ZIRCON/FLUID TRACE ELEMENT PARTITIONING

Hydrothermal and some metamorphic zircons are known to have trace element characteristics such as low Th/U that are different from magmatic zircons.  Zircon/fluid double capsule trace element partitioning experiments were conducted with fO₂ buffers (NNO = Ni + NiO and HM = Hematite Fe₂O₃ – Magnetite Fe₃O₄) in cold seal pressure vessel systems at 2 kbar pressure and temperatures of 450, 600 and 750°C to explore the origin of these distinct trace element signatures.  Starting materials included natural zircon crystals ~200 um in diameter and aqueous fluids containing 0 – 1m NaOH and 100 ppm of Th, U, Mg (for charge balance), and 14 different Rare Earth Elements (REE).  Zircon crystals from before and after experiments were analyzed by LA-ICPMS, and quenched solutes were analyzed by solution ICPMS.  Zircons incorporated trace elements by recrystallizing in the fluid to form homogeneous run product grains.  Measured D_zircon/fluid values range from ~0.01 for Eu to ~7500 for Hf.  Measured values of the fractionation factor f = D_Th/D_U range from 0.045 to 4, with most values < 1.  At 600°C, f decreases from 1.4 at NNO to 0.08 at HM, but no significant fractionation or fO₂ effect is observed at 450°C.  If fractionation is responsible for the low Th/U observed in hydrothermal zircons, then they must precipitate from very oxidizing fluids at temperatures > 450°C.  At all measured conditions D_zircon/fluid values for REE show no Ce anomaly.  At 450°C a negative Eu anomaly occurs at NNO but not at HM.  The negative Eu anomaly is present in all D_zircon/fluid values measured at 600 and 750°C and fO₂ = NNO and HM.  D_zircon/fluid values are lowest for LREE and peak at Dy.  At the conditions of our experiments, fractionation of trace elements may result in negative Eu (but no Ce) anomalies and low Th/U in zircons precipitated from high-temperature, oxidizing aqueous fluids.