Garnets upper mantle composition

The variety of symmetries in the garnet structure (coordinations 4, 6, and 8) allows considerable compositional range. Table 5.16 lists the elements commonly present in positions X, X and Z. The diadochy of Al, Ti" ", and Fe in the tetrahedral site has been confirmed by Mossbauer spectroscopy on natural Fe-Ti-bearing garnets (Schwartz and Burns, 1978), and the presence of phosphorus in these sites, observed in upper mantle garnet, is attributable, according to Bishop et al. (1976), to coupled substitutions of the type... 

Finnerty, a. a. Boyd, F. R. 1987. Thermabaro-metry for garnet peridotites basis for the determination of thermal and compositional structure of the upper mantle. In Nixon, P. H. (ed.) Mantle Xenoliths. Wiley, New York, 381-402. 

The residue of crust formation, on the other hand, has a distinct trace element and isotopic composition. This material, a mixture of Fe-rich pyroxene and garnet under upper-mantle conditions, would contain higher concentrations of incompatible elements, particularly if it had become contaminated with crustal rocks. As proposed by Arndt Goldstein (1989), the presence of this material in the source of basalts would impart a distinct isotopic signature to resultant melts. 

The Earth s mantle is peridotitic in composition and is significantly depleted in silica relative to primitive chondrites. Seismological evidence shows that the mantle is layered and can be divided into an upper and lower mantle, separated by a transition zone at 400-660 km depth. Above the transition zone the mantle is dominated by olivine and orthopyroxene with minor garnet and clinopyroxene. The lower mantle is made up of phases Mg- and Ca-perovskite and magnesiowustite. Seismic velocity contrasts between the upper and lower mantle are thought to reflect the ph ase transformations between the two and are not related to differences in bulk chemical composition. The lower mantle is separated from the outer core by the D" layer, a hot thermal boundary layer of enigmatic composition. 

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