The Abundance Of Live ⁶⁰Fe In The Early Solar System

Iron‐60 decays to ⁶⁰Ni with a half‐life of 1.49 million years (Myr). Because ⁶⁰Fe is produced only in stars, its initial abundance in the solar system provides a constraint on the stellar contribution of radionuclides to the solar system and on the nature of the stellar source. The initial abundance of ⁶⁰Fe in the solar system has been only loosely constrained from sulfides, of which ⁶⁰Fe‐⁶⁰Ni systems are easily disturbed, in primitive chondrites. In this study, we show that ⁶⁰Fe was present in chondrules, silicate spherules commonly found in chondrites, with initial ⁶⁰Fe/⁵⁶Fe ratios ((⁶⁰Fe/⁵⁶Fe)₀) of (2.2–3.7) × 10⁻⁷ at the time of their formation. By applying the time difference of 1.5–2.0 Myr between formation of the oldest known solar‐system solids (Ca‐Al‐rich inclusions) and chondrules, a (⁶⁰Fe/⁵⁶Fe)₀ ratio for the initial solar system of (5–10) × 10⁻⁷ is estimated. This new solidly based (⁶⁰Fe/⁵⁶Fe)₀ ratio is consistent with predictions for nucleosynthesis in a supernova or in an intermediate‐mass AGB star just before the solar‐system formation, but too high for the source to have been a low‐mass AGB star. Because encounters between a molecular cloud and an AGB star are rare, our results appear to be the best evidence found to date for a contribution of material from a nearby supernova to the solar system.