Thematic Articles

Numerous cases have been reported where zircon may have precipitated from a hydrothermal fluid or a fluid-saturated residual melt. Temperatures for hydrothermal zircon formation range from 600°C in late-magmatic systems at the magmatic-to-hydrothermal transition down to 300°C in mesothermal ore-forming systems. Late-magmatic to hydrothermal zircon may precipitate from fluid-saturated magma and possibly from the fluids exsolved from mineralized granites and pegmatites. For example, in the Sn–Wmineralized Mole Granite, New South Wales, Australia, zircon occurs in growth zones in hydrothermal quartz, along with monazite, xenotime and thorite

Natural zircon crystals often show complex secondary textures that cut across primary growth zones. In zircon showing structural damage caused by self-irradiation, such textures are the result of a diffusion– reaction process in which a hydrous species diffuses inwards and “catalyzes” structural recovery. Nanoscale pores develop, solvent elements such as Ca, Al and Fe are gained, and radiogenic Pb is lost. In both aqueous fluids and melts, replacement of zircon with undamaged structure by a coupled dissolution– reprecipitation process can produce similar textures. The reacted domains usually have lower trace element contents and may contain micrometer-sized pores and inclusions of uranium, thorium and/or yttrium phases, originally in solid solution. Both processes have considerable implications for zircon geochronology.

Natural zircon crystals incorporate rare earth elements (REE) into their structure at concentrations determined by the pressure, temperature, and composition of their growth environment. In principle, REE concentrations in magmatic zircon crystals can be used to infer their conditions of growth and the composition of the melt from which they grew—provided accurate information is available about the distribution of REE between zircon and melt. Currently available zircon–melt partitioning data show a range in values covering several orders of magnitude for some REE. Further experimental work and studies using carefully selected natural samples are required to fully understand REE incorporation in zircon.

Zircon is of fundamental importance in the investigation of deeply subducted crustal rocks in which it is a trace constituent. Tiny mineral inclusions within zircon may be the only indicators that rocks were subducted to a depth of up to 150 km. Because zircon is resistant to physical and chemical changes, it preserves stages of the subduction and exhumation history within submillimetre-size grains. Advanced in situ techniques allow us to date zircon domains and to determine their trace element composition. We can thus acquire a detailed knowledge of the temperature–pressure–time paths that these extraordinary rocks have experienced. Zircon studies provide evidence that subduction and exhumation act at plate tectonic speeds of 1–3 cm/year.

Using the U–Pb geochronology of zircon we can understand the growth and collapse of mountain chains, both recent and ancient. In the hightemperature metamorphic rocks that underlie mountain ranges, zircon may survive from precursor rocks, recrystallize, or grow anew. All these possibilities must be considered in the interpretation of zircon ages. Microtextural characterisation and microanalysis, coupled with considerations of mineral equilibria and trace element distributions between zircon and neighbouring silicate minerals, provide insights into the factors controlling zircon modification and growth. Zircon ages do not usually correspond to the peak of metamorphism but instead provide information on the history of cooling from high temperatures, including the timing and rates of exhumation of the deep roots of mountain chains.