ZIRCON/WHOLE-ROCK TRACE ELEMENT PARTITIONING MODELS FOR POLY-METAMORPHIC GNEISSES PROVIDE BROAD CONSTRAINTS ON PETROGENESIS (AT BEST)

Element partitioning in mineral/melt can be modeled as the elastic energy required to force a sphere of radius r0 (1+e) into a site of radius r0 in a solid with a known Poisson ratio and compressibility. This behavior is predictable (in theory) for the size and the elasticity of the crystal site in which substitution occurs. Igneous zircons have high HREE/LREE due to the lanthanide contraction and relative HREE compatibility in the zirconium site. In principle, REE concentrations in zircons should place robust constraints on growth conditions. Alas, experimental partition data show that REEs can range over several orders of magnitude and because D-values are also controlled by P, T, fO2, crystal and melt composition, inverse models used to reconstruct melt compositions based solely from zircon compositions are unsatisfactory. Studies of natural rock samples are warranted to further understand zircon D-values and their petrogenetic meaning. Minor- and trace element contents of metamorphic zircon vs. WR deviate strongly from predicted magmatic partitioning; Dzircon-WR for different zircon populations are useful to elucidate the petrogenetic history of complex poly-metamorphic terranes. A suite of 4.02–3.75 Ga gneisses illustrate this: for in situ Dzircon-WR measurements of trace elements (REEs, Ti, Th, U), minor elements (P, Fe, Y) and Hf in petrographic thin sections and mineral separates show how these rocks experienced protracted deformational and P-T histories which ranged from 0.2-0.5 GPa at ~630°C, to 0.7-1.0 GPa at ~800°C. Only for the igneous zircon populations do Onuma diagrams show systematic near-parabolic dependence on cation radius in Dzircon-WR: r0 +3 curves peak near Lu (0.977Å), while r0 +4 curves peak at 0.91Å (near Hf and Zr). Metamorphic zircons exhibit more horizontal Dzircon-WR vs. ionic radius (>La/Lu; variable Th/U). Integrated Dzircon-WR, (Th/U)zircon, and U-Pb geochronology can effectively discriminate between igneous, inherited and metamorphic zircon populations in gneisses and reveal much (but not enough) of the petrogenetic history of complex gneissic terranes including original crystallization ages vs. inheritance.