Continental crust composition

This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents. Earth differentiation, and its geochemical inventories. 

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).

Table 11 Comparison of the upper, middle, lower and total continental crust compositions recommended here.

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. 

Weaver B. L. and Tarney J. (1980) Continental crust composition and nature of the lower crust constraints from mantle Nd-Sr isotope correlation. Nature 286, 342-346. 

Another striking feature of the data in Figure 20 is that the upper, lower and bulk continental crust compositions all have similar Eu/Sr ratios and thus define a distinct, near-vertical trend that is... 

Zandt, G. Ammon, C. J. 1995. Continental crust composition constrained by measurements of crustal Poisson s ratio. Nature, 374, 152-154. 

The 20 largest rivers on Earth carry about 40% of the total continental runoff, with the Amazon alone accounting for about 15% of the total. These rivers give the best indication of global average riverwater chemical composition, which can be compared with average continental crust composition (Table 5.1). Three features stand out from this comparison ... 

FIGURE 9 Plot of K20 versus crustal heat flow, comparing various estimates of bulk continental crust composition. Heat-flow constraints are shown in the vertical dashed lines. The lower limit assumes that the lower crust contributes no heat and all heat-producing elements are contained within the upper crust. The absolute upper limit is given by the total average heat flow from stabilized continental crust and thus assumes no mantle contribution to heat flow. A more realistic upper limit is model-dependent and adopts modest mantle heat-flow contributions suggested by detailed geochemical studies of deeply exposed crustal cross sections. 

Fig. 7.7 Fe and CO concentrations of the Antarctic Vostok ice core for the past 160,000 years (adopted from De Angelis et al. 1987). Measured Al concentrations were converted to Fe concentrations according to the average continental crust composition. The negative correlation of COj and Fe supports the iron hypothesis (see text).

Taylor S.R., McLennan S.M. The continental crust Its composition and evolution, Oxford Blackwell, 1985. 

Yan, M.C. Chi, Q.H. 2005. The Chemical Compositions of the Continental Crust and Rocks in the Eastern Part of China. Beijing Science Press. 171 p. 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

Clastic sediments are reservoirs of information about weathering processes, but are sufficiently complex that no study has yet to realize their potential. Despite a number of initial reports of relatively isotopically heavy samples, the majority of data for clastic sedimentary rocks have an average 8 Li 0, equivalent to the estimated average isotopic composition of the continental crust. 

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... 

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... 

Taylor SR, McLennan SM (1985) The Continental Crust Its Composition and Evolution. Blackwell, Boston Tuit CB, Ravizza G (2003) The marine distribution of molybdenum. Geochim Cosmochim Acta 67 A4950 Tumlund JR, Keyes WR, Peiffer GL (1993) Isotope ratios of molybdenum determined by thermal ionization mass spectrometry for stable isotope studies of molybdenum metabolism in humans. Anal Chem 65 1717-1722... 

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. 

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 ). 

Magma types 2006). A significant part in formation of magmatic complexes accompaning riftogenesis belongs to the sources of different nature and to characteristics of the continental crust contaminated by those complexes. These very data accounted for the formation of a contrasting volcanism which is widely developed in the zone of the Central-Asian fold belt. The paper considers a bimodal volcano-plutonic complex of the end of the Late Cretaceous. It is spatially located within the continuation of the formations with similar composition which compose the Central-Asian fold belt. 

Taylor S.R. McLennan S.M. 1985. The Continental Crust Its Composition and Evolution. Blackwell Science Publisher, Oxford. 

Mantle-derived basalts, on the other hand, have a relatively uniform composition with 8 Li values of 4 2%o (Tomaszak 2004 Elliott et al. 2004). The continental crust generally has a lighter Ei isotope composition than the upper mantle from which it was derived (Teng et al. 2004). Considering the small Li isotope fractionation at high temperature igneous differentiation processes (Tomaszak 2004), pristine... 

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. 

Since hthium and boron isotope fractionations mainly occur during low temperature processes, Li and B isotopes may provide a robust tracer of surface material that is recycled to the mantle (Elhott et al. 2004). Heterogeneous distribution of subducted oceanic and continental crust in the mantle will thus result in variations in Li and B isotope ratios. Furthermore, dehydration processes active in subducdon zones appear to be of crucial importance in the control of Li and B isotope composition of different parts of the mantle. For the upper mantle as a whole Jeffcoate et al. (2007) gave an estimated 8 Li-value of 3.5%o. 

Continental basalts tend to be enriched in relative to oceanic basalts and exhibit considerably more variability in O-isotope composition, a feature attributed to interaction with 0-enriched continental crust during magma ascent (Harmon and Hoefs 1995 Baker et al. 2000). 

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