Bulk silicate Earth

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 temperature of 50% condensation of a given element in the Solar Nebula defined by Wasson (1985) is 1037 K for Cu and 660 K for Zn. The much more volatile character of Zn with respect to Cu conditions the relative abundances of the two elements among the dififerent classes of chondrites. Copper concentrations vary from 80 to 120 ppm in carbonaceous and ordinary chondrites (Newsom 1995). In contrast, Zn concentrations decrease from 310 ppm in the volatile-rich Cl to 100 ppm in CO and CV, and to 50 ppm in ordinary chondrites. McDonough and Sun (1995) estimate the Cu and Zn content of the Bulk Silicate Earth to be 30 and 55 ppm, respectively. 

Lithium isotope geochemistry is characterized by a difference close to 30%c between ocean water (5 Li + 31%c) and bulk silicate earth with a 8 Li-value of 3.2%c (Seitz et al. 2007). In this respect lithium isotope geochemistry is very similar to that of boron (see p. 45). The isotopic difference between the mantle and the ocean can be used as a powerful tracer to constrain water/ rock interactions (Tomaszak 2004). Figure 2.6 gives an overview of Li-isotope variations in major geological reservoirs. 

One of the most important qnestions in the genesis of ore deposits is the origin of the metals. Recent analytical developments have provided a new tool for the analysis of metal isotopes (Fe, Cu, Zn, Mo). Since the bulk silicate earth (crust+mantle) shows a uniform mean isotope composition of the metals, different metal reservoirs with distinct isotopic compositions are not easily recognizable. Thus, investigations by Markl et al. (2006a, b) on Cu and Fe ores have indicated that tracing of ore sources is ambiguous. 

The Pb-Pb isochron was made famous by the determination of the age of the Earth Patterson (1956) grouped meteorite samples with a sediment sample that is supposed to represent the bulk silicate Earth in terms of Pb isotopes (Figure 5-8). The assumption is that the Earth formed at roughly the same time as the meteorites. The colinearity of the data in Figure 5-8 is viewed as verification of the assumption. The age given by Patterson (1956) is 4.55 Ga. 

Osmium isotopic ratio of the bulk silicate Earth overlaps measurements of ordinary chondrites but is distinct from other chondrite groups. Adapted from Righter et al. (2006). 

Figure 2 Comparison between the K/U and Rb/Sr ratios of the silicate Earth compared with other solar system objects. ADOR Angra dos Reis HED howar-dite-eucrite-diogenite parent body BSE bulk silicate Earth Cl, CM, CV, CO, H, L, and LL are all classes of chondrites (source Halliday and Porcelli, 2001).

The six most abundant, nonvolatile rock-forming elements in the Sun are Si (100), Mg (104), Fe (86), S (43), Al (8.4), and Ca (6.2). The numbers in parentheses are atoms relative to 100 Si atoms. They are derived from element abundances in Cl-meteorites which are identical to those in the Sun except that Cl-abundances are better known (see Chapter 1.03). From geophysical measurements it is known that the Earth s core accounts for 32.5% of the mass of the Earth. Assuming that the core contains only iron, nickel, and sulfur allows us to calculate the composition of the silicate fraction of the Earth by mass balance. This is the composition of the bulk silicate earth (BSE) or the primitive earth mantle (PM). The term primitive implies the composition of the Earth s mantle before crust and after core formation. 

In the early days of mantle geochemistry, the composition of the bulk silicate earth, also called primitive mantle (i.e., mantle prior to the formation of any crust see Chapter 2.01) was not known for strontium isotopes because of the obvious depletion of rubidium of the Earth relative to chondritic meteorites (Gast, 1960). 

Calculated bulk rock trace-element systematics of eclogites have wider implications for mantle recycling models and bulk silicate earth mass balance. The subchondritic Nb/Ta, Nb/La, and Ti/Zr of both continental cmst and depleted mantle require the existence of an additional reservoir with superchondritic ratios to complete the terrestrial mass balance. Rudnick et al. (2000) have shown that rutile-bearing eclogites from cratonic mantle have suitably superchondritic Nb/Ta, Nb/La, and Ti/Zr such that if this component formed 1 -6% by weight of the bulk silicate earth, this would resolve the mass imbalance. This mass fraction far exceeds the likely mass of eclogite in the continental lithosphere and so the material is proposed to reside in the lower mantle, possibly at the core-mantle boundary. 

Kellogg et al. (1999), however, have suggested, on the basis of a transition in seismic heterogeneity observed at —1,600 km depth, the possibility of a very deep layer extending hundreds of kilometers above the core-mantle boundary. One possibility is that a relic layer of dense, primordial crystalline differentiates (e.g., magnesium- and calcium-silicate perovskite) may have remained buried in the deep lower mantle until the present. Such a layer is a potential storehouse for trace elements, including radioactive heat-producing elements, and potentially could provide an important reservoir for bulk silicate Earth chemical mass balance... 

If the fertile mantle protolith to oceanic and subcontinental mantle is nonchondritic in refractory elements, and if the bulk silicate Earth is chondritic in refractory elements, then a complimentary reservoir must exist elsewhere, presumably buried in the deep lower mantle and isolated from mantle convection (e.g., Anderson, 1989 Kellogg et al., 1999 Albarede and van der HUst, 1999). The primitive upper mantle could have acquired a superchondritic ratio as a consequence of crystal fractionation in a magma ocean, or perhaps by extraction of an early crust with low CaO/AbOs. 

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Eaure and Hurley, 1963 Hart et al, 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental cmst. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental cmst is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1979). 

The Lu- Hf isotopic system (half-life —37 Gyr) is, in many ways, chemically similar to Sm- Nd. In both isotopic schemes the parent and daughter elements are refractory lithophile elements, such that their relative abundances in the Earth were probably not modified during accretion, nor did they participate in core formation. Thus, as for the Sm-Nd system, the compositions of chondritic meteorites can, in principle, be used to establish bulk silicate Earth isotopic compositions and Lu/Hf ratios directly. The potential, therefore, exists for establishing a precise isotopic baseline to use for recognizing fine-scale deviations in isotopic compositions, which can then be used to reveal... 

Potassium is a moderately volatile element and is depleted by a factor of —8 in the bulk silicate Earth compared to Cl chondrites, but a precise and unambiguous concentration is difficult to obtain. Estimates have been made by comparison with uranium, which like potassium is highly incompatible during melting and so is not readily fractionated between MORE and the upper mantle. There is little debate regarding the concentration of uranium, which is obtained from concentration in carbonaceous chondrites and, by assuming that refractory elements (e.g., calcium, uranium, thorium) are unfractionated from solar values... 

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