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Extended Data Fig. 5 | Structure of the lithospheric keel beneath the Lac de
Gras area, Slave craton, northern Canada^72. 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 metasomatism^14. The Al 2 O 3 -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 diamonds^59 are regarded as
evidence that the deeper layer represents a plume head, but other
interpretations are possible^14 ,^81 ,^82.