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A new technique has recently been developed to investigate the strength of high-pressure mantle phases during high-strain plastic deformation (see 1996 Annual Report). Experiments are performed up to 16 GPa and 1800 K using a multi-anvil apparatus. The samples are thin disks, 200 microns thick, which are deformed in simple shear between hard (Al₂O₃) pistons. This development has now enabled the first deformation experiments to be performed on ringwoodite (the spinel phase of (Mg,Fe)₂SiO₄) under pressure-temperature conditions of the mantle transition zone (16 GPa, 1400-1600 K). The maximum shear strains achieved exceeded 100%. Samples were prepared by transforming Mg_1.2Fe_0.8SiO₄ olivine at various temperatures at a pressure of 16 GPa. A sample transformed to ringwoodite at 1400 K has a small grain-size (0.5 microns) and shows the following evidence for diffusion creep (or superplastic flow): (1) the density of dislocations in most grains is very low and (2) four-grain junctions are common and are indicative of active grain-switching events. Samples transformed at 1600 K are relatively coarse-grained (grain-sizes of several microns) and show evidence of dislocation creep (high densities of dislocations with characteristics similar to dislocations observed in deformed spinels of other compositions (e.g. Mg₂GeO₄) at low pressures). A comparison of the present results with results obtained on germanate spinel indicates that the latter is a good analogue of ringwoodite for plastic deformation and suggests that (1) a ringwoodite-rich layer in the mantle transition zone will be significantly stronger than the olivine-rich upper mantle if deformation occurs by dislocation creep and (2) ringwoodite-rich lithologies are likely to become very weak in subduction zones when significant grain-size reduction occurs as a result of phase transformations (see Section 3.1c).