Chemical composition of the Mantle

The chemical composition of the Earth s interior determined the character (the oxidation state) of the primeval atmosphere. If metallic iron had collected in the Earth s core in the early phase of the accretion, the exhalations from the interior of the Earth would have consisted mainly of CO2 and H20, since the gas from the interior could only have come into contact with FeO and Fe203 silicates in the mantle. If, however, metallic iron had been distributed throughout the mantle, the iron and the FeO silicates would have had a reductive influence on the gases the gas exhaled into the atmosphere would then have consisted of CH4, H2 and NH3 (Whittet, 1997). 

The compositions of the crusts of the Moon and Mars are distinct - one is dominated by feldspathic cumulates from an early magma ocean, and the other by basaltic lavas. Regional patterns reflect differences in subjacent mantle compositions. The compositions of the mantles and cores of these bodies can be constrained by chemical analyses of mantle-derived basalts. The interiors of both bodies have remained geochemically isolated, because of the absence of plate tectonics. 

Some estimates of the bulk chemical compositions of the other terrestrial planets are compared with the Earth s in Table 14.2. In this compilation, the compositions and relative proportions of the silicate (mantle plus crust) and core fractions are separated. 

The chemical composition of the Lower Mantle below 670 km is essentially unknown. It has often been assumed to be the same as the Upper Mantle with the seismic discontinuity at 670 km representing a phase change to denser polymorphs rather than a chemical boundary (Liu and Bassett, 1986). However, some models of the Earth s interior suggest that the Mantle is stratified with the Upper Mantle and Lower Mantle convecting separately, leading to compositional density differences between these two regions. There is a commonly held view that the Lower Mantle has a higher Fe/(Mg+Fe) ratio than the Upper Mantle (Liu and Bassett, 1986 Jeanloz and Knittle, 1989). 

Jackson I. (1983) Some geophysical constraints on the chemical composition of the Earth s lower mantle. Earth Planet. Sci. Lett. 62, 91-103. 

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... 

Allegre et al (1995) took the same Mg/Al ratio as Hart and Zindler (1986) and assumed an Mg/Si ratio of 0.945 which they considered to be representative of the least dilferentiated sample from the Earth s mantle to calculate the bulk chemical composition of the upper mantle. 

Structure and dynamics of the lowermost mantle. This region includes the D layer, which is characterized by major chemical and thermal variations. It is likely of fundamental importance to the chemical evolution of the mantle and may function as a (temporary) resting place for subducted slabs. It is also expected to influence the stability of mantle plumes (Davaille et al., 2002 Jellinek and Manga, 2002), the entrainment and residence times of chemical heterogeneity (Olson and Kincaid, 1991 Schott et al., 2002, and the thermal, chemical, and seismological characteristics compositional variations (Kellogg et al., 1999 Tackley, 2002). 

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... 

It is generally conceded that ultramafic to mafic magmas are the products of partial melting of ultramafic rocks in the Earth s upper mantle (Kushiro, 2001 see Chapter 2.08). The composition of primary magmas generated by partial melting of ultramafic mantle rocks are, in turn, controlled by the mineralogy and chemical composition of the ultramafic rocks, as well as by the pressure, temperature, and extent of partial... 

Ronov A. B. and Yaroshevsky A. A. (1969) Chemical composition of the Earth s crust. In Earth s Crust and Upper Mantle, (ed. J. P. Hart). American Geophysical Union Monograph. 

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at midocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals. 

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