Upper mantle xenoliths are abundant in the basanites of the Pliocene Kozakov volcano. The studied samples of spinel lherzolites come from the depth of about 50–75 km. Their mineral assemblage preserved subsolidus temperatures of 1165–1052 ?C from the time of xenolith entrapment. Oxygen fugacity varies from +0.14 to +0.93 log unit relative to fayalite-magnetite-quartz (FMQ) buffer. Major bulk-rock oxides and variations in mineral chemistry indicate a continual depletion trend mainly associated with extraction of basaltic melt from the mantle. Mineralogical features and the absence of highly oxidized lherzolites suggest a negligible degree of modal metasomatic overprint. On the contrary, the LREE upward patterns and U-shaped REE patterns of clinopyroxenes, as well as of the bulk lherzolite compositions are indicators of cryptic metasomatic event(s) in the upper mantle. The U-shape REE patterns corroborates to enrichment mechanisms in the mantle by reactive porous flow and chromatographic fractionation. A possible cryptic metasomatic event(s) could have occurred in pre-Cenozoic times, probably during the Variscan orogeny.