ICARUS 69, 1--13 (1987) Original Structures, and Fragmentation and Reassembly Histories of Asteroids: Evidence from Meteorites G. JEFFREY TAYLOR, PETER MAGGIORE, EDWARD R. D. SCOTT, ALAN E. RUBIN, 1 AND KLAUS KEIL Institute of Meteoritics and Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 Received March 17, 1986; revised AugUst 18, 1986 If chondritic meteorites were internally heated after accretion had ended, then the hottest mate- rial would have been buried the deepest and should have cooled the slowest. If this is correct, there ought to be a correlation between cooling rate and petrographic type, a measure of the extent to which chondrites were metamorphosed (i.e., heated). Published and new cooling rates derived from the compositions of metallic iron-nickel grains do not display this correlation, implying either that chondrite parent asteroids never had onion-shell structures or that bodies with onion-shell structures were broken up and reassembled prior to cooling to below 500°C, the temperature at which cooling-rate information is recorded in metallic iron-nickel. Chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids. Metalfic iron-nickel grains in H- and L-chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to > 1000°K/myr. Based on thermal calculations, these cooling rates suggest that the materials spread out on the surfaces of H- and L-chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers. Craters deep enough to excavate tens of kilometers cannot form on typical asteroidai bodies only 100 to 300 km in diameter without disrupting them. Therefore, it appears that at least some asteroids, namely, the parent bodies of H and L chondrites, were disrupted after cooling to below 300°C, and then reassembled to create surfaces containing rocks that originated at a wide range of depths. These results support theoreti- cal calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles. © 1987Academic Press,inc. INTRODUCTION Meteorites record the geologic evolution of the parent bodies, presumably asteroids, in which they formed. They store informa- tion about accretion mechanisms, original compositions and structures, thermal histo- ries, and fragmentation histories. This pa- per focuses on the geology of the parent asteroids of ordinary chondrites. The rates at which chondritic meteorites cooled can be determined quantitatively by measuring the compositions of metallic iron-nickel grains (Wood, 1967) or by counting the abundances of nuclear particle tracks resulting from the spontaneous fis- sion of 244pu (Pellas and Storzer, 1981). 1 Present address: Institute of Geophysics and Plan- etary Physics, University of California, Los Angeles, Calif. 90024. Such measurements have elucidated the thermal histories of chondrite parent bodies. In this paper we use metallographic cooling rates to obtain information about the original structures and fragmentation histories of the parent objects of ordinary chondrites (Taylor et al., 1982). Many authors (e.g., Dodd, 1969; Anders, 1978; Minster and Allegre, 1979; Pellas and Storzer, 1981) believe that the parent aster- oids of H, L. and LL chondrites had onion- shell structures with the most metamor- phosed meteorites [called type 6 by Van Schmus and Wood (1969)] formed in their cores, surrounded by successive shells of less metamorphosed type 5 through type 3 rock. Because the cooling rate would de- crease as depth increased, this model pre- dicts an inverse relation between cooling rate and petrologic type. Other investiga- 0019-1035/87 $3.00 Copyright© 1987 by Academic Press,Inc. All rightsof reproduction in any formreserved.