60 Fe: A Heat Source for Planetary Differentiation from a Nearby Supernova Explosion

Abstract

From a sample of the Semarkona (LL 3.0) ordinary chondrite we report the in situ discovery of 60 Ni isotopic anomalies attributable to the decay of short-lived 60 Fe (half-life 1.5 Myr) in the mineral phases troilite (FeS) and magnetite (Fe 3 O 4 ). The troilite shows a 60 Ni excesses of up to ∼100 parts per thousand (%0) relative to its solar isotopic abundance. A positive correlation between 60 Ni excesses and 56 Fe/ 58 Ni ratios provides evidence for live 60 Fe in the early solar system. The inferred 60 Fe/ 56 Fe ratio of (0.92 ±0.24) × 10- -6 is the highest measured in any meteorite sample so far. This ratio is higher than predictions for production within asymptotic giant branch stars, but falls within the range expected for a Type II supernova source. This result is strongly suggestive of injection of freshly synthesized 60 Fe into the nascent solar nebula by a nearby supernova explosion. Such a high abundance of 60 Fe will exclude irradiation with solar energetic particles as the sole mechanism responsible for the production of short-lived radio-nuclides. It further shows that the decay of 60 Fe was an important heat source for early planetary melting and differentiation and for keeping asteroids thermally active for much longer than would be possible from the decay of 26 Al alone. © 2005. The American Astronomical Society. All rights reserved.