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

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:Mg3Al2Si3O12)-grossular (Gr:Ca3Al2Si3O12) join, doped with a suite of trace elements (Li, B, K, Sc, Ti, Sr, Y, Zr, Nb, Cd, In, REE, I-If, 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(0)), the Young's modulus of the site (E), and the (theoretical) partition coefficient D-0 for an ion of radius r(0). For trivalent cations substituting in the garnet X-site (Y, REE, Sc, and In), apparent values of lb fitted to our data vary systematically from 0.935 +/- 0.004 Angstrom (Py-84) to 0.99 +/- 0.01 Angstrom (Py-9), a trend consistent with variations in the size of the X-site. Values of D-0 show an increase from Py-9 (D-0 = 2.8 +/- 0.1) to Py-84 (4.8 +/- 0.1) and Young's modulus E varies from 257 +/- 20 GPa for Py-60 to 590 +/- 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.