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138 Handbook on the Physics and Chemistry of Rare Earths plagioclase, and HREEs are highly partitioned to garnet due to the similarity of ionic size, which is the main cause of HREE depletion in the magmas such as carbonatites that principally form at the garnet-stable field (Harrison, 1981). 2.2 Concentration of REEs in the Earth 2.2.1 Bulk Silicic Earth Chondrites constitute primitive meteorites, which represent chemical properties of the solar nebula, the cloud of gas and dust from which the solar system formed. Chondrites are classified into carbonaceous chondrites, ordinary chon- drites, and enstatite chondrites. Ordinary chondrites are composed of olivine, orthopyroxene, and lesser amounts of nitrogen–iron alloy, while enstatite chondrites are highly reduced, with iron metal and enstatite. Those chondrite classes may have undergone thermal and impact influence in the host stars (Van Schmus and Wood, 1967). In contrast, carbonaceous chondrites are enriched in the abundances of carbon, sulfur, water, and other volatile compounds than the other chon- drite classes. This indicates the most primitive chemical characteristic among the classes of chondrites mentioned earlier. Thus, one of the sub- classes of carbonaceous chondrites, represented by Carbonaceous Ivuna (CI) chondrite, has been regarded to represent the chemical composition of the solar system. This is supported by the fact that the composition of CI chondrites is most similar to that of the sun determined by spectrograph- ically except noncondensing elements and lithium (White, 2013). The chem- ical composition of the CI chondrite shows that the abundance of REEs in the earth follows Oddo–Harkins effect that the elements with even atomic number are more stable and abundant than the neighboring elements of the odd atomic number, and the lighter REEs are more abundant than the heavier ones (Table 3 and Fig. 4). 2.2.2 Mantle The chemical composition of the mantle is assumed by modifying the com- position of CI chondrites, on the basis of the correlation with elements in peridotites, which are dominated in the mantle. Abundances of the refractory lithophile elements including REEs in the primitive mantle are enhanced rel- ative to CI chondrite (Table 3), because volatile and siderophile elements and some oxygen were removed to the core (eg, McDonough and Sun, 1995). HREEs mainly condensed first in aluminate hibonite (CaAl12O19) with litho- phile elements such as Th and U, when it formed from high-temperature gas of solar composition. Somewhat less refractory elements (V, Nb, Ta, LREEs, Sr, and Ba) condensed to calcium titanates such as perovskite when they became present as an additional host. Europium, the most volatile element in this group, condensed when feldspar became stable (Lodders, 2003).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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