Investigating the high seismic velocities and buoyancy of cratonic roots – the possibility of diamond

Abstract 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). However, changing only the major element composition and the Mg# does not fully meet the requirements implied by seismology or gravity observations. We explore various hypotheses, varying the temperature of the lithosphere, considering the presence of eclogite, accounting for chromium or for a minute quantity of diamonds. We also discuss the uncertainties in seismological observations of velocity and radial anisotropy as well as in mineralogical models. Changes in temperature, the presence of eclogite, of Cr incorporated in garnet, or radial anisotropy are inconsistent with shear velocity and gravity constraints when considered individually. Below the graphite/diamond transition, the combination of depletion with a low volume fraction of diamond (< 1% at 150 km) appears to be the simplest way to explain the very high shear velocities, while maintaining the cratonic lithosphere close to neutral buoyancy. This small amount of diamond accounts for only 0.5% of the mantle’s carbon and can be formed within a reasonable geological timescale (> 180 Ma) through the subduction of organic carbon.