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Abstract:
We measured the nickel isotope composition of troilites from chondritic meteorites using the NanoSIMS to put constraints on the abundance of iron-60 in the early solar system. The troilites were selected from petrologic type 3 ordinary and carbonaceous chondrites. Based on petrographic observations and mineral chemistry, the troilites targeted for isotope analysis crystallized from melts, most likely in a nebular setting. Our isotope analyses did not reveal any significant correlation between nickel-60 enrichments and Fe/Ni ratios, either in the entire set of troilite grains or in individual troilites. The average inferred initial 60Fe/56Fe ratio of the studied troilites (i.e., the 60Fe/56Fe ratio calculated for the entire troilite population) is 1.05 (±1.48) ×10−8. This value is very similar to those estimated in the past for Semarkona chondrules, angrites, as well as diogenites and eucrites, based on the isotope analyses of bulk samples (109–108), but about two orders of magnitude smaller than the average initial 60Fe/56Fe ratios inferred previously for Semarkona troilites and many chondrules from ordinary and carbonaceous chondrites (107–106) using in situ analysis techniques. Based on petrographic evidence, and the generally unequilibrated nature of our samples, as well as on the timing of chondrule formation and planetary evolution, the lack of discernible signs of in situ iron-60 decay in the studied troilites is probably unrelated to metamorphic re-equilibration, and it is also not the result of a late formation of the troilites. We suggest that the highest inferred initial 60Fe/56Fe ratios reported in the literature are probably inaccurate.
