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

Abstract

Metallographic cooling rates of group IVA iron meteorites have been recalculated based on the most recent Ni diffusion coefficients and phase diagram. The cooling rates are revised upwards by a factor of ca. 15 relative to previous estimates. A large range in cooling rate is found in the low-Ni part of group IVA (Ni < 8.4 wt%), while the high-Ni part shows approximately constant cooling rates. Undercooling is observed only in the high-Ni IVA members. Some of the taenite lamellae in the high-Ni IVA irons, which were apparently affected by moderate undercooling, can, alternatively, be interpreted to have experienced a nonlinear 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 model and a raisin-bread model but seemingly it does not contradict a model where the parent body is broken up and reassembled after core crystallization but prior to Widmanstatten pattern formation.