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

Wim Van Westrenen*, Jonathan D. Blundy, Bernard J. Wood

*Corresponding author for this work

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    82 Citations (Scopus)

    Abstract

    Garnet-melt trace element partitioning experiments were performed in the system FeO-CaO-MgO-Al2O3-SiO2 (FCMAS) at 3 GPa and 1540°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, 452-454], 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 r0(Z), the strain-free partition coefficient D0(Z) for a cation with radius r0(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 r0(3 +) and E(X)(3 +) as a function of garnet composition (X) and pressure (P): r0(3 +) [Å] = 0.930X(Py) + 0.993X(Gr) + 0.916X(Alm) + 0.946X(Spes) + 1.05(X(And)+X(Uv)) - 0.005(P[GPa] - 3.0)(±0.005Å) E(x)(3 +) [GPa] = 3.5 x 1012 (1.38 + r0(3 +) [Å])-26.7(±30GPa) 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, r0(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. (C) 2000 Elsevier Science B.V. All rights reserved.

    Original languageEnglish
    Pages (from-to)189-201
    Number of pages13
    JournalLithos
    Volume53
    Issue number3-4
    DOIs
    Publication statusPublished - 2000

    Keywords

    • Crystal chemistry
    • Experimental studies
    • Garnet group
    • Partitioning
    • Trace elements

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