The Timing of Potential Last Nucleosynthetic Injections into the Protosolar Molecular Cloud Inferred from Ca-41-Al-26 Systematics of Bulk CAIs

Short-lived radionuclides (SLRs) provide important information about the chronology of the early solar system. Among them, Ca-41, due to its decay to K-41 with a half-life of only 0.1 Ma, is particularly valuable in constraining the timescales and origins of both SLRs and the formation of the oldest solar system materials, the Ca-Al-rich inclusions (CAIs). The initial abundance of Ca-41 in the solar system, expressed as the (Ca-41/Ca-40)(I) ratio, is the key to unveiling the origin of this nuclide. Here, we report a new solar system (Ca-41/Ca-40)(I) ratio of 2.0 x 10(-8) derived from the K isotope compositions of two CAIs. This new ratio is about four times higher than the previous value inferred from a mineral isochron. Such a high (Ca-41/Ca-40)(I) ratio in the CAIs exceeds that expected for the protosolar molecular cloud by similar to 1000x, implying very late injection of the Ca-41 (and possibly other SLRs) into the protosolar molecular cloud. The correlated enrichments of Ca-41 and Al-26 in the bulk CAI samples hint at a common stellar origin of both SLRs. The injection time estimated from our new data depends on the stellar source-it ranges from 0.6 Ma for a Wolf-Rayet wind to 1.0 Ma for a TP-AGB star ejecta.