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REE Mineralogy and Resources Chapter 279 143 100 10 5 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Comparison of chondrite-normalized REE patterns of the total oceanic crust (White and Klein, 2013) and continental crust (Rudnick and Gao, 2003). The comparison of the chondrite-normalized REE patterns in the whole continental and oceanic crusts shows that both are enriched in all REEs rela- tive to primitive mantle as well as CI chondrite, and the continental crust is enriched in LREEs relative to the HREEs. In contrast, the oceanic crust is somewhat enriched in HREEs (Fig. 8). The oceanic crust is produced through comparatively large extents of melting of the upper mantle that has already been depleted in incompatible elements through previous episodes of melting. The strong enrichment of LREEs in the continental crust suggests that relatively small degree of partial melting of primitive mantle material has been important in its evolution (Hofmann, 2005; White, 2013). 2.3 Rare Earth Partitioning to Melts In addition to MORB and OIB, there are two more important tectonic settings for the generation of magmas: subduction zones and continental rift zones (Fig. 5). This section describes the partitioning of REEs to melts along the subduction zones and continental rift zones. 2.3.1 Subduction Zones Generation of magmas along the subduction zones is ascribed to the partial melting of mantle peridotite under hydrous condition. This hydrous condition is realized by the supply of aqueous fluids from subduction slab, which is metamorphosed and dehydrated when it descends beneath the continental or island arc margin. Aqueous fluids exsolved from the slab, which have been subducted to about 100–120 km depth beneath the mantle wedge below the magmatic arc, ascend into the mantle wedge asthenosphere, of which medium part attains temperatures of more than 1400°C. This addition of fluids to Continental crust Oceanic crust FIG. 8 REE concentration/chondritePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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