- © The Mineralogical Society Of America
A number of features of zircon (ZrSiO4), including high U-Th concentrations, high abundance in a wide range of lithologies, refractory nature under metamorphic and some magmatic conditions, and resistance to physical and chemical weathering, make it highly suitable for geochronology and thermochronology and thus a versatile tool for examining a wide range of earth processes. Like apatite and many other minerals, radioisotopic dating of zircon was first performed using the (U-Th)/He system, but the thermochronologic significance of zircon He ages has emerged only in the last few years. In this chapter, I review the current status of zircon He dating in the earth sciences, primarily as applied to thermochronology, including the controls on He diffusivity, the role of radiation damage, analytical techniques for measuring zircon He ages, special considerations unique to zircon He dating, and a series of case studies. Several examples from the literature are briefly summarized to illustrate the diversity of geologic problems accessible by zircon He dating and highlight the future potential of the system and outstanding unresolved issues. Exemplary applications include determining the timing and rates of orogenic exhumation and constraining provenance, depositional ages, and source terrain histories using He-Pb double dating of detrital zircons.
Previous geo- and thermochronometric studies of zircon have utilized a wide range of decay schemes, including Pb-α (e.g., Webber et al. 1956), U/Pb, Pb/Pb, Th/Pb (Larsen et al. 1952; Vinogradov et al. 1952; Tilton et al. 1955; Wetherill 1955; Silver and Deutsch 1963; Parrish and Noble 2003; Ireland and Williams 2003; Bowring and Schmitz 2003), U-series (Scharer 1984; Reid et al. 1997), fission-track (Naeser et al. 1981; Brandon and Vance 1992; Bernet and Garver 2005; Tagami 2005), Lu/Hf (in concert with other phases; e.g., Pettingill and Patchett 1981), Sm/Nd …