The Initial Abundance of 60 Fe in the Solar System

60 Fe, which decays to radiogenic 60 Ni ( 60 Ni*), is a now extinct radionuclide. 60 Fe 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 × 10 6 yr (1.49 Myr)] makes it a potential chronometer for the early solar system. We found clear evidence for 60 Ni* 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 60 Fe/ 56 Fe ratios [( 60 Fe/ 56 Fe) 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 ( 60 Fe/ 56 Fe) 0 for a chondrule in an unequilibrated ordinary chondrite, Semarkona, and for troilites in a relatively metamorphosed chondrite, Ste. Marguerite, taking into account their 26 Al- 26 Mg ages. The 60 Fe and 26 Al 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 60 Fe/ 56 Fe 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.