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The lithospheric mantle beneath continents is often the same age as the superjacent crust, but remains less well understood. Analysis based on a large database of xenoliths and xenocrysts shows that mantle domains that stabilized during different geologic eons have distinctly different mean compositions. There is a secular evolution from depleted Mg-rich lowdensity Archean mantle to more fertile, denser Phanerozoic mantle; the most significant differences are between the Archean and Proterozoic mantle. The compositional variations produce differences in the density and elastic properties of lithospheric mantle of different age. Archean and Proterozoic

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We have mapped the deep structure of the Slave craton by combining analysis the effective elastic thickness (Te) with data on mantle samples from numerous kimberlites. Three‐dimensional mapping of the subcontinental lithospheric mantle (SCLM), using mantle‐derived xenoliths and xenocrysts in kimberlites, has shown that much of the craton is underlain by a strongly layered SCLM; a highly depleted upper layer (low in basaltic components Ca, Al, Fe) is separated from a relatively fertile lower layer by a sharp boundary. This boundary lies at 140–150 km depth in the Lac de Gras area and shallows to ≤100 km in the northern and southern parts of the Craton. Weak lithosphere (Te < 25 km) on the northern edge of the craton may reflect the intrusion of the Mackenzie Plume (circa 1270 Ma). The strongest lithosphere (Te > 56 km), in the younger eastern part of the craton, is separated from the older western part by a zone of steep Te gradient parallel to the major locus of kimberlite intrusion, which may map the deep extension of the boundary between the two domains. Another strong Te gradient across the Kilohigok Basin accompanies a marked compositional change in the upper layer of the lithospheric mantle; the Basin probably marks a major translithospheric fault. Correlations between Te and mantle composition suggest that Te is strongly influenced by the rheology of the upper mantle.

[1] The spatial distribution of large-scale lithospheric domains and the boundaries between them may control the emplacement of large ore bodies, and as such, regional mapping of the lithosphere is relevant to mineral exploration. In this study we combine potential-field geophysical data and mantle petrology to map major lithospheric structures on the eastern part of the Siberian platform. The platform consists of several Archean and Proterozoic terranes that have been mapped from regional magnetic data and basement exposures in the Anabar shield. We use garnet and chromite concentrates from a chain of Paleozoic to Mesozoic kimberlites across the platform to construct mantle sections, which show significant lateral variation in rock type distribution within the lithospheric mantle. These lateral variations correspond to the terranes mapped at the surface and indicate that the terrane boundaries are translithospheric. Archean terranes are underlain by depleted Archean lithosphere more than 200 km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere. Geophysical data show more strongly negative Bouguer anomalies and a more heterogeneous magnetic anomaly pattern over the Archean terranes than on the Proterozoic terranes. The pattern of the gravity data reflects the lateral variation in mantle composition beneath the terranes, as shown by mantlepetrology studies. We invert gravity and topography data to estimate the flexural strength, or elastic thickness (Te), of the lithosphere across the area. Although on a stable Precambrian craton, the Te is relatively low (<30 km) across most of the area, suggesting a relatively weak lithosphere comparable to that of tectonically much younger areas around the world. A 150-km-wide zone of very weak lithosphere (Te < 10 km) runs N-S across the western part of the study area. This weak zone coincides with a zone of thickened lower crust, and abnormally high sub-Moho P wave velocities which suggest anisotropy in the upper mantle. The kimberlite fields in the Archean part of the platform are localized on the flanks of this zone of weak lithosphere. We suggest that the low-Te zone may be a mantle shear zone which has been a preferred conduit for the emplacement of magmas into the lower crust and later has controlled the emplacement of kimberlites in the study area.Components: 9175 words, 12 figures.