HR: 12:10h
AN: V22B-08    [Abstracts]
TI: New Experimental Measurements of Zircon/Melt Trace Element Partition Coefficients
AU: * Ayers, J C
EM: john.c.ayers@vanderbilt.edu
AF: Department of Earth and Environmental Sciences, Vanderbilt University, 2301 Vanderbilt Place Station B 35-1805, Nashville, TN 37235, United States
AU: Luo, Y
EM: yan.luoyan@gmail.com
AF: Department of Earth and Environmental Sciences, Vanderbilt University, 2301 Vanderbilt Place Station B 35-1805, Nashville, TN 37235, United States
AB: Zircon was grown from trace element-doped peralkaline melt of the same composition used by Watson (1980). Cold-seal experiments at 1 and 2 kbar and 800°C and piston cylinder experiments at 15 kbar and 900- 1300°C were run for 160-960 hours. Oxygen fugacity was buffered at NNO, HM or IW. Zircon and glass were present in all run products, and small monazite crystals were present in 7 of the 13 experiments. Diameters of run product zircon crystals ranged from 5-15 um at 800°C to 30-50 um at 1300°C. Zircons were analyzed by electron microprobe and run product glass by LA-ICP-MS. Zircon crystals show slight zoning in CL images and in core-to-rim EMP traverses. Trace element partition coefficients D(zircon/melt) were measured for Sc, Ti, V, Y, Zr, Nb, La, Ce, Pr, Nd, Eu, Gd, Ho, Yb, Lu, Hf, Ta, Th and U. At 800°C D values for some elements change slightly between 216 and 336 h, then change little between 336 and 792 h, suggesting that near-equilibrium is achieved after 336 h; less time should be required to reach equilibrium at higher temperatures. Replicate experiments show good agreement, with differences in D values smaller than those between experiments performed at different P-T-fO2-t conditions. In most cases Lu was the most (D 15-59) and Nb (< 0.05 - 0.37) or La (0.01 - 0.97) the least compatible elements. Our measured D values are lower than those estimated by Watson (1980) using mass balance, and unlike Watson (1980) our values usually do not peak in REE diagrams but smoothly increase from La to Lu. Measured D values for REE fall in the middle of other previously published values. D values for Zr (9.8-25) and Hf (9.6-19) are lower than previously measured values because we purposefully chose a melt composition in which zircon is highly soluble to promote crystal growth. Many elements show a steady decrease in D from 1100-1300°C. A positive Ce anomaly is evident at all fO2 values but a Eu anomaly is slight or non-existent. Ce and Eu do not show a strong dependence of D on fO2, but D(U) increases strongly as fO2 decreases from HM to NNO to IW, indicating that U4+ must fit better in the zircon lattice than U6+. The Th/U fractionation factor f = D(Th)/D(U) ranges from 0.43 to 4.8; it is > 1 for all experiments at HM but < 1 for all experiments at NNO and IW, suggesting that high oxygen fugacity may be partly responsible for the relatively high Th/U of magmatic zircons. Watson, E. B., 1980. Note: Some experimentally determined zircon/liquid partition coefficients for the rare earth elements. Geochim. Cosmochim. Acta 44, 895-7.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting