We systematically examined seismic data from deep earthquakes in the western and northwestern Pacific, Tonga/Fiji, and South America for signs of sharp gradients in seismic properties that could represent either a mineralogical phase change or the boundary between geochemical reservoirs in the Earth's lower mantle. This involved the stacking of short-period, vertical component teleseismic waveforms from the dense Pacific Northwest Seismic Network and searching for evidence of S-to-P conversions at sharp mantle structures between 800 and 2000 km depth. With these data, we estimate we should be able to detect seismic discontinuities with ΔVs > 2%, transition width <20 km, and limited local topography. Using data from over 45 earthquakes, we found no evidence of a horizontal global discontinuity, neither near 920 km nor near 1700 km, beneath the four convergent margins studied here. Consistent with previous results, we detected a deep structure with topography near 1600 km beneath the Marianas region. Because we observed the signal over a large geographical region beneath the Marianas (10¿N to 20¿N), it is likely a coherent local structural and not a point source. The absence of such a structure beneath the northwest Pacific, the Tonga/Fiji region, or South America, argues against a global feature near 1600 km depth, however. The lack of convincing and consistent scattering suggests that no composition boundary or thermal boundary layer exists between 800 to 2000 km depth beneath the convergent zones studied that has ΔVs < 2%, locally limited topography, and a transition interval <20 km.