The Initial Abundance of ⁶⁰Fe in the Solar System

⁶⁰Fe, which decays to radiogenic ⁶⁰Ni (⁶⁰Ni*), is a now extinct radionuclide. ⁶⁰Fe is produced only in stars and thus provides a constraint on the stellar contribution to solar system radionuclides. Its short half-life [t_1/2 = 1.49 × 10⁶ yr (1.49 Myr)] makes it a potential chronometer for the early solar system. We found clear evidence for ⁶⁰Ni* in troilite (FeS) grains from the Bishunpur and Krymka chondrites, two of the least metamorphosed (LL3.1) ordinary chondrites. The weighted means of inferred initial ⁶⁰Fe/⁵⁶Fe ratios [(⁶⁰Fe/⁵⁶Fe)₀] for the troilites are (1.08 ± 0.23) × 10^-7 and (1.73 ± 0.53) × 10^-7 for Bishunpur and Krymka, respectively. We compare our data with upper limits established previously on (⁶⁰Fe/⁵⁶Fe)₀ for a chondrule in an unequilibrated ordinary chondrite, Semarkona, and for troilites in a relatively metamorphosed chondrite, Ste. Marguerite, taking into account their ²⁶Al-²⁶Mg ages. The ⁶⁰Fe and ²⁶Al chronometers can be combined to produce a consistent chronology for Ca-Al-rich inclusions, which are thought to be the earliest solar system solids, chondrules, troilites, and Ste. Marguerite. The initial ⁶⁰Fe/⁵⁶Fe for the solar system is inferred from this chronology to have been 2.8 × 10^-7 to 4 × 10^-7. This is at or below the low end of predictions for a supernova source.