Garnets mantle

Dm (and Du) vary inversely with reciprocal temperature (Fig. 15). For mantle solidus garnets the correlation is reasonably good and can be used to make a first-order estimate of Dm- A more comprehensive model for Du and Dm, as a function of pressure, temperature and melt composition is provided by Salters et al. (2002). Their full expressions (for the molar partition coefficients, D ) are ... 

Figure 15. Variation in Dxh for garnet versns reciprocal temperature for experimental data sources listed in Table lb at a variety of pressures n = 33). A distinction is made between mantle solidus partition coefficients (Salters and Longhi 1999 Salters et al. 2002 McDade et al. 2003a,b) and the rest. Note the strong temperatnre dependence, which is qnalitatively similar to that incorporated in Equation (25b). The scatter is due to additional compositional controls.

Along the mantle solidus this ratio is approximately 10 ". Dsa itself is not well constrained for garnets because of difficulties in analysing such trace quantities. The most robust estimates are probably those of Beattie (1993b), which are around 10 . Suffice to say Z)Ra in garnet is vanishingly small. 

Guo J, Green TH (1990) Experimental study of barium partitioning between phlogopite and sihcate liquid at upper-mantle pressure and temperature. Lithos 24 83-96 Harrison WJ, Wood BJ (1980) An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defects. Contrib Mineral Petrol 72 145-155 Hart SR, Duim T (1993) Experimental cpx/melt partitioning of 24 trace elements. Contrib Mineral Petrol 113 1-8... 

At shallow pressure (less than 1 GPa), Th becomes slightly more compatible than U in clinopyroxene (Landwehr et al. 2001) and garnet is not stable even in pyroxenite or eclogite. On the other hand, it is believed (mostly based on observations) that Pa remains more incompatible than U through the entire length of the melt column. Thus, melting at intermediate pressure should yield small °Th or U excesses, but significant Pa excess. This has been observed in the lithospheric mantle in the Colorado plateau by... 

Beattie (1993) argued rather convincingly, that only zircon could have a significant effect, but zirconium is also present in garnets and clinopyroxenes such that zircon is extremely unlikely in mantle assemblages. 

In contrast to the southern volcanic zone, Parinacota volcano lies on very thick continental crust (> 70 km) in the central volcanic zone of Chile. Bourdon et al. (2000a) showed that young Parinacota lavas encompass a wide range of U-Th disequilibria. excesses were attributed to fluid addition to the mantle wedge but °Th-excesses in lavas from the same volcano are more difficult to explain. The lavas with °Th-excesses also have low ( °Th/ Th) (< 0.6) characteristic of lower continental crust characterized by low Th/U and in their preferred model. Bourdon et al. (2000a) attributed the °Th-excesses to contamination by partial melts, formed in the presence of residual garnet, of old lower crustal materials. 

If that sort of simplistic model is acceptable, the extent of fractionation hints at rather small degrees of melting in the mantle and/or the presence of a residual phase that fractionates Sm from Nd, probably garnet, o... 

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... 

Because of the importance of garnet in upper-mantle parageneses, the compressibility of this phase has been the subject of many experimental studies. Ha-zen and Finger (1978), studying the compressibility of pyrope, observed that the... 

Water in the mantle is fonnd in different states as a fluid especially near sub-duction zones, as a hydrous phase and as a hydroxyl point defect in nominally anhydrous minerals. 8D-values between -90 and -110%c have been obtained by Bell and Ihinger (2000) analyzing nominally anhydrous mantle minerals (garnet, pyroxene) containing trace quantities of OH. Nominally anhydrous minerals from mantle xenoliths are the most D-depleted of all mantle materials with 5D-values 50%c lower than MORE (O Leary et al. 2005). This difference may either imply that these minerals represent an isotopically distinct mantle reservoir or that the samples analyzed have exchanged hydrogen dnring or after their ascent from the mantle (meteoric/water interaction ). 

The oxide is used in phosphors that form red color in color television tubes. Also, it is used in gas mantles and acetylene hghts. Other uses are in yttrium-iron garnets for microwave filters in lasers, and as a stabdizer for high temperature in refractories. 

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