FORMATION OF THE SHORT-LIVED RADIONUCLIDE 36Cl IN THE PROTOPLANETARY DISK DURING LATE-STAGE IRRADIATION OF A VOLATILE-RICH RESERVOIR

Short-lived radionuclides (SLRs) in the early solar system provide fundamental insight into protoplanetary disk evolution. We measured the Cl-36-S-36-isotope abundance in wadalite (<15 mu m), a secondary chlorine-bearing mineral found in calcium-aluminum-rich inclusions (CAIs) in the Allende CV chondrite, to decipher the origin of the SLR Cl-36 (tau(1/2) similar to 3 x 10(5) yr) in the early solar system. Its presence, initial abundance, and the noticeable decoupling from Al-26 raise serious questions about the origin of SLRs. The inferred initial Cl-36 abundance for wadalite, corresponding to a Cl-36/Cl-35 ratio of (1.81 +/- 0.13) x 10(-5), is the highest Cl-36 abundance ever reported in any early solar system material. The high level of 36Cl in wadalite and the absence of Al-26 (Al-26/Al-27 <= 3.9 x 10(-6)) in co-existing grossular (1) unequivocally support the production of Cl-36 by late-stage solar energetic particle irradiation in the protoplanetary disk and (2) indicates that the production of Cl-36, recorded by wadalite, is unrelated to the origin of Al-26 and other SLRs (Be-10, Mn-53) recorded by primary minerals of CAIs and chondrules. We infer that Cl-36 was largely produced by irradiation of a volatile-rich reservoir in an optically thin protoplanetary disk adjacent to the region in which the CV chondrite parent asteroid accreted while the Sun was a weak T Tauri star. Subsequently, Cl-36 accreted into the Allende CV chondrite together with condensed water ices.