On the thermal history of chondrites

In order to investigate shock and reheating effects in ordinary chondrites, 71 bronzite, 26 hypersthene, and 6 amphoterite chondrites were examined by X-ray diffraction and 39 bronzite and 19 hypersthene chondrites were studied metallographically. In addition, the inert gas contents of 12 bronzite and 4 hypersthene chondrites were measured mass spectrometrically. It was found that virtually all chondrite with short gas- retention ages (< 2 by.) are shocked and reheated, whereas chondrites with ages > 3 by. are not reheated to any appreciable extent, although some are heavily shocked. It is concluded that few, if any, severe collisions took place on the chondrite parent bodies between 3 and 4.5 by. ago. Moreover, the bronzite and amphoterite parent objects apparently escaped fragmentation, such as was suggested by Anders and Heymann for the hypersthene object.