Thorium bulk Earth composition

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). 

The U (uranium)-Th (thorium)-Pb (lead) isotopic system represents three independent decay schemes and is a powerful but complex tool with which to unravel the history of the Earth s mantle (Text box 3.2). During planetary accretion U and Th are refractory, lithophile elements and will reside in the mantle. Pb on the other hand is a volatile and chalcophile/ siderophile element and may in part, be stored in the core. Initial U and Th concentrations are derived from chondritic meteorites, and initial Pb isotope compositions are taken from the iron-sulfide troilite phase in the Canyon Diablo meteorite. The initial bulk Earth U/Th ratio was 4.0 0.2 (Rocholl Jochum, 1993). 

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