The continental LAB: Can we sample it?

The subcontinental lithospheric mantle (SCLM) beneath cratonic areas is chemically depleted, buoyant and rigid relative to asthenospheric mantle, and is characterised by a conductive geotherm. The average degree of depletion, measured by eg Y in peridotitic garnets, or XMg in olivine, tends to decrease slowly downward, and to drop more rapidly in a zone 10-20 km thick, typically at depths of 150-200 km. In some localities, eclogites may constitute 30-50% of this basal zone. Detailed studies of sheared peridotite xenoliths, and the chemistry of peridotitic garnets, show that the base of the SCLM has been strongly modified by infiltration of (presumably) asthenosphere-derived melts, which may be represented by the eclogites. This boundary zone is commonly taken as the lithosphere-asthenosphere boundary (LAB), but there is little evidence that the mantle below this zone is convective, or of "asthenospheric" composition. We have few xenoliths from below the cratonic "LAB", a low-velocity zone is rarely observable at this depth, and seismic tomography suggests that material with velocities less than the SCLM, but greater than "asthenosphere", may extend well below this level. Instead, this basal zone probably represents a rheologically-controlled level of melt accumulation, marking a boundary that will move upward through time, as melt infiltration from below gradually modifies the cratonic roots and reduces the differences between them and younger SCLM.
Beneath Phanerozoic mobile belts and rift zones, the SCLM is typically only 50-80 km thick, and may be bounded by a seismic low-velocity zone. This young SCLM is typically much less depleted than cratonic SCLM, and shows little variation with depth. Volumes of more depleted peridotite occur at shallow depths in some sections, but Re-Os data suggest that these are relicts of older, buoyant SCLM, preserved during continental extension. Phanerozoic SCLM typically has (while young) strongly advective geotherms, related to the passage of basaltic melts that pond near the crust-mantle boundary. Other mafic rocks (pyroxenites) occur throughout the sections, rather than being concentrated at the base of the SCLM as in cratonic areas. This may reflect a different rheology; the chemical and thermal contrast between SCLM and "asthenosphere" is much less in the Phanerozoic situations. Below ca 60 km depth, young SCLM typically consists largely of garnet peridotites with undepleted (≈ PUM) compositions. We interpret these rocks as frozen asthenosphere, accreted to the base of the SCLM by upwelling that accompanied lithosphere thinning or delamination. Xenolith-bearing volcanic rocks erupted in such off-craton areas allow us to sample a "fossil" LAB, but one that probably bears little similarity to the cratonic situation.