Abstract
Abstract— Metallographic cooling rates have been calculated for all five members of the iron meteorites group IIF using two different techniques. We have determined cooling rates of ∼5 °C/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles. Ni profiles through the kamacite phase are less precise cooling rate indicators, but suggest a cooling rate of ∼1 °C/Ma within an order of magnitude at lower temperatures (360–400 °C). Based on the kamacite bandwidth and the Ni profiles through the taenite, we estimate that the kamacite nucleated 130–200 °C below the temperature predicted from the phase diagram. The size of and the distance between the large kamacite spindles is found to be consistent with the thermal history that we have determined on the basis of Ni profiles in kamacite and taenite. We find that previously published kamacite bandwidth cooling rates for the five group IIF members are most likely in error because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite was ignored. Contrary to previous workers we find that the metallographic cooling rates are consistent with cooling in a common core.
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