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The ability of crystalline zircon (ZrSiO4) to incorporate and retain trace element and isotopic information, due to sluggish diffusion, makes it an indispensable tool for geochemists and geochronologists (Valley, this volume; Cherniak and Watson, this volume) in deciphering the Earth’s geologic history. The stability of crystalline zircon over long periods of geologic time led researchers to prefer this mineral for geochronology (Heaman and Parrish 1991, Dickin 1995, Davis et al., this volume; Parrish and Noble, this volume). In fact, much of what is known about the timing of major geologic events has been accomplished through geochronology using U/Pb isotopic analyses of zircon (Froude et al. 1983, Bowring and Housch 1995, Buick et al. 1995, Bowring et al. 1998, Amelin et al. 1998, Amelin et al. 1999, Bowring and Williams 1999, Amelin et al. 2000, Mojzsis et al. 2001, Wilde et al. 2001, Peck et al. 2001, Bowring and Schmitz, this volume). Because of its durability, zircon has also been proposed as a potential storage material for weapons-grade plutonium from dismantled nuclear weapons (Ewing et al. 1995, Ewing 1999, Ewing et al., this volume).
In addressing the petrogenesis of granitoid rocks, a thorough understanding of the role of accessory minerals is crucial for constraining the distribution of trace elements and isotopes that are hosted mainly by those minerals. In particular, in granitoid rocks that were derived from melting of preexisting crustal sediments or rocks, it is often desirable to understand the role of Zr (and Hf), which is primarily concentrated in zircon. This includes determining whether the zircon crystals are single generation growth having crystallized in the magma in which they were most recently found. Or, if preexisting zircon was entrained (i.e., did not dissolve) in …