The initial abundance of 60Fe in the solar system

Fe-60, which decays to radiogenic Ni-60 (Ni-60*), is a now extinct radionuclide. Fe-60 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 x 10(6) yr (1.49 Myr)] makes it a potential chronometer for the early solar system. We found clear evidence for Ni-60* 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 Fe-60/Fe-56 ratios [(Fe-60/Fe-56)(0)] for the troilites are (1.08 +/- 0.23) x 10(-7) and (1.73 +/- 0.53) x 10(-7) for Bishunpur and Krymka, respectively. We compare our data with upper limits established previously on (Fe-60/Fe-56)(0) for a chondrule in an unequilibrated ordinary chondrite, Semarkona, and for troilites in a relatively metamorphosed chondrite, Ste. Marguerite, taking into account their Al-26-Mg-26 ages. The Fe-60 and Al-26 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 Fe-60/Fe-56 for the solar system is inferred from this chronology to have been to 2.8 x 10(-7). This is at or below the low end of predictions for a supernova source.