ZIRCON/FLUID TRACE ELEMENT PARTITION COEFFICIENTS MEASURED BY RECRYSTALLIZATION OF MUD TANK ZIRCON AT 1.5 GPA AND 800-1000°C

New experiments aimed at measuring zircon/fluid trace element partition coefficients Dz/f involved recrystallizing natural Mud Tank zircon with low trace element concentrations in the presence of aqueous fluid with ~1000 ppm of REE, Y, Sc, Nb, Ta, U and Th at 1.5 GPa and 800-1000°C at fO₂ = NNO in a piston cylinder apparatus using the double capsule method. Run product zircon crystals have rims ~5 µm thick with much higher trace element concentrations than in Mud Tank zircon as determined from LA-ICP-MS analysis. Solute mass was too low to allow for collection and analysis, so Dz/f was estimated from rim analyses and bulk composition using mass balance. Dz/f for REE show a continuous increase with decreasing ionic radius from La to Lu.

Previous experiments in our lab involved growth of zircon from the oxides in the presence of a trace element-doped fluid. Zircon crystals in run products were < 10 µm in diameter and were intermixed with unreacted oxides, so 20-30 µm diameter LA-ICP-MS spot analyses of crystalline run products produced mixed analyses. Analyses of collected solutes were used to calculate fluid composition, which combined with analyses of crystalline run products yielded Dz/f values that were highly variable for each experiment and showed little evidence of REE fractionation. Dz/f values calculated from fluid compositions and mass balance yield a subset of experiments with DLa/DLu similar to recrystallization experiments, with average Dz/f of 0.4 for La and 4.8 for Lu. With a few exceptions Th and U showed no significant fractionation in both types of experiments.