Chlorine-36 (t_(1/2)=0.3 Myr) decays to either ^(36)Ar (98%, β-) or ^(36)S (1.9%, ε and β+). This radionuclide
can be produced by either charged particle irradiation
[1,2] or stellar nucleosynthesis [3]. Evidence for the prior
existence of ^(36)Cl in the Early Solar System (ESS) comes
from radiogenic excesses of ^(36)Ar [4,5] and/or ^(36)S [6-9] in secondary phases (e.g., sodalite and wadalite) of ESS materials such as Ca, Al-rich inclusions (CAIs) and chondrules. However, the inferred initial ^(36)Cl/^(35)Cl ratios vary over three orders of magnitude among different chondrite constituents (5×10^(-6)-9×10^(-3)) [6-9]. Interestingly, although the initial ^(36)Cl/^(35)Cl ratios inferred in previous studies vary widely, all secondary phases bearing evidence for live ^(36)Cl in the ESS measured so far lack resolvable ^(26)Mg excesses
due to the decay of ^(26)Al (t_(1/2) = 0.7 Myr), implying that ^(36)Cl and ^(26)Al may have been produced by different processes and/or incorporated into ESS solids at different times. Given that secondary phases may have formed late, the ^(36)S anomalies in secondary phases point to either a very high ^(36)Cl/^(35)Cl initial ratio (~10^(-2)) in the ESS, or a late irradiation scenario for the local production of ^(36)Cl (> 3 Myr after CAI formation) [9]. The elevated ESS ratio of ^(36)Cl/^(35)Cl ~10^(-2) inferred from [9] far exceeds the predictions from any
model of stellar nucleosynthesis; therefore, a late irradiation scenario producing ^(36)Cl is currently the favored idea. In this framework, ^(36)Cl would be be produced in the nebular gas and then incorporated into the CAIs via aqueous alteration, which formed secondary phases
