Understanding the mantle

In this first section we examine the nature of the modern Earth s mantle. We examine its physical structure, its chemical composition, and the processes operating within it. This approach is necessary to a study of the early Earth, for not only does this provide an insight into how our planet works but also shows that the modern mantle contains a history of much earlier events. We will find that the parts of the present-day mantle are in fact very old and contain an important memory of earlier Earth events. 

Initially we shall explore ways in which we can obtain information about the modern Earth s mantle. This will then provide a firm foundation for an understanding of the composition of and processes in the Earth s mantle as we find it today. 

Before we consider in detail what we know about the Earth s mantle, first we review how we know about the mantle. Here we review the evidence from seismology, heat flow measurements, and mineral physics, and 

A coefficient 1.0 implies that the element prefers the melt phase these are the incompatible elements. 

The principal rock-type of the upper mantle comprising the minerals olivine and pyroxene. There are a number of different types of peridotite - see dunite, 

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Polve M. and Allegre C. J. (1980) Orogenic Iherzolite complexes studied by Rb- Sr a clue to understand the mantle convection processes Earth Planet. Sci. Lett. 51, 71-93. 

Apatite is a widespread accessory mineral in xenoliths of Phanerozoic lithospheric mantle brought to the surface by volcanic processes. It is important in understanding the mantle residence of volatiles such as Cl and F further, it is a major host for REE and some LILE (such as U, Th, Sr) in the mantle (O Reilly and Griffin 2000). 

Australia, is the doyen of materials scientists who study the elastic and plastic properties of minerals under hydrostatic pressure and also phase stability under large shear stresses (Paterson 1973). J.-P. Poirier, in Paris, a professor of geophysics, was trained as a metallurgist one of his special skills is the use of analogue materials to help understand the behaviour of inaccessible high-pressure polymorphs, e.g., CaTi03 perovskite to stand in for (Mg, FelSiOi in the earth s mantle (Poirier 1988, Besson el al. 1996). 

Thermodynamic properties at high pressures are of great interest for instance to Earth scientists who wish to understand the behaviour of the Earth s mantle, where pressures reach 100 GPa. To carry out energy minimizations in the static limit at non-zero pressures we minimize the enthalpy H = U + pV with respect to all the variables that define the structure, where p is the applied pressure and V the volume. When p is zero we regain eq. (11.7). 

Xenoliths from Siberian continental lithosphere, with Archean model ages, had b Li as low as +0.5 (Eouman et al. 2000). If these values accurately represent the Archean mantle, they suggest the potential for Li isotopic evolution in the Earth, from lighter compositions in the ancient mantle to what is seen in present-day MORE. In spite of the analytical challenges presented by ultramafic rocks, more data from these materials are crucial to an understanding of Li in the mantle, and in resolving questions about the appropriateness of the accepted MORE mantle range. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

Della Vedova B, Marson I, Panza G, Suhadolc P (1991) Upper mantle properties of the Tuscan-Tyrrhenian area a key for understanding the recent tectonic evolution of the Italian region. Tectonophysics 195 311-318... 

There is now a large amount of noble gas diffusion data obtained for rocks or minerals. However, very few studies have been done on noble gas diffusion in silicate melts. The latter bears a central importance in understanding the noble gas evolution in the mantle. Figure 2.11 shows one such scarce example (Lux, 1987), where the diffusion coefficients obtained for a tholeiite basalt melt at 1350°C are plotted as a function of noble gas radius. Diffusion obeys more or less the same linear relationship with r as does the solubility. As Lux (1987) noted, it is remarkable that the... 

These results then lead us to some important questions, which are directly related to a fundamental issue in mantle geochemical dynamics. When and how did the failure of the open system condition take place Did the loss or the gain of He occur in the samples or dining magma production-transportation processes Were they pertinent in the mantle sources We may safely rule out the first possibility because there seems to be no plausible process to input a significant amount of He into a solid material. Unlike solid materials, both addition and loss of He would be possible in melt. However, it is very difficult to understand why the failure of a closed system... 

Among the five noble gases, Ne and Xe deserve special attention because their isotopic compositions are unique (as far as we know) to the Earth, suggesting that their evolution processes are fundamentally related to some specific processes of Earth evolution. In Section 7.4, we will discuss Ne in that a key issue is to understand the distinct difference between mantle neon and atmospheric neon isotopic compositions. In Section 7.5, we discuss a long-standing missing Xe problem. 

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