Formation of short-lived radionuclides during late-stage irradiation of protoplanetary disk
The origin of short-lived radionuclides (SLRs: 10Be, 26Al, 36Cl, 41Ca, 53Mn) in the early solar system is the subject of on-going debate: proposed mechanisms include injection of fresh stellar nucleosynthetic debris into the nascent solar nebula or irradiation of the protoplanetary disk by energetic particles. The external stellar origin of SLRs is currently favored, because the irradiation mechanism fails to explain the uniform initial abundance of 26Al, correlated occurrences of 26Al and 41Ca, and consistent chronologies based on the short-lived and long-lived radionuclides in the early solar system objects, such as CAIs and chondrules. Here we show that 36Cl-36S and 26Al-26Mg isotope abundances and petrographic observations of wadalite-sodalite-grossular paragenesis in the Allende CAIs provide evidence for a late-stage irradiation origin of 36Cl and possibly other short-lived radionuclides in the protoplanetary disk. The inferred irradiation epoch recorded by the aqueously produced secondary minerals, wadalite and sodalite, is unrelated to the early, stellar origin of 26Al, 41Ca, and 53Mn or irradiation origin of 10Be and, possibly, 7Be recorded by the primary minerals of CAIs and chondrules.