Fred Richards , Mark James Hoggard , Siavash Ghelichkhan, Paula Koelemeijer , Harriet Lau 

Two continent-sized features in the deep mantle, the large low-velocity provinces (LLVPs), influence Earth's supercontinent cycles, mantle plume generation, and its geochemical budget. Seismological advances have steadily improved LLVP imaging, but several fundamental questions remain unanswered, including: What is their vertical extent? And, are they purely thermal anomalies, or are they also compositionally distinct? Here, we investigate these questions using a wide range of observations. The relationship between measured geoid anomalies and long-wavelength dynamic surface topography places an important upper limit on LLVP vertical extent of ~900 km above the core-mantle boundary (CMB). Our 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 modes. Our thermodynamic calculations indicate that a ~100 km-thick layer of early-formed, chondrite-enriched basalt is the chemical configuration most compatible with these geodynamic, geodetic and seismological constraints. By reconciling these disparate datasets for the first time, our results demonstrate that, although dominantly thermal structures, basal sections of LLVPs represent a primitive chemical reservoir that is periodically tapped by upwelling mantle plumes.