^(36)Cl-^(36)S Systematics in Curious Marie: A ^(26)Mg-Rich U-Depleted Fine-Grained CAI from Allende

Abstract

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 either by local irradiation of gas and/or dust of solar composition [1-2] or by stellar nucleosynthesis in AGB stars or Type II supernovae [3]. Evidence for the presence of 36Cl in the early Solar System (ESS) comes from radiogenic excesses of ^(36)Ar [4] and/or ^(36)S [5-9] in secondary phases (e.g., sodalite and wadalite) in ESS materials such as Calcium, Aluminum-rich inclusions (CAIs) and chondrules. Though the presence of ^(36)Cl in the ESS has been demonstrated, the inferred initial ^(36)Cl/^(35)Cl ratios vary a lot (from 1.0×10^(-7) to 2×10^(-5)) from one inclusion to another [5-9]. Interestingly, all secondary phases measured so far lack resolvable ^(26)Mg excesses that could be due to the decay of ^(26)Al (t_(1/2) = 0.7 Myr), implying that ^(36)Cl and ^(26)Al may not have been derived from the same source. Given that ^(26)Al could have come from a stellar source [10] and that secondary phases should have formed late, we are left with either a very high ^(36)Cl/^(35)Cl initial ratio (~ 10^(-2)) in the ESS, or a late (> 3 Myr after CAI formation) irradiation scenario for the production of ^(36)Cl [9]. ^(36)Cl/^(35)Cl ~10^(-2) far exceeds the predictions from any model (stellar nucleosynthesis or irradiation); therefore, a late irradiation scenario producing ^(36)Cl at the observed level is favored. In this framework, ^(36)Cl is produced in the early solar nebula and incorporated into CAIs via aqueous activities, which could also lead to the formation of sodalite.