Extended Data Fig. 5: Structure of the lithospheric keel beneath the Lac de Gras area, Slave craton, northern Canada⁷².

This is one of the most striking examples of a layered sub-cratonic lithospheric mantle. An ultradepleted layer extends <100–150 km, where there is a sharp boundary to a more fertile layer, with a high degree of melt-related metasomatism¹⁴. The Al₂O₃-enriched boundary zone corresponds to a high concentration of eclogites, which are notably diamond-rich. Re-Os data indicate that the upper layer experienced depletion around 3.4 Ga, and the lower layer at about 3.27 Ga (refs. ^78,79). Chromite is most abundant, and most Cr-rich, but also most Ti-rich, just below the boundary between layers. Lower-mantle diamonds⁵⁹ are regarded as evidence that the deeper layer represents a plume head, but other interpretations are possible^14,81,82.