Ultradeep, ultramafic mantle xenoliths: transmission electron microscopy preliminary results

Abstract

The defect microstructure of ultradeep garnets from the Jagersfontein kimberlite pipe (South Africa) was investigated by transmission electron microscopy. These crystals exhibit a high density of dislocations, most of which form well-organized and regularly spaced subgrain boundaries. Free dislocations are also detected; most of them have a Burgers vector 1 2 〈111〉 (shortest lattice repeat), but a few 〈100〉 Burgers vectors are also characterized, especially in junctions. Small needle-shaped pyroxene exsolutions are also detected. They exsolved from the Si-rich garnets during assent to the Earth's surface. Their nucleation and 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 deformation regime of garnets at very high pressure and temperature and this dislocation 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 progressively released.