Continental crust average trace element composition

Table 2 Estimates of the trace-element composition of the upper continental crust. Columns 1-4 represent averages of surface exposures. Columns 5-8 are estimates derived from sedimentary and loess data. Column 9 is a previous estimate, where bracketed data are values derived from surface exposure studies. Column 10 is our recommended value (see Table 3). 

Figure 10 Comparison of the trace-element composition of the middle continental crust as determined by sampling of surface exposures (Shaw et al., 1994 Weaver and Tarney, 1984) and inferred from middle-crustal seismic velocities combined with surface and xenolith samples (Rudnick and Fountain, 1995 Gao et al, 1998a). All values normalized to the new composition provided in Table 5 ( R G ), which is an average of the values of Gao et al. (1998a) and Rudnick and Fountain (1995). Gray shaded field represents 20% variation from this value, (a) transition metals, (b) high-field strength elements, (c) alkali, alkaline earth and actinides, and (d) REEs.

In every model for the composition of the upper-continental crust, major-element data are derived from averages of the composition of surface exposures (Table 1). Several surface-exposure studies have also provided estimates of the average composition of a number of trace elements (Table 2). For soluble elements that are fractionated during the weathering process (e.g., sodium, calcium, strontium, barium, etc.), this is the only way in which a reliable estimate of their abundances can be obtained. 

It has been known for over a century that the continental crust has an average composition approximating to andesite (when cast as an igneous rock type) (Clarke, 1889, Clarke and Washington, 1924). The myriad studies on continental crust composition carried out in the intervening years have refined our picture of the crust s composition, particularly for trace elements. 

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. 

Figure 4.23 Trace element concentrations normalized to the composition of the primordial mantle and plotted from left to right in order of increasing compatibility in a small fraction melt of the mantle. The normalizing values are those of McDonough et aL (1992) — Table 4.7, column 4. (a) Upper and lower continental crust from Weaver and Tamey (1984) — data in Table 4.8 (b) Average N-type MORE from Saunders and Tamey (1984) and OIB from Sun (1980) -— data in Table 4.8.

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