The values of individual rare earth element (REE) partition coefficients (weight ratio, CREEvapor/CREEcrystal) = KREEvapor−crystal) range between 1 and 200 for garnet, clinopyroxene, orthopyroxene, and olivine in equilibrium with H2O-rich vapor under experimental conditions corresponding to those of the upper mantle. The KREEvapor−crystal increases rapidly with increasing PH2O (e.g.,KSmvapor−cpx = 0.01, 3, 12 and 31, respectively, at 5, 10, 20, and 30 kbar). Calculated bulk partition coefficients (as defined by Shaw, 1970) range from 5 to 250 for an average garnet peridotite assemblage in the pressure range 5-40 kbar, with DCevapor−rock > DTmvapor−rock

The portions of the upper mantle sampled and brought to the surface by kimberlite magma characteristically display garnet contents ranging from near zero to about 20 percent. If a suite of such garnet peridotites equilibrated with H2O-rich vapor, a wide range of absolute REE contents and degree of light REE enrichment would result. It is notable that the more depleted garnet peridotite nodules in kimberlite (also the most garnet-poor) commonly show great light-REE enrichment (Ce/Yb ≥ 10), as would be expected if these rocks have suffered metasomatic alteration.

Andesites in island arcs commonly show considerable light-REE enrichment (Ce/Yb =2.5-10). It is argued here that the peridotite wedge overlying a descending plate of oceanic basaltic composition with an eclogitic mineral assemblage would attain considerable light-REE enrichment as a result of equilibration with H2O-rich vapor released by dehydration of the material in the descending slab. If, for example, 0.5 weight percent H2O was released and initially equilibrated with an eclogitic rock before migrating into the overlying peridotite, the peridotite (with an initially unfractionated REE pattern) could attain Ce/Sm and Ce/Tm ratios near 3 and 5 respectively. Partial melting of this hydrated peridotite would result in melts of andesitic bulk composition with Ce/Sm = 3-4 and Ce/Tm ∼ 10, closely resembling the REE patterns found in andesites in island arcs.

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