The presence of ancient subducted oceanic crust contributes to seismic anomalies in Large Low Shear Velocity Provinces.

Large Low Shear Velocity Provinces (LLSVPs) near the core-mantle boundary (CMB) are key yet enigmatic structures. Their origin is often linked to the accumulation of subducted mid-ocean ridge basalt (MORB), but computational models question MORB as the sole source due to its predicted high shear wave velocity compared to normal mantle. This uncertainty is compounded by the lack of direct sound velocity measurements at CMB pressures. Here we address this gap through ultrahigh-pressure shear wave velocity measurements on CaCl- and α-PbO-type SiO, major phases in MORB, at pressures exceeding those of the CMB. Our results show shear velocities in dense SiO phases are ~ 7-14% lower than previous predictions under these conditions. Incorporating these values into MORB models suggests that the typical seismic anomaly of -1.5% (δln ) observed in LLSVPs can be explained by ~ 23-33 vol.% oceanic crust along a cold slab geotherm, without invoking extreme thermal anomalies (+1500 K). Considering a subduction history exceeding 2 billion years, this scenario supports long-term MORB accumulation at the lowermost mantle. These findings provide new constraints on LLSVP composition and offer critical insights into deep mantle dynamics and the evolution of Earth's interior.