Metallographic cooling rates of group IIF iron meteorites

Metallographic cooling rates have been calculated for all five members of t he iron meteorites group IIF using two different techniques. We have determ ined cooling rates of similar to5 degreesC/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles. Ni profiles through the kamaci te phase are less precise cooling rate indicators, but suggest a cooling ra te of similar to1 degreesC/Ma within an order of magnitude at lower tempera tures (360-400 degreesC). Based on the kamacite bandwidth and the Ni profil es through the taenite, we estimate that the kamacite nucleated 130-200 deg reesC below the temperature predicted from the phase diagram. The size of a nd the distance between the large kamacite spindles is found to be consiste nt with the thermal history that we have determined on the basis of Ni prof iles in kamacite and taenite. We find that previously published kamacite ba ndwidth cooling rates for the five group IIF members are most likely in err or because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite was ignored. Contrary to pre vious workers we find that the metallographic cooling rates are consistent with cooling in a common core.