The Initial Abundance of 60 Fe in the Solar System

Abstract

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 [t1/2 = 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.