Cooling Rate Variations of Group Wa Iron Meteorites

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 ma jor modification of the Wood method was to include the effect of? on the Ni solubility limits and Ni diffusion co efficients 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 forma tion within the core of a single parent body.