An evaluation of the methods to determine the cooling rates of iron meteorites

Abstract

A numerical model using the most recent phase diagram and diffusion coefficients was developed to simulate the growth of Widmanstätten pattern in iron meteorites, to study the effect of impingement on the determination of metallographic cooling rates and to compare the three available techniques for the determination of metallographic cooling rates (taenhe central Ni content method, the profile-matching method and the kamacite bandwidth method). The iron meteorites used in this study were Toluca (group IA, 8.14 wt% Ni, 0.16 wt% P), Tazewell (group IIID, 16.9 wt% Ni, 0.3 wt% P) and Bristol (group IVA, 8.0 wt% Ni, 0.05 wt% P). The tacnite central Ni content method gave the slowest cooling rates and the bandwidth method gave the fastest cooling rates. For Tazewell and Toluca, the taenite central Ni method and the profile-matching method gave very similar cooling rates. Impingement effects have a significant influence on the value of the measured cooling rate. The bandwidth method does not consider impingement and hence cannot be used for cooling rate measurements. Thus only the taenite central Ni content method and the profile-matching method should be used in the determination of cooling rates. The estimated cooling rates for Tazewell, Toluca and Bristol arc 10°C/Myr 25°C/Myr and 150–300°C/Myr, respectively. The cooling rates obtained in this study are lower by a factor of 50 to 100 than those of Narayan and Goldstein, who used the bandwidth method, and greater by a factor of 5 compared to older cooling rates (1–10°C/Myr).