Effect of Fe2+ on garnet-melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet-melt trace element partitioning experiments were performed in the system FeO-CaO-MgO-Al2O3-SiO2 (FCMAS) at 3 GPa and 1540 degrees C, aimed specifically at studying the effect of garnet Fe2+ content on partition coefficients (D-Grt/Melt). D-Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994, Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452-354], which describes partitioning of an element i with radius r(i) and valency Z in terms of three parameters: the effective radius of the site r(0)(Z), the strain-free partition coefficient D-0(Z) for a cation with radius r(0)(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E-X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838-847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r(0)(3 +) and E-X(3 +) as a function of garnet composition (X) and pressure (P): r(0)(3 +) [Angstrom] = 0.930 X-Py + 0.993 X-Gr + 0.916(Alm) + 0.946X(Spes) + 1.05( X-And + X-Uv) - 0.005(P [GPa] - 3.0)(+/- 0.005 Angstrom) E-X(3 +) [GPa] = 3.5 x 10(12)(1.38 + r(0)(3 +) [Angstrom])(-26.7) (+/-30 GPa) Accuracy of these equations is shown by application to the existing garnet-melt partitioning database, covering a wide range of P and T conditions (1.8 GPa < P < 5.0 GPa; 975 degrees C < T < 1640 degrees C). D-Grt/Melt for all 3 + elements entering the X-site (REE, Sc and Y) are predicted to within 10-40% at given P, T, and X, when D-Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D-Sm(Grt/Melt)/D-Nd(Grt/Melt)) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r(0)(3 +) for garnets ranges from 0.939 +/- 0.005 to 0.953 +/- 0.009 Angstrom. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet-melt partitioning for the REE, Y and Sc. As such. they represent a major advance en route to predicting D-Grt/Melt for these elements as a function of P, T and X. (C) 2000 Elsevier Science B.V. All rights reserved.