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-iro n meteorites to help resolve the apparent conflict between their metal lographic cooling rates, which are highly diverse, and their chemical trends, which favor crystallization in a single core. Transmission ele ctron microscopy of the disordered clinobronzite in the stony-iron, St einbach, using electron diffraction and high resolution imaging techni ques indicates that this meteorite was rapidly cooled at approximate t o 100 degrees C/hr through 1200 degrees C. The IVA irons cooled much s lower 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 reaccrete d 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 di ameters), and short reaccretion times (similar to a few hours). We cal culate that global heating effects were also minor (Delta T < 100 degr ees C for a body with a diameter of <400 km) and that the mean tempera ture of the IVA parent body before and after the impact was 450-700 de grees C. We infer that Steinbach cooled rapidly from 1200 degrees C at the edge of a core fragment by thermal equilibration with cooler sili cates during and after reaccretion. Metallographic cooling rates of IV A irons and stony-irons for the temperature range 600-350 degrees C (R asmussen et al., 1995) strongly support this model and indicate that t he IVA meteorites are derived from only a few core fragments. The larg e 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 s ilicates, and the limited sintering of this fragmental material.