60Fe in chondrites:: Debris from a nearby supernova in the early solar system?

Fe-60 decays to Ni-60 with a half-life of 1.49 x 10(6) yr, so all of the original Fe-60 atoms incorporated into the solar system have decayed. Because Fe-60 is produced only in stars, its initial abundance in the solar system provides a constraint on the stellar contribution of radionuclides to the early solar system and on the nature of the stellar source. Because of its short half-life, Fe-60 is also a potential high-resolution chronometer of early-solar-system events. The presence of Fe-60 in primitive meteorites has been confirmed in sulfides, but the initial abundance of Fe-60 in the solar system has been only loosely constrained because it is uncertain when the sulfides formed. We show that Fe-60 was present with abundance ratios of Fe-60/Fe-56 = (2.2 - 3.7) x 10(-7) when ferromagnesian chondrules formed. By applying the time difference of 1.5 - 2.0 million years between formation of ferromagnesian chondrules and Ca-Al-rich inclusions (CAIs), the oldest known solar system solids, a solar system initial Fe-60/Fe-56 ratio [(Fe-60/Fe-56)(0)] of (5-10) x 10(-7) is estimated. This new solidly based (Fe-60/Fe-56)(0) ratio is consistent with predictions for nucleosynthesis in a supernova or in an intermediate-mass asymptotic giant branch (AGB) star just before the solar system formation, but is too high for the source to have been a low-mass AGB star. Considering the rarity of encounters between a molecular cloud and an AGB star, our results can be considered strong evidence of a contribution of material from a nearby supernova and of a role for a supernova in the origin of the solar system.