No globally detectable seismic interfaces within Earth's mantle transition zone: Evidence for basalt enrichment

Earth's mantle transition zone (MTZ) is characterized by multiple mineral phase changes. The MTZ is bounded seismically by the 410- and 660 -km elastic discontinuities, which are interpreted as phase changes from olivine to wadsleyite and from ringwoodite to bridgmanite, respectively. Within the transition zone, the wadsleyite-toringwoodite phase change has been assigned to a seismic feature at 520 -km depth. Here we show that SS precursors, the most common seismic tool for imaging MTZ structure at global scale, are susceptible to signalprocessing artifacts within the transition zone. Owing to the narrow frequency range in which the SS wave is typically examined, S waves scattered by the bounding MTZ discontinuities generate Gibbs -effect overshoots that map into features within the transition zone, including at 520 -km depth. These results suggest that inferences of global interfaces at 520 -km depth and, in some studies, at 560 -km depth, are problematic. If such interfaces are not detectable, Earth's mantle transition zone may depart significantly from the pyrolite composition, enriched in a subducted basalt component that contributes a high proportion of garnet. A basalt fraction (f = -0.4) in the MTZ, consistent with some estimates, can render the 520 -km discontinuity indistinguishable from signalprocessing artifacts.