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

We performed experiments at 3.0 GPa and 1530-1565 °C to investigate the effects of crystal composition on trace element partitioning between garnet and anhydrous silicate melt. Bulk compositions along the pyrope (Py: Mg₃Al₂Si₃O₁₂)-grossular (Gr: Ca₃Al₂Si₃O₁₂) 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 Py₈₄Gr₁₆ to Py₉Gr₉₁, 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 Py₈₄ to Py₉, 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 (r₀), the Young's modulus of the site (E), and the (theoretical) partition coefficient D₀ for an ion of radius r₀. For trivalent cations substituting in the garnet X-site (Y, REE, Sc, and In), apparent values of r₀ fitted to our data vary systematically from 0.935 ± 0.004 Å (Py₈₄) to 0.99 ± 0.01 Å (Py₉), a trend consistent with variations in the size of the X-site. Values of D₀ show an increase from Py₉ (D₀ = 2.8 ± 0.1) to Py₈₄ (4.8 ± 0.1) and Young's modulus E varies from 257 ± GPa for Py₆₀ to 590 ± 40 GPa for Py₈₄. 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.