Mantle chondritic composition

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. 

Estimating the composition of the Bulk Silicate Earth is strongly dependent upon an appropriate model for the origin of the Earth and accordingly involves a number of assumptions. These are, that for nonvolatile elements, the Earth has a Cl chondritic composition, the mantle is homogeneous, that the upper and lower mantle have the same composition, and that the Earth s core is a Fe-Ni alloy with the addition of 10-15% light elements (Allegre etal., 1995). 

Peridotite-chondrite models Rather than assuming a composition for the mantle, its composition may be estimated by directly measuring the composition of mantle samples. The biggest difficulty here is... 

We can estimate how much water was dissociated by considering the oxidation of the mantle. If the Earth started out with the oxidation state of chondrites (although not necessarily of total chondritic composition), it would take the oxygen from about one present-day ocean volume to reach the oxidation state of the mantle inferred from mantle-derived rocks. Alternatively, if we start with a more reduced ensemble, suggested by condensation calculations from a solar nebula, we will have primarily enstatite (MgSiOs) and metallic iron as the original source to be oxidized. The oxidation reaction would then be as shown in eqn [1]. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

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). 

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). 

With these caveats, one can deduce the following. Early single grains appear to have recorded hafnium isotopic compositions that provide evidence for chondritic or enriched reservoirs. There is no evidence of depleted reservoirs in the earliest (Hadean) zircons dated thus far (Amelin et al., 1999). Use of alternative values for the decay constants or values for the primitive mantle parameters increases the proportion of hafnium with an enriched signature (Amelin et al., 2000), but does not provide evidence for early mantle depletion events. Therefore, there is little doubt that the Hadean mantle was extremely well mixed. Why this should be is unclear, but it probably relates in some way to the lack of preserved continental material from prior to 4.0 Ga. 

Noble gases and nitrogen in martian meteorites reveal several interior components having isotopic compositions different from those of the atmosphere. Xenon, krypton, and probably argon in the mantle components have solar isotopic compositions, rather than those measured in chondrites. However, ratios of these noble gas abundances are strongly fractionated relative to solar abundances. This decoupling of elemental and isotopic fractionation is not understood. The interior ratio in martian meteorites is similar to chondrites. 

---