Hidden ultralow velocity zone beneath paleoslabs near the core-mantle boundary

The interaction between subducted slabs and the core-mantle boundary (CMB) is critical for understanding lower mantle heterogeneity. Here, we investigate a localized ultralow velocity zone (ULVZ) beneath the South Sandwich Islands using core-diffracted shear waveforms recorded by the USArray. Our waveform modeling reveals a pile-like ULVZ, approximately 300 km in diameter and 30 km in height, situated directly beneath a high-velocity paleoslab. The feature exhibits an extreme shear wave velocity reduction of 18%. These constraints are best explained by an iron-rich (Mg,Fe)O body, possibly containing partial melt, accumulated at the slab edge. This juxtaposition suggests that subducted slabs play a key role in concentrating dense, chemically distinct materials at the CMB, potentially serving as roots for mantle plumes even within high-velocity regions.