A TRANSMISSION ELECTRON-MICROSCOPY (TEM) INVESTIGATION OF OPAQUE PHASES IN SHOCKED CHONDRITES

Shock defects in the most common silicate minerals of chondrites (oliv ine, pyroxenes and feldspars) have been investigated in detail, but th ere have been almost no studies of the shock defects in other componen ts, like metal and sulfide. This probably stems from the fact that the se latter phases are opaque in the optical microscope. The same reason explains why veins and melt pockets, which are constituted of microcr ystalline or glassy phases (i.e., isotropic) are also poorly documente d. We have investigated such phases by analytical transmission electro n microscopy (ATEM) in two shocked chondrites, Tenham (L6) and Gaines County (H5). We have characterized shock defects in troilite very simi lar to those occurring in silicates (i.e., a mosaic texture and sets o f straight and very narrow, approximate to 10 nm, lamellae of amorphiz ed FeS). There are many small regions in shocked chondrites that are c omposed of very fine grained (approximate to 1 mu m) mixtures of metal and sulfide or of various silicates. They must result from local melt ing followed by a rapid cooling that prevented grain growth. We have d etermined the chemical compositions and the volume proportions of the tiny grains in these veins and melt pockets, which has allowed their t emperature and pressure (T, P) history to be partially deciphered. Fin ally, we have observed a dense network of very narrow fractures (down to 10 nm) in the olivine and enstatite grains. These fractures are sys tematically filled with an amorphous (or cryptocrystalline) material t hat stems from the melt pockets and was injected when the fractures we re opened by the rarefaction wave. This material was then quenched at the contact with the colder crystalline rims.