THE CHEMISTRY OF ZIRCON - VARIATIONS WITHIN AND BETWEEN LARGE CRYSTALS FROM SYENITE AND ALKALI BASALT XENOLITHS

Single grains of zircon can contain zones indicating several generations of crystal growth, each of which should reflect the chemical and physical conditions occurring at the time of its formation. Trace element analyses have been made of large zircon crystals from rocks of alkaline affinities by ion microprobe. The chondrite-normalised rare earth element (REE) concentrations increase rapidly from La to Lu, as would be expected from the decrease in ionic radius and consequent easier substitution into the Zr site within the zircon lattice. Lanthanum, praseodymium, and neodymium are considerably lower than values observed in bulk analyses of zircon. The partition coefficients for the light rare earth elements (LREEs), between zircon and melt or whole rock, must therefore be significantly lower than those calculated using bulk analyses. Cerium is enriched relative to neighbouring REEs due to the presence of Ce4+. Estimates of partition coefficients of Ce3+ and Ce4+ between zircon and melt demonstrate that although the Ce anomalies are large the Ce4+/Ce3+ ratio is very small (less than 3 x 10(-3)). The size of the Ce anomaly is variable and should be capable of providing information on oxygen fugacity changes.