Lithosphere, composition

Figure 22 Pressure-temperature diagram showing estimated conditions of melt extraction for average lithosphere compositions. Estimates are based on generalized phase relations for batch melting of fertile mantle as depicted in Figure 1 and shown in the background. Lithosphere labels are as in Figure 21.

What does the secular variation of lithosphere composition tell us about mantle temperatures ... 

The details of lithospheric composition are best considered for individual locations. Here it is only worth emphasizing that the chemical composition of xenoliths reflect both major element depletion events and subsequent enrichment processes, and so have had complex open-system histories. The sources and characteristics of noble gases must consider this environment, as discussed further below. 

Present attention is focussed on the lithosphere which is the solid part of the Earth s crust. Details as regards its components and composition are shown in Table 1.2. It is... 

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. 

Regardless of the ultimate sources of these compositions, these results clearly show that strongly isotopically fractionated Li from crustal sources plays a role in the mantle. Processes active in subduction zones appear to be cardinal in the control of the Li isotopic composition of different parts of the mantle. The results to date imply that both isotopically enriched (8 Li > MORE) and depleted (5T i < MORE) material are available for deep subduction, and that areas of the continental lithosphere may retain these records on long time scales. 

Sources of information about the isotopic composition of the upper portion of the lithospheric mantle come from the direct analysis of unaltered ultramafic xenoliths brought rapidly to the surface in explosive volcanic vents. Due to rapid transport, these peridotite nodules are in many cases chemically fresh and considered by most... 

Boyd SR, PUlinger CT, Mflledge HJ, Mendelssohn MJ, Seal M (1992) C and N isotopic composition and the infrared absorption spectra of coated diamonds evidence for the regional uniformity of CO2-H2O rich fluids in lithospheric mantle. Earth Planet Sci Lett 109 633-644... 

Ion microprobe analyses of hydrous minerals in Martian meteorites reveal two different sources of hydrogen. One is interpreted as magmatic water, with 5D = 900 permil, and thought to reflect the mantle composition the other is thought to reflect the atmospheric composition, with 5D =4000 permil (Leshin, 2000). The incorporation of atmospheric water into these meteorites suggests some kind of cycling of water between the atmosphere and lithosphere on Mars. 

Occurrence in Nature. About 99.6% of the earth s mass results from 32 of the chemical elements. The remaining 0.4% is apportioned among 64 elements, all of which are present as traces. Iodine is one of these 64. Estimates about abundance of the constituent elements of the lithosphere place iodine 46th on a restricted list of 59 elements (37 very rare elements are excluded) and 61st on a list in which 96 elements are included. Iodine is, indeed, one of the scarcest of the nonmetallic elements in the total composition of the earth (3). 

The separations between the layers as indicated in figure 7.1 are not as clear as shown, but are gradual, which results in a gradual change in density and mineral composition. The thickness of the continental earth s crust or lithosphere (Greek lithos = stone) varies between appr. 20 and appr. 60 km, with an average thickness of appr. 35 km. The elementary composition of the lithosphere is well-known,... 

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