The continental lithospheric mantle: characteristics and significance as a mantle reservoir

The continental lithospheric mantle (CLM) is a small-volumed (ca. 2.5% of the total mantle), chemically distinct mantle reservoir that has been suggested to play a role in the source of continental and oceanic magmatism. It is our most easily identifiable reservoir for preserving chemical heterogeneity in the mantle. Petrological and geophysical constraints indicate that the maximum depth of the CLM is ca. 250 km. There is a clear secular variation of CLM composition, such that CLM formed in the last 2 Gyr is less depleted and therefore less dynamically stable than ancient CLM formed in the Archean. We present new trace-element data for kimberlite-hosted lithospheric peridotites and metasomites. These data, combined with other data for spinel peridotites from non-cratonic regions, show that neither hydrous nor anhydrous lithospheric mantle xenoliths make suitable sources for continental or oceanic basalts. Addition of a hydrous phase, either amphibole or phlogopite, to depleted peridotite results in positive Nb and Ti anomalies that are the opposite of those predicted for some flood-basalt sources on the basis of their trace-element abundances. Overall, the Sr and Nd isotopic composition of cratonic and non-cratonic CLM is close to bulk Earth, with cratonic CLM showing small numbers of extreme compositions. Thus, while the CLM is certainly ancient in many locations, its average composition is not significantly 'enriched' over primitive upper mantle, in terms of either radiogenic isotopes or trace elements. These characteristics, plus a change in lithospheric chemistry with depth, indicate that the elemental and isotopic composition of lithospheric mantle likely to be re-incorporated into convecting mantle via delamination/thermal erosion processes is probably not very distinct from that of the convecting mantle. These observations lead us to question the requirement for CLM participation in the source of oceanic magmas and to promote consideration of a mantle that is chemically heterogeneous on all scales. Hf and Nd isotope compositions identify a distinctive source component in deeply derived alkaline volcanics associated with continents. This component cannot be constrained to the CLM but may originate from a deeper reservoir of ancient, subducted oceanic crust stored in the mantle.

Deflection of mantle plume material by cratonic keels

Lithosphere that formed in Archaean and possibly early Proterozoic time is thicker, more buoyant, and geochemically distinct from lithosphere that formed after about 2.3 Ga. Mantle xenolith and seismic data indicate that some cratonic roots, or ‘keels’, extend to depths of c. 250 km, compared with normal continental lithosphere of thickness 150 km or less; yet many cratons have experienced uplift, dyking and kimberlite emplacement, suggesting interactions with hot, rising asthenosphere referred to as mantle plumes. Plumes supply additional heat to the base of the lithospheric plates, whose base can be heated and entrained in the flow (thermal erosion). How have these cratonic keels persisted despite their interactions with mantle plumes? The geometry of cratonic keels during their interactions with mantle plumes is a critical factor controlling keel preservation. To a laterally spreading plume head, cratonic keels appear as major obstacles, and the hot, buoyant plume material ponds beneath thinner lithosphere. Our model simulations show that deep keels deflect mantle plume material and that steep gradients at the lithosphere-asthenosphere boundary between Archaean keels and ‘normal’ lithosphere will focus flow, leading to localized adiabatic decompression melting. Plume processes can lead to a reduction in the breadth of a cratonic root where the plume rises beneath the craton, regardless of the initial breadth of the craton. Where the plume rises beneath a craton the hot plume material will spread laterally beneath the keel and attain thicknesses of tens of kilometres. This transfers heat to the base of the lithosphere and could generate small volumes of melt at considerable depth, depending on the composition of the lower lithosphere. We have used model simulations of plumes beneath Africa to predict the magnitude and direction of seismic anisotropy caused by lateral flow of hot plume material beneath and around a cratonic keel. The shear-wave splitting in our models is greatest at the edge of the cratonic keel, and its azimuth is parallel to the plume flow direction.

Geodynamic evidence for a chemically depleted continental tectosphere

The tectosphere, namely the portions of Earth's mantle lying below cratons, has a thermochemical structure that differs from average suboceanic mantle. The tectosphere is thought to be depleted in its basaltic components and to have an intrinsic buoyancy that balances the mass increase associated with its colder temperature relative to suboceanic mantle. Inversions of a large set of geodynamic data related to mantle convection, using tomography-based mantle flow models, indicate that the tectosphere is chemically depleted and relatively cold to 250 kilometers depth below Earth's surface. The approximate equilibrium between thermal and chemical buoyancy contributes to cratonic stability over geological time.