Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Authors:
  • MPMichail I. Petaev
    Michail I. Petaev
  • AKAlexander N. Krot
    Alexander N. Krot
  • JWJohn A. Wood
    John A. Wood
  • KKKlaus Keil
    Klaus Keil
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Abstract
Some FeNi metal grains, ∼150 μm in apparent diameter, in CH carbonaceous chondrites are concentrically zoned in Ni (∼5–10 wt%), Co (0.2–0.4 wt%), and Cr (0.3–0.8 wt%). Silicon is present at the ∼0.1 wt% level. These observations are consistent with predicted gas‐solid condensation from a gas of solar composition at temperatures of ∼ 1370–1270 K and total pressure of ∼10−4 bar. Estimates of FeNi metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula. Compositionally similar FeNi metal grains have also been reported in CR and Bencubbin‐like chondrites. Because FeNi metal is highly susceptible to secondary alteration (i.e., metamorphism, melting, oxidation), the observed FeNi metal condensates in CH, Bencubbin‐like, and CR chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections.

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