Subducted sediments hidden in the upper-mantle X-discontinuities

Sequestered sediments drive mantle chemical heterogeneity and enriched volcanism, yet their deep-mantle detection and geophysical interpretation remain enigmatic. Here, we identify sediment-related processes as a viable contributor to the upper-mantle X-discontinuities, through seismic velocity modeling of various sediment compositions coupled with mid-ocean ridge basalts and pyrolite in comparison to seismological observations. The sediment-derived formation of liebermannite (KAlSi3O8) and stishovite (SiO2) at 250–350 km depths can generate substantial seismic anomalies (4.0–19.4% and 3.2–15.3% increases in shear and compressional wave velocities, respectively), with minimal sensitivity to sediment compositional variations. These results suggest that the upper-mantle X-discontinuities may represent transient phase-transition windows through which subducted sediments migrate and become seismically detectable. Once these sediments are entrained by upwelling mantle plumes, they may yield distinct enriched geochemical signatures observed in ocean island basalts originating from hotspots associated with the X-discontinuities. Sediment-related phase transitions enhance the seismic detectability of recycled sediments in the upper mantle, linking intermittent X-discontinuities to hidden sediment-bearing domains and providing a geophysical means of probing mantle chemical heterogeneity.