Zircon/fluid trace element partition coefficients measured by recrystallization of Mud Tank zircon at 1.5 GPa and 800–1000 °C. (15th February 2018)

• Record Type: Journal Article
• Authors: Ayers, John C.; Peters, Timothy J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 223(2018)
• Page Start: 60
• Page End: 74
• Publication Date: 2018-02-15
• Subjects: Zircon -- Fluid -- Trace element partitioning -- Rare earth elements; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2017.11.025
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Hydrothermal zircon grains have trace element characteristics such as low Th/U, high U, and high rare earth element (REE) concentrations that distinguish them from magmatic, metamorphic, and altered zircon grains, but it is unclear whether these characteristics result from distinctive fluid compositions or zircon/fluid fractionation effects. New experiments aimed at measuring zircon/fluid trace element partition coefficients D z / f involved recrystallizing natural Mud Tank zircon with low trace element concentrations in the presence of H2 O, 1 m NaOH, or 1 m HCl doped with ∼1000 ppm of rare earth elements (REE), Y, U and Th and ∼500 ppm of Li, B, P, Nb, Ba, Hf, and Ta. Experiments were run for 168 h at 1.5 GPa, 800–1000 °C, and fO2 = NNO in a piston cylinder apparatus using the double capsule method. LA-ICP-MS analysis shows that run product zircon crystals have much higher trace element concentrations than in Mud Tank zircon starting material. D z / f values were estimated from run product zircon analyses and bulk composition using mass balance. Most elements behave incompatibly, with median D z / f being highest for Hf = 8 and lowest for B = 0.02. Addition of NaOH or HCl had little influence on D z / f values. D z / f for LREE are anomalously high, likely due to contamination of run product zircon with quenched solutes enriched in incompatible elements, so D LREE were estimated using lattice strain theory. Brice curves for +3 ions yield zircon/fluid D Lu / D La of ∼800–5000. A Brice curve fit to +4 ions yielded D Ce4+ values. Estimated concentrations of Ce 3+ and Ce 4+ show that the average Ce 4+ /Ce 3+ in zircon of 27 is much higher than in fluid of 0.02. Th and U show little fractionation, with median D Th / D U = 0.7, indicating that the low Th/U in natural hydrothermal zircon is inherited from the fluid. Natural fluid compositions estimated from measured D z / f and published compositions of hydrothermal zircon grains from aplite and eclogite reflect the mineralogy of the host rock, e.g., fluid in equilibrium with eclogite garnet is depleted in heavy REE relative to middle REE, and has low Th/U.