Are lithospheres forever? Tracking changes in subcontinental lithospheric mantle through time

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 low-density 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 mantle roots are highly buoyant; they cannot be delaminated but require mechanical disaggregation (lithospheric thinning and/or rifting) and infiltration of upwelling fertile material to be destroyed or transformed. In contrast, Phanerozoic subcontinental lithospheric mantle is denser than the asthenosphere for observed thicknesses (∼100 km) and can "delaminate" under stress. The contrasting properties of different mantle domains require lateral contrasts in composition, density, thickness, and seismic response in the present-day subcontinental lithospheric mantle. They also suggest a secular evolution in Earth's geodynamics from Archean to Proterozoic time, and an increased importance for lithosphere-delamination processes in Phanerozoic orogens.