THE THERMAL EVOLUTION OF IVA IRON-METEORITES - EVIDENCE FROM METALLOGRAPHIC COOLING RATES

Metallographic cooling rates of group IVA iron meteorites have been re calculated based on the most recent Ni diffusion coefficients and phas e diagram. The cooling rates are revised upwards by a factor of ca. 15 relative to previous estimates. A large range in cooling rate is foun d in the low-Ni part of group IVA (Ni < 8.4 wt%), while the high-Ni pa rt shows approximately constant cooling rates. Undercooling is observe d only in the high-Ni IVA members. Some of the taenite lamellae in the high-Ni IVA irons, which were apparently affected by moderate underco oling, can, alternatively, be interpreted to have experienced a nonlin ear cooling history. The variation in cooling rate of the entire group IVA spans two orders of magnitude (19-3400 K/My). This span is still so large that it constitutes severe problems for both a core origin mo del and a raisin-bread model but seemingly it does not contradict a mo del where the parent body broken up and reassembled after core crystal lization but prior to Widmanstatten pattern formation.