THERMAL HISTORIES OF IVA STONY-IRON AND IRON METEORITES: EVIDENCE FOR ASTEROID FRAGMENTATION AND REACCRETION

Abstract

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 # 100oC/hr through 1200oC. The IVA irons cooled much slower in the range 1200-1000oC: absence of dendrites in large troilite nodules indicate cooling rates of <300oC/y. We infer that the parent asteroid was catastrophically fragmented and reaccreted when the core had cooled to 1200oC 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 (# a few hours). We calculate that global heating effects were also minor (ΔT < 100oC for a body with a diameter of <400 km) and that the mean temperature of iva parent body before after impact was 450-700oC. We infer that Steinbach cooled rapidly from 1200oC 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 600-350oC (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 (20-3000oC/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.