Mantle metasomatism: The REE story

Refractory rocks with light REE/heavy REE ratios greater than chondrite are common as xenoliths in basalts and kimberlites and are found in some oceanic peridotite massifs. This has led to the supposition that large parts of the upper mantle have been metasomatically altered by addition of light REE and other hyperfusible constituents.

Structural and major-element geochemical evidence from xenoliths and alpine peridotites, however, suggest that the metasomatic effects are local and are related to emplacement of partial melts. The melts are represented by dikes of pyroxenite, hydrous minerals, and gabbro that occur in mantle peridotites of all origins and were emplaced in them in the same sequence as indicated by crosscutting relations. REE distributions in both the peridotite and the dikes may be explained as a result of metasomatic interaction between dikes and peridotite wall rock in which the peridotite is enriched in light REE and the dikes depleted in light REE relative to the original liquid. Differentiation of the intrusions and separation of residual liquids may further reduce the light REE/heavy REE ratio in pyroxenite dikes; these residual liquids (hydrous mineral veins) enriched in light REE extend the volume of metasomatized peridotite as they too interact with their wall rocks. Differences in the relative abundances of altered peridotite (reacted wall rock) in xenoliths and massifs are seen as a sampling problem rather than a difference in process.