Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt

University of Bristol, Department of Earth Sciences, Bristol, United Kingdom

We performed experiments at 3.0 GPa and 1530-1565 degrees C to investigate the effects of crystal composition on trace element partitioning between garnet and anhydrous silicate melt. Bulk compositions along the pyrope (Py: Mg ₃ Al ₂ Si ₃ O ₁₂ )-grossular (Gr: Ca ₃ Al ₂ Si ₃ O ₁₂ ) join, doped with a suite of trace elements (Li, B, K, Sc, Ti, Sr, Y, Zr, Nb, Cd, In, REE, Hf, Ta, Th, and U) produced homogeneous garnets, ranging in composition from Py ₈₄ Gr ₁₆ to Py ₉ Gr ₉₁ , in equilibrium with melt. Trace element partition coefficients (D-values), measured by SIMS, depend greatly on the Mg/(Mg+Ca) of garnet. For example, from Py ₈₄ to Py ₉ , D _La increases from 0.004 to 0.2, whereas D _U increases from 0.029 to 0.42. These variations can be explained by the lattice strain model of Blundy and Wood (1994), which describes trace element partitioning of an element i in terms of the ionic radius of i (r _i ), the size of the lattice site on which i partitions (r _o ), the Young's modulus of the site (E), and the (theoretical) partition coefficient D _o for an ion of radius r _o . For trivalent cations substituting in the garnet X-site (Y, REE, Sc, and In), apparent values of r _o fitted to our data vary systematically from 0.935+ or -0.004 Aa (Py ₈₄ ) to 0.99+ or -0.01 Aa (Py ₉ ), a trend consistent with variations in the size of the X-site. Values of D _o show an increase from Py ₉ (D _o = 2.8+ or -0.1) to Py ₈₄ (4.8+ or -0.1) and Young's modulus E varies from 257+ or -20 GPa for Py ₆₀ to 590+ or -40 GPa for Py ₈₄ . These results allow a quantitative assessment of the influence of crystal chemistry on garnet-melt D-values, thereby forming the basis for a predictive model similar to that recently developed for clinopyroxene-melt partitioning by Wood and Blundy (1997). Our new data emphasize the importance of taking into account crystal composition when modeling trace element behavior in natural systems.

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