Seismic evidence for the presence of diamonds in the deepest parts of continental roots

High seismic velocity keels extending to depths greater than 200 km underlie the oldest parts of continents, the Precambrian cratons. These keels have probably been formed early in Earth’s history, and the preservation of these deep, cold, and highly viscous roots in a convective mantle remains enigmatic. A classical view is that the excess density due to colder temperatures is compensated for by a light composition. Here, we map the magnesium number (Mg#, a proxi for mantle depletion) within cratonic keels, based on the thermochemical interpretation of a global shear velocity model. Our interpretation suggests that depletion is strong above 150 km (Mg#>92), and decreases with depth down to the lithosphere-asthenosphere boundary (LAB). Below the graphite/diamond transition, the combination of depletion with a low volume fraction of diamond (<1% at 150 km) is necessary to explain the very high shear velocities, while maintaining the cratonic lithosphere close to neutral buoyancy. Our results suggest that a small amount of diamonds is present in the deep part of continental roots, particularly beneath Australia, North America, South Africa, Scandinavia, and Antarctica. Their presence is not exclusively linked to volcanism on the periphery of cratons, where they have been discovered at the surface.