Isotopic homogeneity of iron in the early solar nebula

The chemical and isotopic homogeneity of the early solar nebula, and the pr ocesses producing fractionation during its evolution, are central issues of cosmochemistry. Studies of the relative abundance variations of three or m ore isotopes of an element can in principle determine if the initial reserv oir of material was a homogeneous mixture or if it contained several distin ct sources of precursor material. For example, widespread anomalies(1-4) ob served in the oxygen isotopes of meteorites have been interpreted as result ing from the mixing of a solid phase that was enriched in O-16 with a gas p hase in which O-16 was depleted(1-3), or as an isotopic 'memory' of Galacti c evolution(5). In either case, these anomalies are regarded as strong evid ence that the early solar nebula was not initially homogeneous. Here we pre sent measurements of the relative abundances of three iron isotopes in mete oritic and terrestrial samples. We show that significant variations of iron isotopes exist in both terrestrial and extraterrestrial materials. But whe n plotted in a three-isotope diagram, all of the data for these Solar Syste m materials fall on a single mass-fractionation line, showing that homogeni zation of iron isotopes occurred in the solar nebula before both planetesim al accretion and chondrule formation.