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

Abstract

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., ²⁶Al, ⁴¹Ca, and ¹⁸²Hf) 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 ²⁶Al corresponding to ²⁶Al/²⁷Al of ∼5 × 10⁻⁵, rare CAIs with fractionation and unidentified nuclear isotope effects (FUN CAIs) record nucleosynthetic isotopic heterogeneity and ²⁶Al/²⁷Al 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 ¹⁸²Hf–¹⁸²W chronometer, we show that a FUN CAI recording a condensation origin from a solar gas formed coevally with canonical CAIs, but with ²⁶Al/²⁷Al of ∼3 × 10⁻⁶. The decoupling between ¹⁸²Hf and ²⁶Al requires distinct stellar origins: steady-state galactic stellar nucleosynthesis for ¹⁸²Hf and late-stage contamination of the protosolar molecular cloud by a massive star(s) for ²⁶Al. Admixing of stellar-derived ²⁶Al to the protoplanetary disk occurred during the epoch of CAI formation and, therefore, the ²⁶Al–²⁶Mg systematics of CAIs cannot be used to define their formation interval. In contrast, our results support ¹⁸²Hf homogeneity and chronological significance of the ¹⁸²Hf–¹⁸²W clock.