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

Abstract

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-to-ringwoodite 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 signal-processing 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 signal-processing artifacts.