³⁶Cl-³⁶S in Allende CAIs: Implication for the origins of ³⁶Cl in the early solar system

Abstract

Chlorine-36 (t_(1/2)=0.3 Myr) decays to either ³⁶Ar (98%, β-) or ³⁶S (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 ³⁶Cl in the Early Solar System (ESS) comes from radiogenic excesses of ³⁶Ar [4,5] and/or ³⁶S [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 ³⁶Cl/³⁵Cl ratios vary over three orders of magnitude among different chondrite constituents (5×10⁻⁶-9×10⁻³) [6-9]. Interestingly, although the initial ³⁶Cl/³⁵Cl ratios inferred in previous studies vary widely, all secondary phases bearing evidence for live ³⁶Cl in the ESS measured so far lack resolvable ²⁶Mg excesses due to the decay of ²⁶Al (t_1/2 = 0.7 Myr), implying that ³⁶Cl and ²⁶Al 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 ³⁶S anomalies in secondary phases point to either a very high ³⁶Cl/³⁵Cl initial ratio (~10⁻²) in the ESS, or a late irradiation scenario for the local production of ³⁶Cl (>3 Myr after CAI formation) [9]. The elevated ESS ratio of ³⁶Cl/³⁵Cl ~10⁻² inferred from [9] far exceeds the predictions from any model of stellar nucleosynthesis; therefore, a late irradiation scenario producing ³⁶Cl is currently the favored idea. In this framework, ³⁶Cl would be be produced in the nebular gas and then incorporated into the CAIs via aqueous alteration, which formed secondary phases.