Continental crust incompatible element ratios

Figure 1 Systematics of Nd- and Hf-isotopic evolution in the bulk Earth, continental crust, and mantle. Daughter elements Nd and Hf are more incompatible during mantle melting (more likely to go into a partial melt of mantle rock) than Sm and Lu, respectively. As a result, the continental crust has a lower Sm/Nd and Lu/Hf ratio than the mantle, and lower Nd- and Hf-isotope ratios. Young continental crust has isotope ratios similar to the mantle, and the older the continental terrain, the lower the Nd- and Hf-isotope ratios. Rb-Sr behaves in the opposite sense, such that the parent element Rb is more incompatible than the daughter element Sr. (a) Schematic example of the evolution of Nd- and Hf-isotope ratios of a melt and the melt residue from a melting event around the middle of Earth history from a source with the composition of the bulk Earth, (b) The same scenario as in (a), but with the isotope ratios plotted as e d and snf. The bulk Earth value throughout geological time is defined as e d and SHf = 0> and e-value of a sample is the parts per 10 deviation from the bulk Earth value.

Halides (fluorine, chlorine, bromine, and iodine). Fluorine as F substitutes readily for OH in hydroxy minerals, implying that it probably occurs in all nominally anhydrous minerals in the same way as OH. Fluorine is not significantly soluble in seawater. Both these properties make its geochemical behavior quite different from the other halides. F/K and F/P ratios in basalts are reasonably constant (Smith et al, 1981 Sigvaldason and Oskarsson, 1986) with ratios of 0.09 0.04 and 0.29 0.1, respectively. Both ratios yield the same value of 25 ppm for the PM abundance which is listed in Table 4. However, the F/K ratio in the continental crust appears distinctly higher and the F/P ratio lower (Gao et al, 1998), indicating that the incompatibility of these elements increases in the order P < F < K. 

FIGURE 4.15 (a) Primitive mantle-normalized compositions for the present day upper continental crust and average Archaean upper crust, from McLennan et al. (2005). The comparison show the relative enrichment in incompatible elements in present-day upper continental crust (see Text Box 2.2). (b) The secular change in Th/La ratio in the upper continental crust is from the compilation by Plank (2005). The grey bands are the Archaean and post-Archaean averages from Taylor and McLennan (1985). 

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