Effect of Fe 2+ on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Abstract

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al 2O 3–SiO 2 (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe 2+ 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, 452–454], 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+) [ A ̊ ]=0.930X Py +0.993X Gr +0.916X Alm +0.946X Spes +1.05(X And +X Uv )−0.005(P [ GPa]−3.0)(±0.005 A ̊ ) E X (3+) [ GPa]=3.5×10 12(1.38+r 0(3+) [ A ̊ ]) −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°C< T<1640°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 Å. 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.