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The abundance of (Mg, Fe)O in the lower mantle and its stability over a wide range of pressure conditions makes MgO a model material to investigate the influence of pressure on crystal plasticity. Recent calculations have shown that the elastic properties of MgO are strongly affected by pressure when under the Earth's mantle conditions. This has important potential implications for the rheology of the lower mantle (see 1998 Annual Report).

MgO is available as large, pure single crystals so it is possible to orient samples to promote glide on a given slip system. In this study, we have used specimens oriented along [001] to activate easy glide on the slip systems (Fig 3.1-15a). Experiments were performed at 15 GPa, 1500°C in a 18/11 multianvil assembly. The large volume assembly has allowed us to use 3x1.5x1.5 mm³ specimens in which sections could be cut in several orientations (Fig. 3.1-15b). This is crucial to enable reconstruction of 3-dimensional dislocation patterning from the 2-D information provided by TEM thin foils.

Optical examination under crossed nicols of sections (F₁) parallel to the large faces of the specimens (Fig. 3.1-16) shows that duplex slip on has been activated as expected from Figure 3.1-15a. The wavy nature of the deformation bands suggest that considerable

Fig. 3.1-15: (a) Orientation of the specimens to promote glide on four (only two are represented) of the slip systems. (b) orientation of the thin sections cut in the deformed specimens to investigate dislocation microstructures in transmission electron microscopy.

cross-slip of dislocations took place. There is evidence (in Fig. 3.1-16) that slip on conjugate (90°) systems is able to interpenetrate, as expected in this temperature range. TEM investigations on (F₁) and (F₂) thin foils show a large density of 1/2<110> dislocations that form numerous subgrain boundaries. A better view of dislocation motion can be obtained by examination of foils (F₃) parallel to one slip plane (Fig. 3.1-17a). In particular, the equilibrium shape of a gliding dislocation loop (Fig. 3.1-17b) can be used as a local probe of elastic anisotropy in MgO.

Fig. 3.1-16: Optical micrograph (transmitted polarised light) of a thin section of type (F1) showing coarse wavy glide and intersecting conjugate slip. The load is approximately horizontal.

Fig. 3.1-17: TEM micrographs of a thin foil (F3) parallel to one slip plane. (a) Weak-beam dark-field. In-plane interactions of gliding dislocations. (b) Bright field; shape of a dislocation loop