A seismic discontinuity is frequently observed near 300 km depth beneath continents and island arcs. Here we show that this discontinuity is generated by SiO2-stishovite formation in eclogitic assemblages. Such free silica is petrologically anticipated within materials of mid-oceanic-ridge basalt chemistry at these depths, and the 300 km discontinuity is likely associated with either the coesite to stishovite transition or exsolution of stishovite from clinopyroxenes containing excess silica. The presence and amplitude of this seismic feature provide a means for determining how much subducted, or delaminated, formerly basaltic material is present at deep upper-mantle depths. Thus, the distribution of the 300 km discontinuity yields a measure of mantle geochemical heterogeneity and provides a means for locating the residue of ancient subduction within the upper mantle.