Continental crust trace elements

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 5 Compositional estimates of the middle continental crust. Major elements in weight percent. Trace element...

Relationship between the trace elemental composition of phytoplankton, continental crust and seawater. Phytoplankton and crustal abundances are normalized to phosphorus (ppm trace metal ppm P). Seawater trace elements abundances are normalized to phosphate (ppb trace metal ppb P as phosphate). Source-. From Quigg, A., et al. (2003). Nature 425, 291-294. 

There are a number of trace elements commonly described as incompatible because their partition coefficients (for typical mantle mineralogy) are low, often as low as 10 2 or 10 or less. Prominent examples are K, Rb, U, Th, and the rare earth metals. The principal reason for their incompatibility is evidently their large ionic radii. Concentrations of these elements in mantle-derived rocks are relatively low, and the generalization emerges that they have been expelled from the mantle (at least the upper mantle) and concentrated in the crust, especially continental crust. 

Figure 1 Ionic radius (in angstrom) versus ionic charge for lithophile major and trace elements in mantle sihcates. Also shown are ranges of enrichment factors in average continental crust, using the estimate of (Rudnick and Fountain, 1995), relative to the concentrations in the primitive mantle (or hulk silicate Earth ) (source McDonough and Sun, 1995).

The techniques illustrated in Figures 17 and 18 can be used to establish an approximate compatibility sequence of trace elements for mantle-derived melts. In general, this sequence corresponds to the sequence of decreasing (normalized) abundances in the continental crust shown in Figure 2, but this does not apply to niobium, tantalum, and lead for which the results discussed in the previous section demand rather different positions (see also Hofmann, 1988). Here I adopt a sequence similar to that used by Hofmann (1997), but with slightly modified positions for lead and strontium. 

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.

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. 

McLennan S. M. (2001b) Relationships between the trace element composition of sedimentary rocks and upper continental crust. Geochem. Geophys. Geosys. 2 (article no. 2000GC000109). 

Table 1 Compositions of the continental crust and selected trace element ratios referred to throughout this chapter (all values are from Chapter 3.01). Major element oxides in wt.% and trace elements in ppm.

The preferred chemical estimates of the continental crust used throughout this chapter are listed in Table 1. The major element composition of the upper crust is well constrained, since this is the most accessible to sampling, both directly and via erosion and sedimentation, and different studies utilizing diverse databases have yielded remarkably similar results. Si02is —61%, and Mg number (Mg, molar Mg/(Mg - - Fe)) is — 55 for the bulk continental cmst, and so it is more differentiated than any magma in equilibrium with the upper mantle. Trace-element abundances are more variable, as are estimates for the composition and proportion of the middle and lower cmst. As we will see below, the latter are critical to any discussion of the mechanisms of cmst formation and differentiation. 

Figure 2 Primitive-mantle normalized minor and trace-element diagrams for (a) the upper, middle, bulk, and lower continental crust (values from Table 1), and (b) oceanic and island arc basalts and the bulk continental crust (all normalizing values are from McDonough and Sun, 1995). The oceanic basalts (N-MORB, normal mid-ocean ridge basalt and OIB, ocean island basalt) are from Sun and McDonough (1989), whereas the arc basalts are from Turner et al. (1997) (Tonga-Kermadec arc) and Pearce et al. (1995) (South Sandwich arc).

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