Understanding the generation and evolution of cratonic lithospheric keels requires detailed knowledge of their age, timescale of formation, and the relationship between the crust and mantle part of the lithosphere. Re-Os isotope analyses of peridotite xenoliths from Lesotho kimberlites indicate that large portions of the lithospheric keel beneath the SE Kaapvaal craton were stabilized during the late Archean. This process and subsequent additions of new lithospheric mantle coincide with events recorded by lower crustal xenolith studies from this region. ¿ 2001 American Geophysical Union