THE ELUSIVE 60Fe IN THE SOLAR NEBULA

No Ni-60 or Ni-61 anomalies have been detected in troilite inclusions from Muonionalusta, a 4565.3 +/- 0.1 Ma old IVA iron meteorite, to the level of the analytical precision of 10 ppm. Because Muonionalusta troilite is very old, has a high Fe/Ni ratio, and is free of Ni-61 anomalies, it is the ideal material in which to search for potential excesses of Ni-60 produced by the decay of Fe-60 (t(1/2) = 2.62 Ma). The Ni-60/Ni-58 and Fe/Ni ratios (up to 1680) measured here imply an upper limit for the initial Fe-60/Fe-56 for the Muonionalusta troilite of 3 x 10(-9). Assuming that Fe-60 was homogeneously distributed across the nebula, this result suggests that the solar system initial Fe-60/Fe-56 ratio was less than 5 x 10(-9), which is lower than previously measured by at least a factor of 50.