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The transition zone is a region of the mantle from about 300 km to 1000 km in depth that is marked by high seismic velocity gradients. Abrupt changes in velocities, or seismic discontinuities, at depths of 410 km and 660 km, are prominent features of the transition zone and are sometimes thought to represent its upper and lower boundaries. These discontinuities, along with a smaller, intermittent discontinuity at ˜520 km depth, are potentially the most important clues available for understanding the mineralogy chemistry and dynamics of the deep mantle. However, a firm interpretation of these discontinuities remains elusive. It is not known, for example, whether changes in chemical composition occur across the 410-km and 660-km discontinuities, or whether these discontinuities are due solely to polymorphic phase changes in the (Mg,Fe)2Si04 olivine component of an otherwise isochemical mantle. It is further uncertain whether a chemically stratified mantle with compositionally distinct layers is compatible with our understanding of how mantle convection works.