Carbon in iron meteorites and its importance for metallographic cooling rates

Details Carbon in iron meteorites and its importance for metallographic cooling rates Carbon in iron meteorites and its importance for metallographic cooling rates

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.501m&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 501

Physics

4

Carbon, Cooling, Iron Meteorites, Metallography, Diffusion, Phase Diagrams, Temperature Effects, Widmanstatten Structure

Scientific paper

Metallographical cooling rates of iron meteorites are calculated by simulating the growth of the Widmanstatten structure using a ternary Fe-Ni-P phase diagram and diffusion coefficients of Ni in alpha and gamma phase. The effects of C on the phase diagram and Ni diffusion have not been incorporated in the models. Gamma rays from the nuclear reaction (12)C(d,p)(13)C* were recently used to measure the bulk C concentration in a suite of IIIAB irons covering most of the Ni range of the group. We have tentatively used these data to assess the effect of C on the IIIAB metallographic cooling rate. It has been argued that the effect of C on the phase diagram can be neglected because most of the C present is consumed in the formation of carbides and only very small amounts remain in solution in the metal phases below 773 K. This assumption has not been supported by measurements of C in iron meterorites. It has become possible to measure the amount of C in solid solution in the metal with some spatial resolution. We have measured bulk C concentrations in the range of 15-130 micrograms/gram in IIIAB. Due to the low solubility of C in the kamacite most of the C present is concentrated in the taenite. Carbon concentrations 15-50x higher than the bulk C value would thus be expected, and the diffusivity of Ni in the taenite enhanced accordingly. This is effective only at relatively low temperatures. Even at the low bulk C concentrations relevant for most groups of iron meteorites the phase diagram seems to be significantly affected. This effect is augmented at lower temperatures and might well play a dominant role during the formation of tetrataenite and the cooling rates calculated from the island phase.

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