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 (