A new model for the thickness and thermal structure of the African lithosphere: implications for the distributions of kimberlites, carbonatites and critical mineral deposits

The structure, thickness, lateral heterogeneity, and temporal evolution of the lithosphere significantly influence the distribution of kimberlites, carbonatites, and sediment-hosted mineral deposits, including rare earth elements (REE) and critical metals (e.g., Nb and Ti) that are essential for advancing the transition to green energy.

Seismic data provide critical information on the thermal structure of the lithosphere and underlying mantle. However, seismic tomographic models are inherently non-unique. This can be remedied, to a large extent, by thermodynamic inversions, which utilize computational petrology and offer an effective approach to connecting seismic observations to the thermal structure of the lithosphere and mantle.

We present a new model of the African lithosphere’s thickness and thermal structure, derived from state-of-the-art sampling with seismic surface wave data. The model incorporates both Rayleigh and Love waves, to account and correct for seismic anisotropy of the elastic properties. Rayleigh and Love wave data in the 20–300 s range are inverted, on 1°×1° grids, for the upper-mantle temperature and lithospheric thickness, from which upper-mantle density and seismic velocities are calculated, with attenuation corrections. Radial anisotropy, seismic velocities in the crust, transition zone and uppermost lower mantle, and crustal density are also inversion parameters, the latter constrained primarily by the surface elevation. The resulting model reveals distinct regional variations in the lithospheric thickness that reveal deep lithospheric expressions of known crustal geology. Thick lithosphere (>220 km) is found beneath large parts of the West African Craton, Congo Craton, and Zimbabwe Craton. Thin lithosphere (<70 km) is predominantly observed along the East African Rift.

We analyse the new lithosphere model jointly with recent datasets of the distribution of different types of igneous rocks across the continent. These include kimberlites, which were emplaced at locations with thick cratonic lithosphere; basalts, which are emplaced at locations with thin lithosphere; and carbonatites that are commonly found on intermediate-thickness lithosphere (Gibson et al. 2024). Statistics analysis of the locations of these rock samples shows that kimberlites mostly are found within cratons, with some notable exceptions. Most Neogene basalts are in the East African Rift Zone, with a 50–100 km lithosphere. Carbonatite complexes and their associated REE deposits, are typically observed in clusters in the transition regions from cratonic to non-cratonic lithosphere.

This new lithospheric thickness and temperature model enhances our understanding of the dynamics and evolution of the African lithosphere. Furthermore, it provides valuable insights into the processes that govern the generation and spatial distribution of rocks of different types and the associated primary critical mineral deposits.

 

Gibson, S., McKenzie, D. & Lebedev, S. (2024). The distribution and generation of carbonatites. Geology 52, 667–671.