The continental lithosphere–asthenosphere boundary: Can we sample it?

Abstract

The lithosphere–asthenosphere boundary (LAB) represents the base of the Earth's lithosphere, the rigid and relatively cool outer shell characterised by a conductive thermal regime, isolated from the convecting asthenosphere. Chemically, the LAB should divide a lithospheric mantle that is variably depleted in basaltic components from a more fertile asthenosphere. In xenolith suites from cratonic areas, the bottom of the depleted lithosphere is marked by a rapid downward increase in elements such as Fe, Ca, Al, Ti, Zr and Y, and a rapid decrease in the median Mg# of olivine, reflecting the infiltration of mafic melts and related fluids. Eclogites and related mafic and carbonatitic crystallisation products are concentrated at the same depths as the maximum degrees of metasomatism, and may represent the melts responsible for this refertilisation of the lithosphere. This refertilised zone, at depths of ca 170–220 km beneath Archean and Proterozoic cratons, is unlikely to represent a true LAB. Re–Os isotopic studies of the deepest fertile peridotite xenoliths show that they retain evidence of ancient depletion events; seismic tomography data show high-velocity material extending to much greater depths beneath cratons. The cratonic “LAB” probably represents a level where asthenospheric melts have ponded and refertilised the lithosphere, rather than marking a transition to the convecting asthenosphere. Our only deeper samples are rare diamond inclusions and some xenoliths inverted from majoritic garnet, which are unlikely to represent the bulk composition of the asthenosphere. In younger continental regions the lithosphere–asthenosphere boundary is shallower (commonly at about 80–100 km). In regions of extension and lithosphere thinning (e.g. eastern China, eastern Australia, Mongolia), upwelling asthenosphere may cool to form the lowermost lithosphere, and may be represented by xenoliths of fertile garnet peridotites in alkali basalts. The LAB is a movable boundary. It may become shallower due to thermal and chemical erosion of the lithosphere, assisted by extension. Refertilised lithospheric sections, especially where peridotites are intermixed with eclogite, may be capable of gravitational delamination. The lithosphere–asthenosphere boundary may also be deepened by subcretion of upwelling hot mantle (e.g. plumes). This process may be recorded in the strongly layered lithospheric mantle sections seen in the Slave Craton (Canada), northern Michigan (USA) and the Gawler Craton (Australia).