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142 Handbook on the Physics and Chemistry of Rare Earths of the OIB. In addition to this, however, the composition of the source mantle is assumed to be heterogeneous and distinct, and this heterogeneity may derive from the depth of the mantle; MORB are derived from the shallow mantle, and OIB are derived from deep the mantle (Fig. 5; White, 2013). 2.2.3 The Crust The Earth’s crust is the largest reservoir of REEs (Table 3) although the crust represents only a small fraction of mass (about 0.5%) of the Earth. The crust is composed of oceanic and continental crust. The oceanic crust is created by the magmatism at mid-oceanic ridges and basaltic in composition. The conti- nental crust is much thicker and on average andesitic in composition. The composition of the oceanic crust is estimated by the compositions and mineral assemblages of MORB (Table 3). It is remarkably uniform throughout the oceanic basins. The composition of the continental crust, on the other hand, is extremely heterogeneous, and it is difficult to estimate the average composi- tion. Estimates of crustal composition have been based on the compositions of the rocks taken over a large area, or the compositions of the glacial clays and/or loess that are regarded as well-mixed products of rocks in a wide area and time span. The most recent comprehensive study by Rudnick and Gao (2003) indi- cates that the upper and middle crusts are more enriched in REEs than the lower crust, and the upper crust is characterized by a negative Eu anomaly and middle and lower crust by slightly positive Eu anomaly (Fig. 7). 500 100 10 5 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Comparison of chondrite-normalized REE patterns in upper, middle, and lower crust (Rudnick and Gao, 2003). Upper continental crust Lower continental crust Middle continental crust FIG. 7 REE concentration/chondritePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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