Cooling rate variations of group IVA iron meteorites |
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| Authors: | AE Moren JI Goldstein |
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| Institution: | Department of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA 18015 U.S.A. |
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| Abstract: | Cooling rates of eight group IVA iron meteorites were determined using a modification of the Wood method where cooling rate curves are calculated as a function of central taenite Ni content and taenite half-width. The major modification of the Wood method was to include the effect of P on the Ni solubility limits and Ni diffusion coefficients in the kamacite phase for each meteorite studied. The Borg and Lai binary kamacite Ni diffusivities were judged to be the best quality data available for the calculations. The calculated cooling rates range between 3 and 65°C/Myr. A correlation between decreasing cooling rate and increasing Ni content within the group IVA irons is observed. This cooling rate variation agrees closely with the 6–70°C/Myr range calculated by the independent bulk Ni-kamacite bandwidth method. Such a large variation in cooling rate within the group IVA argues against formation within the core of a single parent body.Willis and Wasson in the preceding paper found only a factor of 2 variation in cooling rate for the six IVA irons studied. The differences between the results of Willis and Wasson and this study are due mainly to the choice of the Ni diffusion coefficients in α and to the choice of the expressions for the effect of P on both the diffusion coefficients and the Ni solubility in kamacite. The Hirano et al. diffusivities used by Willis and Wasson were judged to be incorrect particularly because they are significantly higher than diffusivities in kamacite and taenite measured by other investigators. The assumption of Willis and Wasson that the same P content (0.03 wt.%) can be used for the low-Ni IVA's and that the same P content (0.16 wt.%) can be used for the high-Ni IVA's was judged to be a serious oversimplification. |
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