Continental crust, elemental composition

Richter FM, Davis AM, Ehel DS, Hashimoto A (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions Experiments, theoretical considerations, and constraints on their thermal evolution. Geochim Cosmochim Acta 66 521-540 Richter FM, Davis AM, DePaolo DJ, Watson EB (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim Cosmochim Acta 67 3905-3923 Rudnick RL, Fountain DM (1995) Nature and composition of the continental crust—a lower crustal perspective. Rev Geophys 33 267-309... 

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. 

O, H, C, S, and N isotope compositions of mantle-derived rocks are substantially more variable than expected from the small fractionations at high temperatures. The most plausible process that may result in variable isotope ratios in the mantle is the input of subducted oceanic crust, and less frequent of continental crust, into some portions of the mantle. Because different parts of subducted slabs have different isotopic compositions, the released fluids may also differ in the O, H, C, and S isotope composition. In this context, the process of mantle metasomatism is of special significance. Metasomatic fluids rich in Fe +, Ti, K, TREE, P, and other large ion lithophile (LIE) elements tend to react with peridotite mantle and form secondary micas, amphiboles and other accessory minerals. The origin of metasomatic fluids is likely to be either (1) exsolved fluids from an ascending magma or (2) fluids or melts derived from subducted, hydrothermally altered crust and its overlying sediments. 

The Earth s crust and, indeed, the crusts of all differentiated bodies, are enriched in incompatible elements relative to their mantles. This reflects the partial melting of mantle material and extraction and transport of the basaltic melt to the surface. On Earth, further partial melting of the basaltic crust in the presence of water produces magma compositions even richer in silica (andesite and granite), which form the bulk of the continental crust. Because other differentiated bodies are effectively dry, this second level of differentiation did not occur. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

Holland, I. G., Lambert, R. St. J. Major element chemical composition of shields and the continental crust. Geochim. et Cosmochim. Acta 36, 673-683 (1972). 

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).

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.

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

Figure 9 Comparison of the major-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-cmstal 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 between tbe values of Gao et al.

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