Lower crust bulk composition

In column 12 of Table 9 we present a new estimate of the bulk crust composition. This composition derives from our estimates of upper, middle, and lower crust given in Tables 3,5, and 8, mixed in the proportions derived from the global compilation of Rudnick and Fountain (1995) 31.7% upper, 29.6% middle, and 38.8% lower... 

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... 

Whether derived from a mantle or crustal source, the origin of anorthosites must have left a distinct signature in the lower crust. The bulk composition of the lower crust is expected to be of gabbroic composition (metamorphosed in Grenville time) if the anorthosite magma was derived... 

Longhi et al. 1999) and amphibolite (Christensen Mooney 1995 Rudnick Fountain 1995). For any of these compositions, partial melting and extraction of the anorthosite suite had to result in a general increase of restitic minerals (pyroxene and garnet). The occurrence of this process at a large scale, as the volume of mid-Proterozoic meta-anorthosites would require, would have produced a restitic bulk composition of the lower crust and a decrease of VpjVs ratios to values inconsistent with our observations. 

Crust-mantle chemical mass-balance models offer important constraints on compositional variations in the mantle, but their constraints on the size of the various reservoirs involved depend critically on uncertainties in the estimates of the bulk composition of the continental crust, the degree of depletion of the complementary depleted mantle, and the existence of enriched reservoirs in Earth s interior, for example, possibly significant volumes of subducted oceanic crust. This last item was left out of the mass-balance models that suggested that the upper and lower mantle are chemically distinct. Chapter 2.03 makes it clear that much of the chemical and isotopic heterogeneity observed in oceanic volcanic rocks reflects various mixtures of depleted mantle with different types of recycled subducted crust. With this realization, and excepting the noble gas evidence for undegassed mantle, some of the characteristics of what was once labeled... 

A particularly influential model for the average composition of the continental crust is the andesite model of Taylor and McLennan (1981, 1985). This model is based upon the observation that modern continental growth primarily takes place at a convergent margin. The model has two principal assumptions. First, that the continental crust could be divided into geochemically distinct upper and lower portions which are present in the proportions of 1 to 2. Second, that the bulk composition of the continental crust is andesitic. Given these premises and an estimated average composition for the upper continental crust, it was possible for Taylor and McLennan to calculate the composition of the lower continental crust, which they found to be basaltic. 

A related debate focused on heat flow data from different regions of the continental crust. Nyblade and Pollack (1993) showed that average heat flow measurements in Archaean cratons are lower than those for Proterozoic cratons. This observation has, however, been interpreted in two quite different ways. On the one hand, it has been argued that cratons of different age have different bulk compositions, and so have different concentrations of heat producing elements (U, Th and K), hence different levels of heat production. Alternatively, the observed differences in heat flow do not derive from the crust but reflect different lithospheric thicknesses between Proterozoic and Archaean cratons reflecting different mantle heat flow contributions (Rudnick et al., 1998 Nyblade, 1999). 

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... 

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. 

Figure 12. Depth profile of Li isotopic composition (a) and concentration (b) in drilled oceanic crust at ODP Sites 504B (open symbols) and 896A (filled symbols) off Costa Rica (Chan et al. 2002a). The transition zone exhibits mixing between hydrothermal fluids and seawater. Average oxygen isotopic (5 0) composition of bulk samples decreases with depth upper volcanic zone = +7.8, lower volcanic zone = +6.4, transition zone = +5.4, sheeted dikes = +4.3. However, despite many sheeted dike samples having 5 Li less than unaltered MORB, there is no simple relationship between concentration and Li isotopes.

In this chapter we review the composition of the upper, middle, and lower continental crust (Sections 3.01.2 and 3.01.3). We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes (Sections 3.01.4 and 3.01.5). Finally, we compare the Earth s crust with those of the other terrestrial planets in our solar system (Section 3.01.6) and speculate about what unique processes on Earth have given rise to this unusual crustal distribution. 

Rehnements of the Taylor and McLennan (1985) model are provided by McLennan and Taylor (1996) and McLennan (2001b). The latter is a modihcation of several trace-element abundances in the upper crust and as such, should not affect their compositional model for the bulk crust, which does not rely on their upper crustal composition. Nevertheless, McLennan (2001b) does provide modihed bulk-crust estimates for niobium, rubidium, caesium, and tantalum (and these are dealt with in the footnotes of Table 9). McLennan and Taylor (1996) revisited the heat-flow constraints on the proportions of mahc and felsic rocks in the Archean crust and revised the proportion of Archean-aged crust to propose a more evolved bulk crust composition. This revised composition is derived from a mixture of 60% Archean cmst (which is a 50 50 mixture of mahc and felsic end-member lithologies), and 40% average-andesite cmst of Taylor (1977). McLennan and Taylor (1996) focused on potassium, thorium, and uranium, and did not provide amended values for other elements, although other incompatible elements will be higher (e.g., rubidium, barium, LREEs) and compatible elements lower in a cmst composition so revised. 

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. 

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