Untransformed metastable minerals exist under the low-temperature conditions of the subducting plates. Here I report the experimental studies on the transformation kinetics of pyroxene and garnet at high-pressure and hightemperature conditions. Slow kinetics of the pyroxene-garnet transformation due to the low atomic diffusion rate indicates that large low-density metastable regions would exist in the slab, greatly contributing to the slab stagnation around the mantle transition region.