Neodymium crust/mantle ratios

Chondritic relative abundances of strongly incompatible RLEs (lanthanum, niobium, tantalum, uranium, thorium) and their ratios to compatible RLEs in the Earth s mantle are more difficult to test. The smooth and complementary patterns of REEs in the continental crust and the residual depleted mantle are consistent with a bulk REE pattern that is flat, i.e., unfractionated when normalized to chondritic abundances. As mentioned earlier, the isotopic compositions of neodymium and hafnium are consistent with chondritic Sm/Nd and Lu/Hf ratios for bulk Earth. Most authors, however, assume that RLEs occur in chondritic relative abundances in the Earth s mantle. However, the uncertainties of RLE ratios in Cl-meteorites do exceed 10% in some cases (see Table 4) and the uncertainties of the corresponding ratios in the Earth are in same range (Jochum et ai, 1989 W eyer et ai, 2002). Minor differences (even in the percent range) in RLE ratios between the Earth and chondritic meteorites cannot be excluded, with the apparent exception of Sm/Nd and Lu/Hf ratios (Blicher-Toft and Albarede, 1997). 

REEs are classified as lithophiles and are partitioned into the earth s crust and mantle. The name rare earths originated over a century ago when the elements were first identified in minerals that, at the time, were rare. The elements are actually distributed widely over the earth and relatively accessible on the earth s surface. For a comprehensive description of REE geology, geochemistry, and natural abundances, see Geology, Geochemistry, and Natural Abundances of the Rare Earth Elements. In 2010, the United States Geological Survey (USGS) estimated that there were REE reserves of 110 million metric tons (mt). The static depletion index, the ratio of reserves to present-day production, for REEs is approximately 870 years. Thus, the primary immediate consideration is whether REE production can match demand, and particularly whether it will be possible to increase the use of dysprosium and neodymium in wind turbines and the batteries of electric vehicles. 

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