Continental crust, upper

Sr/ Sr ratios which are not greatly different from modem mantle values. This means that a modem granite derived from the lower crust and one derived from the mantle will have very similar Sr/ Sr initial ratios. U/Pb and Th/Pb ratios in the lower crust are lower than modem mande values so that Pb, and Pb 

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Teng FZ, McDonough WF, Rudnick RL, Dalpe C, Tomascak PB, Gao S, Chappell BW (2004) Lifiiium isotopic composition and concentration of the upper continental crust. Geochim Cosmochim Acta (in press)... 

Coleman ML (1971) Potassium-calcium dates from pegmatitic micas. Earth Planet Sci Lett 12 399-405 Condie KC (1993) Chemical composition and evolution of the upper continental crust contrasting results from surface samples and shales. Chem Geol 104 1-37... 

Table 18.1 Average Compositions of the Earth s Upper Continental Crust, Shale, Iron-Manganese Oxides, Phosphorite, and Various Types of Marine Sediments (All in Units of ppm. Unless Noted otherwise), along with Seawater and a Hydrothermal Vent Solution from the East Pacific Rise (both in Units of 10 g L ). 

Although the uppermost part of the continental crust is relatively accessible for sampling, it is very heterogeneous and its overall arsenic concentration is difficult to estimate. Sims, Newsom and Gladney (1990, 302) provides an estimate of 5.1 1 mg kg-1 (1 SD Table 3.3). In comparison, Wedepohl (1995) used chemical data from rocks of the Canadian Precambrian shield to obtain a somewhat lower arsenic concentration of 2.0 mg kg-1 (Table 3.3). Granites and granodiorites, which are very common in the upper continental crust (Wedepohl, 1995), typically contain around 3 mg kg-1 of arsenic (Matschullat, 2000, 299 Table 3.4). 

Like the upper continental crust, the lower portion of the continental crust probably consists of heterogeneous distributions of various metamorphic and igneous rocks (Sims, Newsom and Gladney, 1990). Available chemical data from rock samples suggest that the lower continental crust is depleted in arsenic when compared with the upper crust (Sims, Newsom and Gladney, 1990,303-304). Specifically, Wedepohl (1995) estimated the average arsenic concentration of the lower continental crust at 1.3 mg kg-1, which is somewhat lower than his 2.0 mg kg-1 of arsenic for the upper crust (Table 3.3). For the continental crust as a whole, Wedepohl (1995, 1220) obtained an arsenic value of 1.7 mg kg-1. 

Figures 4-6 show the isotopic compositions of MORBs from spreading ridges in the three major ocean basins. Figures 4(b) and 5(a) also show isotope data for marine sediments, because these are derived from the upper continental crust and should roughly represent the isotopic composition of this crust. In general, the isotopic relationships between the continental and oceanic crust are just what is expected from the elemental parent-daughter relationships seen in Figure 3. The high Rb/Sr and low Sm/Nd and Lu/Hf ratios of continental materials relative to the residual mantle are reflected by high Sr/ Sr and low " Nd/ " Nd and Hf/ Hf ratios (not shown).

Peucker-Ehrenbrink B. and Jahn B.-M. (2001) Rhenium-osmium isotope systematics and platinum group element concentrations loess and the upper continental crust. Geochem. Geophys. Geosys. G 3 2001GC000172. 

The upper continental crust, being the most accessible part of our planet, has long been the target of geochemical investigations (Clarke, 1889). There are two basic methods employed to determine the composition of the upper crust ... 

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. 

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 3 Comparison of different models for the trace-element composition of the upper-continental crust. All values normalized to the new composition provided in Table 3. Gray shaded field represents 20% variation from this value for all panels except (f), in which gray field represents a factor of two variation. Trace elements are divided into the following groups (a) transition metals, (b) high-field strength elements, (c) alkali, alkaline-earth elements, (d) REEs, (e) actinides and heavy metals, and (f) highly siderophile and chalcophile elements (note log scale). Data from Tables 1 and 2 lanthanum estimate from Eade and Fahrig (1973) is omitted from panel D.

Figure 5 Comparison of REE patterns between (a) average post-Archean shales and loess and (b) various estimates of the upper continental crust composition. PA AS = post-Archean Australian Shale (Taylor and McLennan, 1985) NASC = North American shale composite (Haskin et al., 1966) ES = European shale composite (Haskin and Haskin, 1966) ECPAS = Eastern China post-Archean shale (Gao et al., 1998a). The loess range includes samples from China, Spitsbergen, Argentina, and France (Gallet et al., 1998 Jahn et al., 2001).

Plank and Langmuir (1998) also suggested, from their analyses of marine sediments, increasing the upper crustal Ti02 values by —40% (from 0.5 wt. % to 0.76 wt. %). Thus the Ti02 content of the upper-continental crust probably ties between 0.55 wt.% and 0.76 wt.%, a difference of —30%. 

Table 3 Recommended composition of the upper continental crust. Major elements in weight percent.

Borodin L. S. (1998) Estimated chemical composition and petrochemical evolution of the upper continental crust. Geochem. Int. 37(8), 723-734. 

Gallet S., Jahn B.-M., van Vliet Lanoe B., Dia A., and Rossello E. (1998) Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth Planet. Sci. Lett. 156, 157-172. 

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