Transformation textures and kinetics of the olivine to spinel phase transformation were observed in situ in ungasketed samples using a diamond anvil cell (DAC). The low-temperature kinetic limit for reconstructive transformation from olivine (α) to spinel phase (&ggr;) observed in DAC experiments is approximately 150¿C lower than when observed under more hydrostatic conditions. The spinel phase, which in many of the samples is distributed in an annular pattern, exhibits reconstructive textures including grain boundary nucleation, and lack of topotaxy; in some cases it forms lenses similar to those associated with transformation-induced mechanical failure. Although spinel phase lamellae, formed by a martensiticlike mechanism, are observed in the specimens, the lamellae remain extremely thin (~10 nm) and do not produce enough spinel to be optically visible. The observation of reconstructive textures within the annular transformed regions leads us to conclude that high shear stress and plastic strain enables reconstructive transformation at temperatures where transformation rates would otherwise be virtually zero. High transient differential stresses and rapid deformation accompany deep earthquakes. Therefore knowledge of the kinetics of this transformation under these conditions is important for understanding the connection between phase transformation and deep earthquakes in subducting lithospheric slabs. ¿ American Geophysical Union 1995