Crustal component

Fig. 3. Tectonic discrimination diagram of Rb versus Y + Nb from Pearce et al. (1984) and modified by Christiansen and Keith (1996) for syn-collision granites (syn-COLG), volcanic arc granites (VAG), within plate granites (WPG), and ocean ridge granites (ORG). The diagram suggests the granites and pegmatite were contaminated by a crustal component.

The refined source profiles that best reproduced the coarse fraction are listed in table 7. The calculated profiles of the two crustal components follow those of Mason ( ), though the calcium concentration of 20 in the limestone factor is less than the reported value. The paint pigment profile strongly resembles that calculated for the fine-fraction data. The only major difference is that unlike the fine fraction, the coarse-fraction profile does not associate barium with the paint-pigment factor. The calculated sulfur concentration in the coarse-fraction sulfate factor is much less than that in the fine-fraction and there are sizable concentrations of elements such as aluminum, iron, and lead not found in the fine-fraction profile. The origin of this factor is not clear although as described earlier a possible explanation is that a small part of the sulfate particles in the fine fraction ended up in the coarse samples. 

Here the lead contribution is 70 from motor vehicle emissions and 10 from refuse incinerators. In the coarse fraction, the two crustal components account for 80 of the total mass. 

A similar normalization for the coal and various crustal components can be performed using the transition elements Co, Cr, Fe, Mn, Ni, and V. Table VIII shows this normalization to iron for coal, granite, diabase, and crustal average materials. As with the earlier elements, the ratio of transition metals appears to approximate the crustal average composition. Thus, it appears that the concentrations of the transition elements may also be explained by the incorporation of geological materials in the coal. 

Whatever model is assumed for the mantle source of Aeolian magmas, Sr-Nd-Pb-He isotope variations call for the involvement of upper crustal components in magma genesis. It can be calculated that less than 10% of upper crust added to a pyrolite mantle could explain the entire range of radiogenic isotopic compositions encountered in the Aeolian mafic rocks. Sediments transported by the Ionian subducting plate represent the most likely candidates for such a mantle source contaminant (e.g. Ellam et al. 1988 Francalanci et al. 1993b). 

Figure 12 Chemical and isotopic correlations among shergottites, arising from assimilation of a crustal component. Decreasing values of 5 " Nd indicate increasing assimilation. This parameter correlates with magma redox state, indicated by size of the Eu anomaly in pyroxenes (Wadhwa, 2001), and ratio of light-to-heavy rare-earth elements (after McSween, 2002).

Seismic scatterers within the lower mantle are more likely to represent chemical than thermal heterogeneities, with subducted slab material (especially the basaltic crustal component thereof) constituting a likely candidate. The very base of the mantle, nearest the core-mantle boundary, may also be characterized by significant major-element chemical heterogeneity. 

Jacobsen S. B. and Dymek R. F. (1988) Nd and Sr isotope systematics of clastic metasediments from Isua, West Greenland identification of pre-3.8 Ga differentiated crustal components. J. Geophys. Res. 93, 338-354. 

Crustal component of heat flow for a 41 km thick crust. 

Figure 5 Terranes of the Canadian Cordillera. Older continental components are represented by the Miogeocline, which rests on North American Precambrian basement, and the Yukon-Tanana terrane. Of the other terranes, those near to the Miogeocline, such as Kootenay, have major older crustal components. Juvenile characteristics increase in the direction of the Pacific Ocean (after Patchett et aL, 1998 Butler et al., 2001).

DeBari S. M., Anderson R. G., and Mortensen J. K. (1999) Correlation among lower to upper crustal components in an island arc the Jurassic Bonanza Arc, Vancouver Island, Canada. Can. J. Earth Sci. 36, 1371 — 1413. 

Fignre 1 sud evolution of the depleted mantle as defined by juvenile granites and Phanerozoic ophiolites. The inflections in the evolution curve may represent rapid early depletion of the mantle, followed by mixing of more highly depleted mantle reservoirs with either less depleted mantle, or enriched crustal components, between the early and late Archean (sources Jacobsen and Dymek, 1988 Bowring and Housh, 1995 Bennett et al., 1993 CoUerson et al., 1991 Baadsgaard et al., 1986 Moorbath et al., 1997 Vervoort and Blichert-Toft, 1999 and the compilation of Shirey, 1991). 

Here we explore the idea that the discrepancy between the average composition of the continental crust, which is andesitic, and the modern flux from the mantle to the continental crust, which is basaltic, can be explained in terms of a change in the composition of the crust-mantle flux over time. The hypothesis adopted here is that Archaean crust had a TI G composition, formed from a TTG melt, and was not fractionated into lower basaltic and upper felsic components. Modern crust on the other hand has a basaltic bulk composition but has been modified to andesitic through the fractionation and the removal of a mafic lower crustal component (Rudnick Taylor, 1987). Evidence for the absence of a mafic lower crust in the Archaean comes from Archaean lower crust preserved as granulite terrains, such as the Lewisian (Rollinson Tarney, 2005), the Limpopo Belt (Berger Rollinson, 1997), and the lower crust of the Kaapvaal Craton... 

Most of the crust is generated in the Late Archean, with lesser additions from later island-arc volcanism, to make up the present crust. The overall crustal bulk composition was calculated from a 60/40 mixture of the Archean bimodal and the Post-Archean andesitic compositions. These result in the following concentrations for the heat-producing elements in the bulk continental crust 1.1 % K, 4.2 ppm Th, and 1.1 ppm U, which give the crustal component of the heat flow of 29 mWm-2, or slightly over half of the total heat flow measured in the continental crust. Thus there may be little difference in bulk composition between the Archean and Post-Archean bulk crust despite a significant difference in their upper crustal compositions. The bulk crustal compositions in fact are quite similar (Post-Archean values of 1.1% K 4.2 ppm Th, and... 

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