Continental crust volume

Volume 34 Rolf Meissner. The Continental Crust Volume 49... 

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... 

Table 7.2 The most common elements in the continental crust (according to Brian Mason, 1966, Principles of Geochemistry, table 34). The mass % and volume % values have been rounded up or down...

Figure 1 The distribution of U/Pb ages from juvenile crust, with only one age (n) used for each supracrustal succession or pluton to avoid undue weighting from particularly well-studied areas (from Condie, 1998), combined with selected curves representing different models for the changing volumes of stable continental crust with time (A minimum curve based on Nd isotopes in shales (authors compilation) B from Collerson and Kamber (1999), and similar to Kramers and Tolstikhin (1997) and C Taylor and McLennan (1985)).

Beyond the broad major-element constraints afforded by seismic imaging, the abundance of many trace elements in the mantle clearly records the extraction of core (Chapters 2.01 and 2.15) and continental crust (Chapter 2.03). Estimates of the bulk composition of continental cmst (Volume 3) show it to be tremendously enriched compared to any estimate of the bulk Earth in certain elements that are incompatible in the minerals that make up the mantle. Because the crust contains more than its share of these elements, there must be complementary regions in the mantle depleted of these elements—and there are. The most voluminous magmatic system on Earth, the mid-ocean ridges, almost invariably erupt basalts that are depleted in the elements that are enriched in the continental crust (Chapter 2.03). Many attempts have been made to calculate the amount of mantle depleted by continent formation, but the result depends on which group of elements is used and the assumed composition of both the crust and the depleted mantle. If one uses the more enriched estimates of bulk-continent composition, the less depleted estimates for average depleted mantle, and the most incompatible elements, then the mass-balance calculations allow the whole mantle to have been depleted by continent formation. If one uses elements that are not so severely enriched in the continental cmst, for example, samarium and neodymium, then smaller volumes of depleted mantle are required in order to satisfy simultaneously the abundance of these elements in the continental cmst and the quite significant fractionation of these elements in the depleted mantle as indicated by neodymium isotope systematics. 

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... 

The Earth is the only planet amongst the rocky planets of the solar system to possess a well-developed, felsic continental crust. This crust was progressively extracted from the Earth s mantle over the last 4.0 billion years, and although it only represents 0.6% of the mass of the silicate Earth, it contains up 70% of the Earth s budget of highly incompatible elements. Over Earth history, the reservoir of depleted mantle, from which the crust was created, has grown in volume to mirror the growth of the continental crust. 

The age distribution of crustal rocks An estimate of the relative volumes of continental crust of different ages is an obvious way to assess crustal growth models. However, making such an estimate is not straightforward for two reasons. First, it cannot be assumed that the age of the continents is the same at depth as it is at the surface (Corfu, 1987). Second, the reworking of older crust... 

This result could imply that there was an extensive volume of continental crust in existence at 3.5 Ga, strongly supporting the "nogrowth" crustal growth model of Armstrong (1968). However, there are problems with this interpretation. First, because Nb exchange between the crust and mantle is not a simple two-component system (see Section 4.5.1.2.3) and second, because U is mobile in an oxidizing... 

A number of models have been proposed which link crustal evolution with the mantle Nd-isotope evolution curve, the most realistic of which is probably the transport-balance model of Nagler and Kramers (1998). This model is based upon their empirically derived Nd-isotope mantle evolution curve and assumes that the upper mantle melts to form basaltic oceanic crust, which is then reprocessed to form continental crust. An important aspect of the model is that it also includes crustal recycling, such that as the volume of continental crust grows with time, proportionately some crust is recycled back into the mantle through erosion and subduction. 

A refinement of this model (Rudnick Gao, 2003) includes an estimate of the composition and volume of the middle continental crust, ignored in the earlier model of Rudnick and Fountain (1995) and a revised estimate for the composition of the upper continental crust. In this revised model upper, middle, and lower continental crust are in the proportions 31.7%, 29.6%, and 38.8%. The composition is given in Table 4.2 and, to date, it is the best estimate we have for an average composition of the whole crust. 

How many mantle reservoirs are required to make the continental crustl As the volume and quality of isotopic and trace element data for the continental crust and mantle have improved in recent years, it has become increasingly clear that the qualitative, three reservoir model outlined above is inadequate and cannot fully explain all the geochemical features of the crust-mantle system. As more trace element data are considered it is apparent that additional reservoirs need to be considered in addition to the primitive mantle, the depleted mantle, and the continental crust. 

A central question for models of crust extraction from the mantle is whether or not the continental crust has been extracted from the whole mantle or just from the "depleted" upper mantle. The debate between geophysicists and geochemists over whether there is whole mantle convection, or whether the mantle convects as two independent layers was outlined in Chapter 3, Section 3.1.5. Hofmann et al. (1986) argued that the mass balance of trace element ratios for Nb/U and Ce/Pb in the continental crust and upper mantle require that the continental crust be extracted from a volume of mantle that equates to about 50% of the whole mantle - a volume much larger than that of the modern depleted mantle as defined by the 660 km discontinuity. More recently Helffrich and Wood (2001) have confirmed this estimate on the basis of calculations using K, U, and Th concentrations. 

Calculations of this type can, however, be turned on their head and used to argue that since the volume of mantle required to make the continental crust is inconsistent with the volume of the depleted mantle the mantle cannot be chemically layered. In that case we arrive at a model for the mantle in which there is a depleted portion, from which the continental crust has been extracted, and an undepleted portion, but the two are not confined to discrete layers. In addition trace element constraints outlined below will show that other reservoirs are also present. 

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