Iron-60 as a heat source for melting and differentiation of Earth-forming planetesimals and planetary embryos

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The level at which radioactive 60Fe (half-life: 2.62 Myr) was present in the solar nebula and whether its decay could have made an important contribution (in addition to 26Al) to the melting and differentiation of planetesimals, the building blocks of the terrestrial planets, is still highly uncertain. To which extent the building blocks of the Earth were already differentiated has strong impact on how Earth’s core and mantle came into its present configuration and on the abundances of volatile elements (e.g. H, C, N) and water, important components for building a habitable Earth. In view of the predicted production of 60Fe in asymptotic giant branch stars and supernovae it appears plausible that 60Fe could have been present with significant abundance in the solar nebula. The application of a combination of several nano- and microanalytical analysis techniques to meteoritic samples proposed here is aimed to put better constraints on the level of 60Fe in planetesimals that later formed the Earth. Our experimental approach relies on Fe-Ni isotope measurements with high spatial resolution and a thorough mineralogical characterization of analyzed sample spots. We will use the new oxygen primary ion source (Hyperion II from Oregon Physics), which was recently installed on the NanoSIMS ion probe at MPI for Chemistry, to conduct Fe-Ni isotope studies at much smaller scales than was possible before. Objects with high Fe/Ni ratios, a pre-requisite to identify 60Ni excesses from 60Fe decay, will be pre-selected on the basis of combined SEM-EMPA investigations of several meteorites that experienced only minimal aqueous and thermal alteration. Follow-up studies of identified objects with 60Ni excesses will be conducted by FIB/TEM and by RIMS (CHILI at the University of Chicago) to validate inferred 60Fe/56Fe ratios. FIB/TEM will be used to characterize the mineralogical context and to constrain the degree of disturbance of the Fe-Ni isotope system by possible secondary alteration processes and presence of nanometer- to submicrometer-sized metal and sulfide inclusions within the analyzed minerals. Iron-Ni isotope studies with CHILI will be used to verify observed 60Ni excesses to exclude potential unidentified isobaric interferences and extreme intrinsic mass fractionation effects not recognized in the preceding NanoSIMS studies.