusion and the chondrule and confirm the earlier report of 36 S excess in another meteorite. In addition, the 36 Cl is decoupled from 26 Al. The observed and any possible higher levels of 36 Cl 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 10 Be. It is now clear that a blend of several sources is required to explain the short-lived nuclei when the solar system formed.