MINERAL inclusions in diamonds contained in the explosive volcanic rocks known as kimberlites provide an important record of geochemical processes in the continental lithosphere. Samarium-neodymium model ages as old as 3.2 Gyr have been obtained for many peridotitic mineral inclusions(1-4), pointing to the great antiquity of the host ’peridotitic’ diamonds (relative to the age of the associated kimberlites) and hence an extended period of residence in the sub-continental mantle. Here we report trace-element data from garnet inclusions in peridotitic diamonds from the Mir kimberlite pipe, Siberia, which appear to be inconsistent with this interpretation. The heterogeneous distribution of tine trace elements both within and among discrete garnets from single diamonds are indicative of rapid crystal growth leading to solid-melt disequilibrium on a local scale. The conditions for survival of these heterogeneities within individual garnet crystals are tightly constrained by diffusion kinetics, which would rapidly homogenize the trace-element distributions in the sub-continental mantle. Thus, while it seems clear that peridotitic diamonds and their inclusions are derived from ancient lithospheric material, our data require that they crystallized shortly before the eruption of the kimberlite 360 Myr ago(5).