Thermal history of Almahata Sitta

Jason Herrin, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA

Fragments of the Almahata Sitta fall have been investigated by several groups using widely different techniques. The combined results of these studies reveal much of the thermal history of the meteorite and its parent asteroids. A record of sustained high temperatures within the ureilite parent body (UPB) mantle is preserved in two-pyroxene equilibration temperatures. Depletion of incompatible trace elements indicates significant loss of basaltic partial melts in this same timeframe. A late reduction event, recorded in the rims of olivine and pyroxene, confirms that the UPB underwent catastrophic disruption while its mantle was still hot and that the ejected fragments of mantle cooled rapidly. Fragments of original UPB later re-accreted into smaller daughter asteroids, wherein temperatures remained well below olivine closure. Amino acids and PAHÕs found in Almahatta Sitta are only stable at low temperatures, and so must have been introduced after cooling. Diamonds re ported to have formed at high temperatures must be from the UPB stage, and perhaps record the shock event that led to its disruption.