Mapping depletion of continental roots with seismic waves.

High seismic velocity keels extending to depths greater than 200 km underlie the oldest parts of continents, the Precambrian cratons. Although mantle xenoliths suggests that these keels formed early in Earth’s history, the preservation of deep, cold, and highly viscous roots for hundreds of millions of years 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 that stabilizes high viscosity roots. Here, we map the magnesium number (Mg#, a proxi for mantle depletion) and the Modal content of olivine within cratonic keels, based on the thermochemical interpretation of a global shear velocity model. This thermochemical interpretation requires mineralogical depletion and a decrease of compositional density beneath Precambrian cratons. We observe that depletion is strong above 150 km (Mg#>92), and decreases with depth down to the lithosphere-asthenosphere boundary (LAB). These conditions ensure cratons’s preservation for hundred to billions of years in a convective mantle, in agreement with mantle xenoliths suggesting that high viscous keels formed early in the history of cratons