Earth mantle composition

Sturm ME, Goldstein SJ, Klein EM, Karson JA, Mnrrell MT (2000) Uranium-series age constraints on lavas from the axial valley of the Mid-Atlantic Ridge, MARK area. Earth Planet Sci Lett 181 61-70 Sun S, McDonongh WF (1989) Chemical and isotopic systematics of ocean basalts implications for mantle composition and processes. In Magmatism in the Ocean Basins. Saunders AD, Norry MJ (eds) Blackwell Scientific Pnbl. Oxford, p 313-345... 

Major-element compositions (weight ratios of Mg/Si and Al/Si) for mantle rocks (peridotites) and estimates of the primitive mantle composition of the Earth compared with various groups of chondrites and the Sun. No mixture of chondrite types provides an exact match to the primitive mantle composition, although some carbonaceous chondrites provide the closest match. Modified from Righter et al. (2006). 

Is the Upper Mantle Composition Representative of the Bulk Earth Mantle ... 

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. 

From Equations (2) to (4) and by using the solar abundance ratio for FetoM/Mgtotai. the hypothetical composition of the Earth s mantle is obtained as given in Table 1. The Earth s mantle composition derived from the analyses of actual upper mantle rocks, as described in the next section, is listed for comparison. 

In the following section we will derive the composition of the mantle of the Earth from the chemical analyses of upper mantle rocks. The resulting mantle composition will then be compared with the composition of chondritic meteorites. In order to avoid circular arguments, we will use as few assumptions based on the... 

The least fractionated rocks of the Earth are those that have only suffered core formation but have not been affected by the extraction of partial melts during crust formation. These rocks should have the composition of the PM, i.e., the mantle before the onset of crust formation. Such rocks are typically high in MgO and low in AI2O3, CaO, Ti02, and other elements incompatible with mantle minerals. Fortunately, it is possible to collect samples on the surface of the Earth with compositions that closely resemble the composition of the primitive mantle. Such samples are not known from the surfaces of Moon, Mars, and the asteroid Vesta. It is, therefore, much more difficult to reconstruct the bulk composition of Moon, Mars, and Vesta based on the analyses of samples available from these bodies. 

Most seismological constraints on mantle composition are derived by comparison of values of seismic wave velocities inferred for particular regions within the Earth to the values measured in the laboratory for particular minerals or mineral assemblages, with such comparisons being made under comparable regimes of pressure (P) and temperature (T). The primary parameters of interest, then, are the compressional (or P-) wave velocities (Vp) and the shear (or S-) wave velocities (Ej). These wave velocities are simply related to the density (p) and to the two isotropic elastic moduli, the adiabatic bulk modulus (Ks)... 

The long-lived radioactive decay systems commonly used to characterize mantle compositions, their half-lives, and the isotope ratios of the respective radiogenic daughter elements are given in Table 1. The half-lives of Sm, Sr, Hf, Re, and Th are several times greater than the age of the Earth, so that the accumulation of the radiogenic daughter nuclide is nearly linear with time. This is not the case for the shorter-lived... 

Davies G. F. (1998) Plates, plumes, mantle convection and mantle evolution. In The Earth s Mantle-composition, Structure, and Evolution (ed. I. Jackson). Cambridge University Press, Cambridge, pp. 228-258. 

Altogether, tectonically emplaced and abyssal mantle rocks provide insights into upper mantle compositions and processes that are complementary to the information conveyed by mantle xenoliths (See Chapter 2.05). They provide coverage to vast regions of the Earth s upper mantle that are sparsely sampled by mantle xenoliths, particularly in the ocean basins and beneath passive continental margins, back-arc basins, and oceanic island arcs. 

Green D. H. (1973) Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions. Earth Planet. Sci Lett. 19, 37-53. 

Hunter R. H. and McKenzie D. (1989) The equilibrium geometry of carbonate melts in rocks of mantle composition. Earth Planet. Sci. Lett. 92, 347-356. 

Kennett B. L. N. and Hilst R. D. V. d. (1998) Seismic structure of the mantle from subduction zone to craton. In The Earth s Mantle Composition, Structure and Evolution (ed. 

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). 

Although Sr/ Sr isotopic evolution for the mantle is only poorly constrained at present, this system has tremendous potential for revealing details of cmst-formation processes through time as well as for providing precise timescales on Earth formation. The test of this awaits determination of accurate initial strontium isotopic compositions from Precambrian samples. The potential for acquisition of this type of data is high, as advances in techniques including both in situ measurements (e.g., Christiensen et al., 1996) and low-level solution work (e.g., Mueller et al., 2000) now allow precise determination of isotopic compositions from extremely small amounts (<10 g) of strontium. This ability makes feasible analyses of rare, relict high-strontium, low-mbidium phases which may be preserved within ancient rocks and minerals and may accurately record mantle compositions. 

For this review the Earth s composition will be considered to be more similar to carbonaceous chondrites and somewhat less like the high-iron end-members of the ordinary or enstatite chondrites, especially with regard to the most abundant elements (iron, oxygen, silicon, and magnesium) and their ratios. However, before reaching any firm conclusions about this assumption, we need to develop a compositional model for the Earth that can be compared with different chondritic compositions. To do this we need to (i) classify the elements in terms of their properties in the nebula and the Earth and (2) establish the absolute abundances of the refractory and volatile elements in the mantle and bulk Earth. 

Table 5 The composition of the bulk Earth, mantle, and core and atomic proportions for abundant elements.

Ribe N. M. (1985) The generation and composition of partial melts in the Earths mantle. Earth Planet. Sci. Lett. 73, 361-376. 

Zhu and MacDougall (1998) showed that calcium isotope composition of the shells was both species and temperamre dependent and that river water is depleted by 2%o relative to seawater. The temperature dependency of calcium isotope fractionation enables this tracer to be a potential paleotem-peramre proxy (Nagler et al., 2000). The seawater Cal" Ca secular variations are indicated by the results of De La Rocha and De Paolo (2000) for the last 160 Myr, and by the data of Zhu (1999) for the entire 3.4 Gyr of earth history. The latter indicate that, in analogy to strontium isotopes (Veizer and Compston, 1976), the Archean samples have C Ca ratios similar to the earth mantle, with the crustal-like values first appearing at the Archean/Proterozoic transition. However, in contrast to the strontium isotope trend, the calcium isotope ratios appear to dip towards mantle values also at —1.6 Gyr ago. 

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