The Buoyancy of Cratonic Lithospheric Mantle

Abstract Cratons are generally thought to be characterized by stable, long‐lived mantle roots. However, recent studies have suggested that the lithospheric mantle may be prone to removal, implying that it may be denser than the asthenosphere. To address these suggestions, we use a global data set of mantle xenoliths to estimate the density structure of the cratonic lithospheric mantle (CLM), which we then compare with the density of the asthenospheric mantle at equivalent depths. Most of the CLM is either neutrally buoyant or slightly positively buoyant relative to the asthenosphere. The exception is where pyroxenite has accumulated due to melt infiltration. For these pyroxenites to remain within the CLM over geologically significant timescales they must be relatively small, that is, ≤1–20 km in size for a CLM viscosity of 10 21 –10 23 Pa·s. Our results suggest that the majority of cratonic lithospheric roots are long‐lived due to their neutral‐to‐positive buoyancy. Plain Language Summary The stability and longevity of mantle roots beneath cratons remain subjects of ongoing debate. While traditionally considered to be stable over billions of years, recent studies have suggested that these roots may be more frequently removed into the underlying mantle than previously thought, a process that would require them to be denser than the surrounding convecting mantle. To address this debate, we have calculated the densities of a global database of mantle‐derived xenoliths, at the pressures and temperatures that they equilibrated within the lithospheric mantle, which we then compare with the density of the convecting mantle at equivalent depths. Our results show that the cratonic lithospheric mantle (CLM) sampled by our mantle xenolith data set is dominantly neutrally or positively buoyant with respect to the convecting mantle. Our results therefore show that the buoyancy of the CLM results in its long‐term stability, the only exceptions being pyroxenites that have formed by melt infiltration. However, for pyroxenites to have remained in the lithospheric mantle for significant timescales, they must be volumetrically minor. Key Points Most of the cratonic lithospheric mantle is neutrally or positively buoyant relative to the asthenosphere Regions of negative buoyancy develop where pyroxenite has accumulated as a result of melt infiltration Pyroxenite bodies must be smaller than 1–20 km to be retained in the lithosphere over geological time scales