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Published August 15, 2007 | public
Journal Article

⁶⁰Fe–⁶⁰Ni and ⁵³Mn–⁵³Cr isotopic systems in sulfides from unequilibrated enstatite chondrites

Abstract

In situ measurements of ⁶⁰Fe–⁶⁰Ni and ⁵³Mn–⁵³Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of ⁶⁰Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial (⁶⁰Fe/56Fe) [(⁶⁰Fe/⁵⁶Fe)0] ratios show a large variation range, from ∼2 × 10⁻⁷ to ∼2 × 10⁻⁰. The sulfide inclusions with high Fe/Ni ratios yield (⁶⁰Fe/⁵⁶Fe)₀ ratios of ∼(2–7) × 10⁻⁷, similar to most of the (⁶⁰Fe/⁵⁶Fe)₀ values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred (⁶⁰Fe/⁵⁶Fe)₀ ratios (∼1–2 × 10⁻⁶) 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 (⁶⁰Fe/⁵⁶Fe)₀ ratios were probably the result of Fe–Ni re-distribution in the sulfides during later alteration processes. The ⁵³Mn–⁵³Cr system was measured in five of the sulfide assemblages that were examined for their ⁶⁰Fe–⁶⁰Ni systematics. The ⁵³Mn–⁵³Cr isochrons yielded variable initial (⁵³Mn/⁵⁵Mn) [(⁵³Mn/⁵⁵Mn)₀] ratios from ∼(2–7) × 10⁻⁷. There is no obvious correlation between the (⁶⁰Fe/⁶⁰Fe)0 and (⁵³Mn/⁵⁵Mn)₀ ratios. The variable ⁵³Mn–⁵³Cr 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 ⁵³Mn–⁵³Cr systems in these UEC sulfides, our results indicate that ⁶⁰Fe was present in the enstatite chondrite formation region of the early Solar System.

Additional Information

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. Shukolyukov and an anonymous reviewer for their constructive reviews which help to improve this manuscript. This study was supported by NASA Grants NAG5-10523 (L.A.L.), NNG05GH37G (L.A.L./Y.G.), NAG5-11543 (G.R.H.), and NNG05GG48G (G.R.H.).

Additional details

Created:
August 22, 2023
Modified:
October 25, 2023