Continental crust enrichment-depletion

Figure 3 Crust-mantle differentiation patterns for the decay systems Rb-Sr, Sm-Nd, Lu-Hf, and Re-Os. The diagram illustrates the depletion-enrichment relationships of the parent-daughter pairs, which lead to the isotopic differences between continental crust and the residual mantle. For example, the Sm/Nd ratio is increased, whereas the Rb/Sr ratio is decreased in the residual mantle. This leads to the isotopic correlation in mantle-derived rocks plotted in Figure 4(a). The construction is similar to that used in Figure 2, but D values have been adjusted slightly for greater clarity.

An Ej value of greater than one represents enrichment of element i in the sample as compared to the reference whereas a value less than one means depletion. In order to avoid possible confusion, sample and reference names can be added after j. As shown in Figure 1(a), the average shale composition is very similar to that of the upper continental crust (Ej i = 1.0 0.3). The obvious exceptions are Li, and volatile elements B, C, N, S, Se, Te, Br, I, As, Cd, In, Sb, Hg, and Bi, which are enriched in the shale in comparison to the upper continental crust when aluminum is chosen as the normalizing element (Li, 2000). These excess volatile elements came from the interior of the Earth by magmatic... 

Beyond the broad major-element constraints afforded by seismic imaging, the abundance of many trace elements in the mantle clearly records the extraction of core (Chapters 2.01 and 2.15) and continental crust (Chapter 2.03). Estimates of the bulk composition of continental cmst (Volume 3) show it to be tremendously enriched compared to any estimate of the bulk Earth in certain elements that are incompatible in the minerals that make up the mantle. Because the crust contains more than its share of these elements, there must be complementary regions in the mantle depleted of these elements—and there are. The most voluminous magmatic system on Earth, the mid-ocean ridges, almost invariably erupt basalts that are depleted in the elements that are enriched in the continental crust (Chapter 2.03). Many attempts have been made to calculate the amount of mantle depleted by continent formation, but the result depends on which group of elements is used and the assumed composition of both the crust and the depleted mantle. If one uses the more enriched estimates of bulk-continent composition, the less depleted estimates for average depleted mantle, and the most incompatible elements, then the mass-balance calculations allow the whole mantle to have been depleted by continent formation. If one uses elements that are not so severely enriched in the continental cmst, for example, samarium and neodymium, then smaller volumes of depleted mantle are required in order to satisfy simultaneously the abundance of these elements in the continental cmst and the quite significant fractionation of these elements in the depleted mantle as indicated by neodymium isotope systematics. 

Crust-mantle chemical mass-balance models offer important constraints on compositional variations in the mantle, but their constraints on the size of the various reservoirs involved depend critically on uncertainties in the estimates of the bulk composition of the continental crust, the degree of depletion of the complementary depleted mantle, and the existence of enriched reservoirs in Earth s interior, for example, possibly significant volumes of subducted oceanic crust. This last item was left out of the mass-balance models that suggested that the upper and lower mantle are chemically distinct. Chapter 2.03 makes it clear that much of the chemical and isotopic heterogeneity observed in oceanic volcanic rocks reflects various mixtures of depleted mantle with different types of recycled subducted crust. With this realization, and excepting the noble gas evidence for undegassed mantle, some of the characteristics of what was once labeled... 

FIGURE 3.29 eH time diagrams for the evolution of the Earth s mantle, (a) The mantle evolution curve of Vervoort and Blichert-Toft (1999) showing pre-3.0 Ga fractionation followed by a less extreme fractionation event. The mantle curve indicates that the mantle was depleted in Lu relative to Hf. Negative eHf-values would imply derivation from an enriched source such as enriched mantle or ancient continental crust, (b) A proposed eH -mantle evolution curve based upon eH -values calculated using the fast decay sheme of Bizzarre et al. (2003) - A = 1.983 X 10-11. This dataset supports the mantle evolution curve proposed by Vervoort and Blichert-Toft (1999) shown in (a). The data are taken from Blichert-Toft and Arndt (1999), Vervoort and Blichert-Toft (1999), Amelin et al. (1999), and Amelin et al. (2000). Symbols diamonds, most radiogenic samples Arc, modern arc samples ... 

There is good trace element and isotopic evidence to support the view that the continental crust and the depleted upper mantle are complementary geochemical reservoirs relative to the composition of the Earth s primitive mantle. The trace element enriched nature of the continental crust relative to the primitive mantle was illustrated in Fig. 4.11, and, it is argued, the depleted mantle is the complementary reservoir. At first sight the trace element enriched character of the Earth s continental crust looks like a 1 % melt of the upper mantle (O Nions McKenzie, 1988). However, that is not to say that this is the precise mechanism of its extraction, for McKenzie (1989) showed that it is not possible to extract such a small melt fraction from... 

Table 4.3 below summarizes the different reservoirs thought to be present in the modern silicate Earth. However, of these, only the depleted mantle and buried eclogitic slabs have compositions which make them directly complementary to the composition of the continental crust. It is suggested here that Archaean SCLM is not closely related in a geochemical sense to Archaean felsic crust but is the product of basalt extraction, basalt which now is emplaced within the Archaean crust. In contrast, Phanerozoic SCLM may be the restite-complement of the basaltic precursor to modern crust. The proposal that there was an early-formed (pre 3.7 Ga), enriched, basaltic crust (Section 4.5.1.2.2), if confirmed, has important implications for the balance between the major Earth reservoirs, not least because the primitive mantle and the bulk silicate Earth can no longer be regarded as com-positionally identical. 

The evolution of Nd/ isotopes with rime in the mande, the continental crust and the bulk earth. Relative to the bulk Earth (CHUR) in which the fractionation of Sm/Nd is normalized to unity, the depleted mande (DM) has a high Sm/Nd ratio and shows higher Nd. The continental crust has lower Sm/Nd and shows a retarded Nd/ " Nd evolution with time. Enriched mantle (EM) shows some affinity with the continental crust inasmuch as it also has a retarded evolution. Note that in the Sm-Nd system... 

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