Geodynamic, Geodetic, and Seismic Constraints Favour Deflated and Dense-Cored LLVPs

The two large low velocity provinces (LLVPs) that sit atop the core-mantle boundary (CMB) influence mantle plume generation, Earths supercontinent cycles, and its geochemical budget. While advances in global seismology have steadily improved the imaging of these features, deciphering their structure and composition has proven difficult due to resolution limitations and the large number of thermochemical configurations that are compatible with seismic constraints. Consequently, key questions remain unanswered including: How high do LLVPs extend above the CMB? Are LLVPs purely thermal anomalies or do they also contain compositionally distinct material? Here, we provide answers to these questions by using a wide range of geodynamic, geodetic, and seismic data to address the non-uniqueness associated with applying seismological techniques in isolation. We demonstrate that 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 CMB. Our mantle flow modelling indicates that a layer of anomalously dense material must be present at the base of these structures to simultaneously reproduce geoid, dynamic topography and CMB excess ellipticity observations. Significantly, we show that Earth models incorporating this dense basal layer within the LLVPs are also consistent with independent semi-diurnal body tide and Stoneley mode splitting measurements. By performing thermodynamic calculations for several candidate primordial and recycled compositional endmembers, we show that a 100200 km-thick dense layer composed of iron-rich Hadean crust and chondrite-like regolith is the chemical configuration most compatible with the geodynamic, geodetic and seismological constraints. Through reconciling these different datasets, our results demonstrate that, although the LLVPs are dominantly thermal structures, their basal sections likely represent an early-formed chemical reservoir that is periodically tapped by upwelling mantle plumes.