Two continent-sized features in the deep mantle, the large low-velocity provinces (LLVPs), influence Earth's supercontinent cycles, mantle plume generation, and geochemical budget. Seismological advances have steadily improved LLVP imaging, but several fundamental questions remain unanswered, including: What is the true vertical extent of the buoyancy anomalies within these regions? And, are they purely thermal features, or are they also compositionally distinct? Here, we address these questions using a comprehensive range of geophysical observations. The relationship between measured geoid anomalies and long-wavelength dynamic surface topography places an important upper limit on the vertical extent of large-scale, LLVP-related density anomalies at ∼900 km above the core-mantle boundary (CMB). Instantaneous mantle flow modelling suggests that anomalously dense material must exist at their base to simultaneously reproduce geoid, dynamic topography, and CMB ellipticity observations. We demonstrate that models incorporating this dense basal layer are consistent with independent measurements of semi-diurnal Earth tides and Stoneley mode splitting functions. Our thermodynamic calculations indicate that the presence of early-formed, chondrite-enriched basalt in the deepest 100–200 km of the LLVPs is most compatible with these geodynamic, geodetic, and seismological constraints. By reconciling these disparate datasets for the first time, our results demonstrate that, although LLVPs are dominantly thermal structures, their basal sections likely represent a primitive chemical reservoir that is periodically tapped by upwelling mantle plumes.