Mineralogy of the Mantle

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). 

The a - P — y phase transitions involving (Mg,Fe)2Si04 are isochemical. However, below 670 km where the Lower Mantle begins, disproportionation 

Mineral Pressure Site Point symmetry Metal-Oxygen distances (pm) Sitte occupancy Source of data 

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Composition and mineralogy of the Mantle. The Earth s Mantle consists of Upper and Lower regions separated by the Transition Zone at depths between about 350 km and 650 km. Several phase changes occur in the Transition Zone in which common ferromagnesian silicates of the Upper Mantle, all containing Fe2+ ions in distorted six-coordinated sites and tetrahedrally coordinated Si, transform to dense oxide structures with cations occupying regular octahedral... 

Mineral phases in the mantle -evidence from mineral physics The mineralogy of the mantle changes with depth as pressure increases. In the case of the shallow upper-mantle the mineralogy of the mantle is known from mantle xenoliths (see Section 3.1.1.5). The mineralogy of the deeper mantle is known from rare mineral inclusions in diamond (see Section 3.1.1.5), from experimental studies using multianvil and diamond... 

Whilst inferences can be made about variations in the mineralogy of the mantle with depth from basalts as the mantle melts, it is the chemical composition of basalts that is the most powerful source of information about the mantle. In particular, a detailed examination of the trace element and isotopic characteristics of basalts has revealed a number of different basalt types, whose differences can only be explained if they come from chemically different domains within the Earth s mantle. Basalt chemistry, therefore, is the prime source of information about chemical heterogeneity within the Earth s mantle. This heterogeneity is a very important property of the mantle and one which can only be understood when the history of the Earth s mantle is considered over geological time. 

Constraints on the bulk composition of the mantle are mainly density, solar elemental abundances, and compatibility with compositions of lavas derived from the mantle and extruded onto the surface. Lavas come from the upper few hundred km only. Phase changes because of increasing temperature and pressure with depth strongly affect the mineralogy of the mantle. 

The choice of topics is largely governed by the author s interests. Following a brief introduction the crystal field model is described non-mathematically in chapter 2. This treatment is extended to chapter 3, which outlines the theory of crystal field spectra of transition elements. Chapter 4 describes the information that can be obtained from measurements of absorption spectra of minerals, and chapter 5 describes the electronic spectra of suites of common, rock-forming silicates. The crystal chemistry of transition metal compounds and minerals is reviewed in chapter 6, while chapter 7 discusses thermodynamic properties of minerals using data derived from the spectra in chapter 5. Applications of crystal field theory to the distribution of transition elements in the crust are described in chapter 8, and properties of the mantle are considered in chapter 9. The final chapter is devoted to a brief outline of the molecular orbital theory, which is used to interpret some aspects of the sulphide mineralogy of transition elements. 

Hunter R. H., Upton B. G. J., and Aspen P. (1984) Meta-igneous granulite and ultramafic xenohths from basalts of the Midland Valley of Scotland petrology and mineralogy of the lower emst and upper mantle. Trans. Roy. Soc. Edinburgh IS, 75-84. 

The mineralogical and chemical composition of peridotite from subcontinental lithosphere differs from that of peridotite from other parts of the mantle (Boyd 1989 Berstein et al. 1997). Peridotites from subcontinental lithosphere is depleted , which means it contains only a small amount of clinopyroxene and an aluminous phase, which together make up the so-called basaltic component. The lithosphere beneath the oldest Archaean cratons has a composition markedly different from that of younger subcontinental lithosphere (Boyd Mertzman 1987 Griffin et al. 1999). Old unmetasomatized lithosphere is harzburgitic, a mixture of olivine... 

Of particular significance are the density increases which take place at the upper and lower boundaries of the mantle transition zone, at the 410 km and 660 km discontinuities. In the past it has been argued that the large density increase which takes place at 660 km depth reflects a change in the bulk composition of the mantle with depth. However, the present consensus is that the contrasts can be accommodated simply by phase changes in the mantle mineralogy. (This debate has huge consequences for whether or not the mantle is chemically layered, and is an important factor in the current debate about the nature of mantle convection). 

Figure 3.6 illustrates the main contributions of geophysical data, ophiolite complexes, xenoliths, diamond inclusions, and mantle melts to our current understanding of the mantle. What is clear from this summary is that the extent to which we understand the mineralogy and composition of the Earth s mantle decreases with depth, and our only detailed knowledge is of the upper mantle. 

Controls on mantle melting The exact composition of a mantle melt is a function of the source mineralogy, the mantle composition and volatile content, and the degree of melting, as is illustrated in Fig. 3.15. However, a major question in the study of the upper mantle is whether we ever see the direct melting products of the mantle, or whether what is produced in the melting zone of the... 

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