Bulk continental crust

Convergent margins are generally considered to be the principle present-day tectonic setting where new continental crust is formed (-1.1 kmVyr, Reymer and Schubert 1984). As illustrated on Figure 23, this new crustal material is characterized by Th/U ratios that are even lower than the Th/U ratio of the MORB mantle (2.6, Sun and McDonough 1989) yet the Th/U ratio of the bulk continental crust (3.9, Rudnick and Fountain 1995) is close to the Th/U ratio of the bulk silicate earth (see Bourdon and Sims 2003). There are several possible explanations for this paradox. Firstly, it is possible that the processes that formed the continental crust in the past were different to those in operation today. Since... 

The average age of continental crustal rocks is about 2 Ga. The typical upper crustal rock has K/Ca of 0.9 and the bulk continental crust has a ratio of about 0.35 (Rudnick and Fountain 1995, Taylor and Mclennan 1995, Shaw et al. 1986, Condie 1993, Wedepohl 1995). Using these values, the average continental crustal value for Cca is 0.8 and that for upper continental... 

The general, incompatible-element depleted nature of the majority of MOREs and their sources is well explained by the extraction of the continental crust. Nevertheless, the bulk continental crust and the bulk of the MORE sources are not exact chemical complements. Rather, the residual mantle has undergone additional differentiation, most likely involving the generation of OlEs and their subducted equivalents. In addition, there may be more subtle differentiation processes involving smaller-scale melt migration occurring in the upper mantle (Donnelly et al., 2003). It is these additional differentiation processes that have... 

Table 9 Compositional estimates of the bulk continental crust. Major elements in weight percent. Trace element concentration units the same as in Table 2. 

Figure 16 Comparison of the trace-element composition of bulk continental crust from seismological and model-based approaches. All data normalized to the new composition given here (Table 10, R G ). Gray shading depicts 30% variation from Rudnick and Gao composition (this work), (a) Transition metals, (b) high-field strength elements, (c) alkali and alkaline earth metals, and (d) REEs, (e) actinides and heavy metals, and (f) siderophile and...

Bulk continental crust deduced from global crust/mantle chemical budgets provides a useful reference. Various studies lead to a small range of 0.74-0.86 p.W m for the average rate of cmstal... 

Heat production for the bulk continental crust in Table 7 is calculated using the crustal age distribution of model 2 in Rudnick and Fountain (1995) and is in the upper end of the range of values from global crust/mantle budgets (Table 2). 

Table 7 Estimates of bulk continental crust heat production from heat flow data.

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).

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... 

Element Upper continental crust Bulk continental crust Lower continental crust Average andesitic crust Lower crustal granulites Archean upper crust Archean bulk crust... 

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. 

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... 

Figure 9 (a) Chondrite-normalized REE patterns for the upper, bulk, and lower continental crust reservoirs, (b) Upper and lower continental crust normalized to the bulk continental crust, highlighting the fractionation of europium and the more incompatible FREE during intracrustal differentiation... 

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