| Summary: | In situ measurements of â¶â°Fe–â¶â°Ni and âµÂ³Mn–âµÂ³Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of â¶â°Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial (â¶â°Fe/56Fe) [(â¶â°Fe/âµâ¶Fe)0] ratios show a large variation range, from ∼2 × 10â»â· to ∼2 × 10â»â°. The sulfide inclusions with high Fe/Ni ratios yield (â¶â°Fe/âµâ¶Fe)â‚€ ratios of ∼(2–7) × 10â»â·, similar to most of the (â¶â°Fe/âµâ¶Fe)â‚€ values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred (â¶â°Fe/âµâ¶Fe)â‚€ ratios (∼1–2 × 10â»â¶) have low Fe/Ni ratios and the magnitude of the 60Ni excesses is similar in two MAC88136 assemblages in spite of a difference of a factor of two in their Fe/Ni ratios. The inferred high (â¶â°Fe/âµâ¶Fe)â‚€ ratios were probably the result of Fe–Ni re-distribution in the sulfides during later alteration processes. The âµÂ³Mn–âµÂ³Cr system was measured in five of the sulfide assemblages that were examined for their â¶â°Fe–â¶â°Ni systematics. The âµÂ³Mn–âµÂ³Cr isochrons yielded variable initial (âµÂ³Mn/âµâµMn) [(âµÂ³Mn/âµâµMn)â‚€] ratios from ∼(2–7) × 10â»â·. There is no obvious correlation between the (â¶â°Fe/â¶â°Fe)0 and (âµÂ³Mn/âµâµMn)â‚€ ratios. The variable âµÂ³Mn–âµÂ³Cr isochrons probably also indicate later disturbance to the isotopic systems in these sulfides. Even though no chronological information can be extracted from the 60Fe–60Ni and âµÂ³Mn–âµÂ³Cr systems in these UEC sulfides, our results indicate that â¶â°Fe was present in the enstatite chondrite formation region of the early Solar System. We thank the Antarctic Meteorite Curatorial Facility, the Smithsonian Institute, and the Laboratory for Space Sciences of Washington University in St. Louis for providing the samples used in this work. We thank M. Kimura, N. Kita, A. ...
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