Primitive mantle chemical composition

Efforts have been made to determine the compositions of both the primitive mantle and depleted materials in the modern mantle. In the approach of Palme and O Neill (2004), the estimated chemistry of the primitive mantle was determined by subtracting the likely elemental concentrations of the Earth s core from results on the bulk chemistry of the Earth. The chemistry of the bulk Earth may be derived from chemical data on Cl chondrite meteorites, spectrographs of the Sun, and/or analyses of upper mantle rocks. Based on the chemical properties of an element, assumptions can be made on how much of the element was likely to have accumulated in the core. On the basis of this approach, Palme and O Neill (2004, 14) concluded that the arsenic concentration of the primitive mantle was 0.066 0.046 mg kg-1 (Table 3.3). 

The constancy of refractory element ratios in the Earth s mantle, discussed before, is documented in the most primitive samples from the Earth s mantle. Figure 8 plots (modified from Jochum et ai, 1989) the PM-normalized abundances of 21 refractory elements from four fertile spinel Iherzolites. These four samples closely approach, in their bulk chemical composition, the primitive upper mantle as defined in the previous section. The patterns of most of the REEs (up to praseodymium) and of titanium, zirconium, and yttrium are essentially flat. The three... 

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 mantle is the Earth s largest chemical reservoir comprising 82% of its total volume and 65% of its mass. The mantle constitutes almost all of the silicate Earth, extending from the base of the crust (which comprises only 0.6% of the silicate mass) to the top of the metallic core at 2,900 km depth. The chemical compositions of direct mantle samples such as abyssal perido-tites (Chapter 2.04) and peridotite xenoliths (Chapter 2.05), and of indirect probes of the mantle such as basalts from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) (Chapter 2.03), and some types of primitive... 

Figure 9 Examples of models proposed for the chemical structure of the terrestrial mantle, (a) Whole mantle convection with depletion of the entire mantle. Some subducted slabs pass through the transition zone to the coremantle boundary. Plumes arise from both the core-mantle boundary and the transition zone. This model is not in agreement with isotopic and chemical mass balances, (b) Two-layer mantle convection, with the depleted mantle above the 660 km transition zone and the lower mantle retaining a primitive composition, (c) Blob model mantle where regions of more primitive mantle are preserved within a variously depleted and enriched lower mantle, (d) Chemically layered mantle with lower third above the core comprising a heterogeneous mixture of enriched (mafic slabs) and more primitive mantle components, and the upper two-thirds of the mantle is depleted in incompatible elements (see text) (after Albarede and van der Hilst, 1999).

Developing a model for the composition of the Earth and its major reservoirs can be established in a four-step process. The first involves estimating the composition of the silicate Earth (or primitive mantle, which includes the crust plus mantle after core formation). The second step involves defining a volatility curve for the planet, based on the abundances of the moderately volatile and highly volatile lithophile elements in the silicate Earth, assuming that none have been sequestered into the core (i.e., they are truly lithophile). The third step entails calculating a bulk Earth composition using the planetary volatility curve established in step two, chemical data for chondrites, and... 

The Earth s mantle is peridotitic in composition and is significantly depleted in silica relative to primitive chondrites. Seismological evidence shows that the mantle is layered and can be divided into an upper and lower mantle, separated by a transition zone at 400-660 km depth. Above the transition zone the mantle is dominated by olivine and orthopyroxene with minor garnet and clinopyroxene. The lower mantle is made up of phases Mg- and Ca-perovskite and magnesiowustite. Seismic velocity contrasts between the upper and lower mantle are thought to reflect the ph ase transformations between the two and are not related to differences in bulk chemical composition. The lower mantle is separated from the outer core by the D" layer, a hot thermal boundary layer of enigmatic composition. 

Alkali basalt xenoliths Xenoliths in alkali basalts are most commonly spinel-bearing peridotites. They have been divided into two main groups (Frey Prinz, 1978). Primitive xenoliths have chemical compositions which suggest that they have had a melt extracted from them. Menzies (1983) showed that this "depleted" character is similar in alkali basalt xenoliths beneath both the oceans and the continents and proposed that a depleted mantle layer underlies both... 

Estimates of the chemical composition of Earth s mantle normally refer to the composition of the Earth s mantle as it existed immediately after core formation but before the extraction of the continental crust. This composition is known as the bulk silicate Earth (BSE) or the Primitive mantle and is an important reference composition for the study of the mantle. 

Sun S.S., 1982, Chemical composition and origin of the earth s primitive mantle. Geochim. Cosmochim. Acta, 46, 179-192. 

The third kind of evidence is that the upper mantle composition violates the cosmochemical constraints on PM compositions that are obtained from the meteoritic record. A detailed comparison of PM compositions with primitive meteorite compositions is given below and it is shown that the PM composition shows chemical fractionations that are similar to the fractionations seen in carbonaceous chondrites. 

The inclusion of the subjects covered in Volume 1 of this Treatise illustrates the recognition that one critical avenue to understanding geo chemistry is to understand the solar environment in which Earth formed. Chapter 2.01 of this volume compares the composition of Earth with that of various primitive meteorite classes and with the spectroscopically determined composition of the Sun. Chemical variability in these meteorites reflects primarily two processes (i) volatility and (u) affinity for metal (the so-called siderophile elements) over silicate (lithophile elements). Perhaps the most surprising outcome of this comparison is that Earth s mantle has a bulk composition that is close to solar, at least for refractory lithophile elements. As detailed in Chapter 2.01, the mantle s most obvious departures from solar composition are its deficiencies in volatile and siderophile elements. The latter is easily understood in that Earth has a large metallic core that extracted the missing siderophile elements from the mantle (Chapter 2.15). 

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