182 Hf– 182 W age dating of a 26 Al-poor inclusion and implications for the origin of short-lived radioisotopes in the early Solar System

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 110; no. 22; pp. 8819 - 8823
• Main Authors: Holst, Jesper C., Olsen, Mia B., Paton, Chad, Nagashima, Kazuhide, Schiller, Martin, Wielandt, Daniel, Larsen, Kirsten K., Connelly, James N., Jørgensen, Jes K., Krot, Alexander N., Nordlund, Åke, Bizzarro, Martin
• Format: Journal Article
• Language: English
• Published: 28.05.2013
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1300383110

Refractory inclusions [calcium–aluminum-rich inclusions, (CAIs)] represent the oldest Solar System solids and provide information regarding the formation of the Sun and its protoplanetary disk. CAIs contain evidence of now extinct short-lived radioisotopes (e.g., 26 Al, 41 Ca, and 182 Hf) synthesized in one or multiple stars and added to the protosolar molecular cloud before or during its collapse. Understanding how and when short-lived radioisotopes were added to the Solar System is necessary to assess their validity as chronometers and constrain the birthplace of the Sun. Whereas most CAIs formed with the canonical abundance of 26 Al corresponding to 26 Al/ 27 Al of ∼5 × 10 −5 , rare CAIs with fractionation and unidentified nuclear isotope effects (FUN CAIs) record nucleosynthetic isotopic heterogeneity and 26 Al/ 27 Al of <5 × 10 −6 , possibly reflecting their formation before canonical CAIs. Thus, FUN CAIs may provide a unique window into the earliest Solar System, including the origin of short-lived radioisotopes. However, their chronology is unknown. Using the 182 Hf– 182 W chronometer, we show that a FUN CAI recording a condensation origin from a solar gas formed coevally with canonical CAIs, but with 26 Al/ 27 Al of ∼3 × 10 −6 . The decoupling between 182 Hf and 26 Al requires distinct stellar origins: steady-state galactic stellar nucleosynthesis for 182 Hf and late-stage contamination of the protosolar molecular cloud by a massive star(s) for 26 Al. Admixing of stellar-derived 26 Al to the protoplanetary disk occurred during the epoch of CAI formation and, therefore, the 26 Al– 26 Mg systematics of CAIs cannot be used to define their formation interval. In contrast, our results support 182 Hf homogeneity and chronological significance of the 182 Hf– 182 W clock.