Deep carbon cycling into Earth's mantle regulates fundamental processes such as melting, redox state, and volatile transport, yet its geophysical expression within seismic structures has remained elusive. A prominent seismic puzzle is the sporadic splitting of the 520-km discontinuity into a secondary reflector near 560 km depth beneath subduction slabs and mantle plumes. Although previously attributed to davemaoite (CaSiO₃) exsolution, standard shear-wave properties of davemaoite failed to generate the observed 2–4% impedance contrast.

To resolve this discrepancy, researchers conducted high pressure-temperature experiments (~20 GPa, 1,200–1,600 °C) using a large-volume multianvil apparatus on compositions similar to carbonate-altered oceanic crust. The results reveal that carbonate addition drives significant Ca-Mg exchange with silicates and increases Ca incorporation into silicates specifically under Fe-rich conditions. This mechanism dramatically boosts davemaoite exsolution, concentrating the mineral to 12–33(2) vol.% near ~560 km depth—a level sufficient to account for the observed seismic impedance contrast.

Furthermore, davemaoite-rich crust recycled by mantle plumes explains the persistent 560-km seismic reflectors detected beneath hot thermal regions. Ultimately, this study demonstrates that subducted carbonates actively alter mineral phase equilibria, reinforce chemical stratification, and leave distinct, long-lasting seismic signatures of deep carbon cycling within Earth's interior.