Thermal histories of IVA stony-iron and iron meteorites: Evidence for asteroid fragmentation and reaccretion

We have investigated the thermal history of the IVA iron and stony-iron meteorites to help resolve the apparent conflict between their metallographic cooling rates, which are highly diverse, and their chemical trends, which favor crystallization in a single core. Transmission electron microscopy of the disordered clinobronzite in the stony-iron, Steinbach, using electron diffraction and high resolution imaging techniques indicates that this meteorite was rapidly cooled at approximate to 100 degrees C/hr through 1200 degrees C. The IVA irons cooled much slower in the range 1200-1000 degrees C: absence of dendrites in large troilite nodules indicate cooling rates of <300 degrees C/y. We infer that the parent asteroid was catastrophically fragmented and reaccreted when the core had cooled to 1200 degrees C and was 95% crystallized. We argue that radiative heat losses from the debris cloud would have been minor due to its high opacity, small size (only a few asteroid diameters), and short reaccretion times (similar to a few hours). We calculate that global heating effects were also minor (Delta T < 100 degrees C for a body with a diameter of <400 km) and that the mean temperature of the IVA parent body before and after the impact was 450-700 degrees C. We infer that Steinbach cooled rapidly from 1200 degrees C at the edge of a core fragment by thermal equilibration with cooler silicates during and after reaccretion. Metallographic cooling rates of IVA irons and stony-irons for the temperature range 600-350 degrees C (Rasmussen et al., 1995) strongly support this model and indicate that the IVA meteorites are derived from only a few core fragments. The large range of these cooling rates (20-3000 degrees C/My) and the decrease in the metallographic cooling rates of high-Ni IVA irons with falling temperature probably reflect the diversity of thermal environments in the reaccreted asteroid, the low thermal conductivity of fragmental silicates, and the limited sintering of this fragmental material.