Xenoliths chemical composition

Palme H., Kurat G., Spettel B., and Burghele A. (1989) Chemical composition of an unusual xenolith of the Allende meteorite. Z. Naturforsch. 44a, 1005-1014. 

Direct sampling of mantle rocks and minerals is limited to tectonic slices emplaced at the surface (see Chapter 2.04), smaller xenoliths transported upwards by magmatic processes (see Chapter 2.05), and still smaller inclusions in such far-traveled namral sample chambers as diamonds (see Chapter 2.05). Because of such limited direct access to mantle materials, knowledge of mantle structure, composition, and processes must be augmented by geophysical remote sensing. What can various seismological observations tell us about the major-element composition of the upper mantle How can they constrain possible differences in chemical composition between the upper... 

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... 

Leyreloup A., Dupuy C., and Andriambololona R. (1977) Catazonal xenoliths in French Neogene volcanic rocks constitution of the lower crust 2. Chemical composition and consequences of the evolution of the French Massif Central Precambrian crust. Contrib. Mineral. Petrol. 62, 283-300. 

Alkali basalt xenoliths Xenoliths in alkali basalts are most commonly spinel-bearing peridotites. They have been divided into two main groups (Frey Prinz, 1978). Primitive xenoliths have chemical compositions which suggest that they have had a melt extracted from them. Menzies (1983) showed that this "depleted" character is similar in alkali basalt xenoliths beneath both the oceans and the continents and proposed that a depleted mantle layer underlies both... 

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. 

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... 

Peridotite fertilization may also result from the fractional solidihcation of exotic (deep-seated) melts inhltrated in wall rocks of translithospheric magma conduits. This process was hrst described in composite mantle xenoliths (Wilshire and Shervais, 1975 Gurney and Harte, 1980 Irving, 1980 Wilshire et al., 1980 Boivin, 1982 Harte, 1983 Harte et al, 1993 Menzies et ah, 1987), where it is referred to as modal metasomatism when new (generally hydrous) minerals are precipitated (Dawson, 1984 Kempton, 1987), or Fe-Ti metasomatism (Menzies et al., 1987) when the attention is focused on chemical enrichment. In contrast with ultramafic xenoliths, the tectonically emplaced and oceanic peridotites contain only sparse rock types attributable to mantle metasomatism by deep-seated melts. Examples of wall-rock, modal, and Fe-Ti metasomatism were nevertheless described in IP orogenic Iherzolites, notably in the Pyrenees (Fabrics et al., 1989 Bodinier et al., 1988, 1990, 2003 McPherson et al., 1996 Woodland et al., 1996). 

The mantle comprises 68% by mass of Earth, and an accurate estimate for its composition is the very basis for unraveling the origin and differentiation of our planet. The bulk chemical analysis of xenoliths has been central to understanding the composition of the Earth s mantle, the genesis of basalt and the physical properties in the lithosphere that bear on its stability in the rigid part of the mantle system. 

Modeling of the whole-rock and mineral trace-element compositions in xenoliths that have experienced metasomatic infiltration of melts led Ionov et al. (2002a) to conclude that the REE and HFSE element compositions of peridotites adjacent to veins bear the chemical fingerprints of the metasomatic agent closest to its source (e.g., a melt vein). Further away, signatures are increasingly dominated by fractionation processes related to melt percolation. 

Rudnick and Fountain (1995) (and hence our current composition) used the compositions of lower-crustal xenoliths to constrain the mafic end-member of the deep crust. These xenoliths have high Mg and low alkalis (Table 7), and thus may be chemically distinct from mafic rocks exposed on the Earth s surface (the chemical data used by Christensen and Mooney, 1995). 

Eberz G. W., Clarke D. B., Chatterjee A. K., and Giles P. S. (1991) Chemical and isotopic composition of the lower cmst beneath the Meguma Lithotectonic Zone, Nova Scotia evidence from granulite facies xenoliths. Contrib. Mineral. Petrol. 109, 69-88. 

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