Observations of Regional Seismic Discontinuities in the Earth’s Upper Mantle from SS- and PP- precursors

The physical properties of regional seismic discontinuities in the upper mantle yield insights into lateral and radial thermochemical variations, with implications for our understanding of magmatism and convection in the mantle.The global distribution of the 300-km discontinuity (termed the “X” discontinuity) is relatively poorly resolved, as it is detected infrequently, likely due to its small impedance contrast. Reflectors observed near this depth are usually local and primarily detected beneath continent and subduction zones. Several mechanisms suggest that the X is associated with mineral transformations that occur in basalt-enriched material. Thus, imaging the X-discontinuity holds the key to mapping subducted oceanic crust remnants.

Another discontinuity, at around 520 km depth, is detected more frequently and sometimes observed to be split into two signals. Its existence is predicted by the wadsleyite to ringwoodite mineral phase transition. However, the variations in ambient thermochemistry, which influence its visibility, depth variation, reflectivity, and/or splitting, are not fully understood, necessitating further investigations. Improved constraints on the nature of the 520 will inform regarding thermal and compositional gradients within the mantle transition zone.

In this study, we use large global datasets of SS and PP precursors to obtain new maps of these discontinuities. Our observations indicate regionally weak yet clear signals at both depths, linked to variations in basalt fraction and potential temperature. We perform mineral physics modeling and investigate the characteristic temperature and composition associated with the signatures of these signals. These results provide insight into our understanding of the chemical segregation and plume stagnation in the upper mantle.