ULTRADEEP, ULTRAMAFIC MANTLE XENOLITHS - TRANSMISSION ELECTRON-MICROSCOPY PRELIMINARY-RESULTS

The defect microstructure of ultradeep garnets from the Jagersfontein kimberlite pipe (South Africa) was investigated by transmission electr on microscopy. These crystals exhibit a high density of dislocations, most of which form well-organized and regularly spaced subgrain bounda ries. Free dislocations are also detected; most of them have a Burgers vector 1/2[111] (shortest lattice repeat), but a few [100] Burgers ve ctors are also characterized, especially in junctions. Small needle-sh aped pyroxene exsolutions are also detected. They exsolved from the Si -rich garnets during assent to the Earth's surface. Their nucleation a nd growth required bulk diffusion, and this process may have affected the original dislocation microstructure. It is believed, however, that the observed dislocation pattern still reflects the original deformat ion regime of garnets at very high pressure and temperature and this d islocation pattern strongly suggests that garnets were highly ductile. Finally, a number of precipitates of a dense amorphous phase of pure carbon are also detected. They might result from the amorphization of small diamonds exsolved from the garnet as the pressure was progressiv ely released.