Partitioning garnets

P(x) U(x) we see that the variation in Du/Dm with (Fig. 14) is remarkably similar to that for clinopyroxene (Fig. 1). The fact that the plotted garnet partitioning data derive from a wide range of pressures, temperatures and compositions is particularly encouraging. 

Van Westrenen et al. (2001a) present a model of lanthanide and Sc partitioning between the garnet X-site and melt. The model is a variant of the lattice strain model of clinopyroxene-melt partitioning of Wood and Blundy (1997), and is based on 160 experimental garnet-melt pairs in the pressure-temperature range 2.5-7.5 GPa and 1450-1930°C. The model includes composition-sensitive expressions for and accounts for the non-linear variation in with composition, as follows ... 

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.

Perhaps the biggest challenge in estimating U-series partition coefficients is the case of protactinium partitioning into garnet. The difficulty arises because the ionic radius of... 

Figure 16. Electrostatic model fitted to partition coefficients for cations entering the X-site in garnet, based on the experiments of Van Westrenen et al. (1999, 2000) and Klemme et al. (2002). The curves are fits to Equation (7) and can be used to estimate and f>o(x) > which f>pa(x) can be calculated via the lattice strain model. The fit parameters are given in Table 5.

Beattie P (1993b) Uranium-thorium disequihbria and partitioning on melting of garnet peridotite. Nature 363 63-65... 

Green TH, Blundy JD, Adam J, Yaxley GM (2000) SIMS determination of trace element partition coefficients between garnet clinopyroxene and hydrous basaltic liquids at 2-7.5 GPa and 1080-1200°C. Lithos 53 165-187... 

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... 

Hauri EH, Wagner TP, Grove TL (1994) Experimental and natural partitioning of Th U Pb and other trace elements between garnet clinopyroxene and basaltic melts. Chem Geol 117 149-166 Hazen RM, Finger LW (1979) Bulk Modulus-volume relationship for cation-anion polyhedra. J Geophys Res 84 6723-6728... 

Table 2. Experimentally determined mineral/melt partition coefficients (x 10 ) for U, Th, and Ba in clinopyroxene and garnet and calculated bulk partition coefficients (xlO ) for a garnet peridotite source.

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