AN EXPERIMENTAL AND ANALYTICAL STUDY OF TI PARTITIONING IN ZIRCON

Zircon (ZrSiO₄) is a common accessory phase and robust repository of trace elements (e.g., Y, P, REEs) in igneous rocks. Trace element concentrations within a single zircon can provide compositional information on the sources of the parental melts as well as on extrinsic variables—pressure and temperature (T)—during crystal growth.

The concentration of Ti in zircon has been proposed as a temperature proxy in magmatic systems where zircon and a Ti-rich phase such as rutile or ilmenite have co-crystallized (Watson et al., 2006; Ferry & Watson, 2007). This application assumes that Ti concentrations in zircon reflect equilibrium partitioning between zircon and melt. However, NanoSIMS images of Jack Hills (Hofmann et al., 2009) as well as younger zircons indicate that Ti is generally not uniformly distributed but shows oscillatory zoning that spatially correlates with distributions of other trace elements (Y, P, Ce), which in turn track growth features seen in cathodoluminescent banding.

These correlated zonations could reflect: oscillating magmatic temperatures if partitioning is T-dependent and an equilibrium process; episodic diffusion-limited enrichment of incompatible trace elements in the crystal-melt boundary layer; and/or kinetically controlled, non-equilibrium crystal-melt partitioning caused by trace element enrichments in the boundary layer melt surrounding fast-growing grains. In order to test these hypotheses, we initiated a series of 1 GPa experiments in which zircons were synthesized from trace-element-free oxide-based mixes and trace-element-bearing natural rock-based powders—all granitic in composition. Overgrowth rims (~ 0.5 to 2 µm) on the seed zircons were analyzed using the NanoSIMS, while the coexisting glass was analyzed using an electron microprobe.

Our results to date (1200–1400°C) yield the first experimental partition coefficient for Ti between zircon and melt: D_Ti is ~ 0.02 with little or no T-dependence and no observed differences between the trace-element-free and trace-element-bearing mixes. If these results hold with decreasing T, then the ~ 2 orders of magnitude variations in Ti contents in zircons over Ts of 1400°C (experimental) to ~ 800°C (magmatic) would, to first order, reflect the T-dependence of Ti solubility in silicate melts (e.g., Gaetani et al., 2008).