AN ANALYTICAL ELECTRON-MICROSCOPY (AEM) INVESTIGATION OF OPAQUE INCLUSIONS IN SOME TYPE-6 ORDINARY CHONDRITES

A large number of ordinary chondrites contains micron-sized particles of metal and/or troilite dispersed in their silicate grains. Such meta llic phases are responsible for the so-called darkening of the silicat e grains and might be either precipitates, which formed during reducti on of the silicates, or inclusions injected as a melt during a shock e vent. We have investigated these tiny foreign phases by analytical tra nsmission electron microscopy in three unweathered, metamorphosed ordi nary chondrites (Saint Severin, LL6, Tsarev, L6 and Kernouve, H6). We also looked for remnant shock indices. Our TEM observations suggest th e following sequence of events in the three meteorites. First, a numbe r of relatively strong shock events occurred on the parent body/bodies producing an Fe-FeS melt that was injected into silicate grains along a dense network of open fractures. Most of these shock defects were s ubsequently erased by high-temperature (700-900 degrees C) thermal met amorphism. Some remnants of the shock events are the observed trails o f tiny metal and/or sulfide inclusions that formed as a result of frac ture healing. Chemical homogenization of the silicates and limited oxi dation of the metallic blebs also occurred during this high-temperatur e annealing event, resulting in Ni-rich inclusions. This effect was es pecially pronounced in the L and LL-chondrites studied. During subsequ ent cooling of the body/bodies, inclusions of chromite and phosphate p recipitated, nucleating preferentially on lattice defects (dislocation s, subgrain boundaries) and on the metal and sulfide inclusions. A lat er shock event of moderate intensity, probably corresponding to the se paration of the meteorite from its parent body, produced new shock fea tures in the silicate grains of the Saint Severin meteorite, including mechanical twins in diopside and straight free screw dislocations in olivine.