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Refractory rocks with light REE/heavy REE ratios > chondrites are common as xenoliths in basalts and kimberlites and are found in some oceanic peridotite massifs. Structural and major-element geochemical evidence from these rocks suggest that the metasomatic effects resulting in addition of light REE are local and are related to emplacement of partial melts. The melts are represented by dykes of pyroxenites, hydrous minerals and gabbro that were emplaced in mantle peridotites of various origins. Metasomatic interaction between dykes and peridotite wall rock results in light REE enrichment in peridotite and depletion in dykes relative to the original liquid. Differentiation of the intrusions and separation of residual liquids may further enhance the REE exchange and extend the volume of metasomatized peridotite. Differences in the relative abundances of altered peridotite in xenoliths and massifs are seen as a sampling problem rather than a difference in process.-L.diH.