- © The Mineralogical Society Of America
Despite its low abundance in most rocks, zircon is extraordinarily useful in interpreting crustal histories. The importance of U-Th-Pb isotopic dating of zircon is well and long established (Davis et al., this volume; Parrish et al., this volume). Zircon also tends to incorporate trace elements useful as geochemical tracers, such as the REE, Y, and Hf. A number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations, often on extremely fine scale, which provide valuable insight into thermal histories and past geochemical environments. The relative insolubility of zircon in crustal melts and fluids, as well as its general resistance to chemical and physical breakdown, often result in the existence of several generations of geochemical information in a single zircon grain. The fact that this information is so frequently retained (as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine-scale zoning down to the sub-micron scale) has long suggested that diffusion of most elements is quite sluggish in zircon.
In this chapter, we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon. Because of its importance as a geochronometer, attempts have been made to measure diffusion (especially of Pb) for over 30 years. But only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies, resulting in a plethora of new diffusion data. These findings have important implications for isotopic dating, interpretation of stable-isotope ratios, closure temperatures, and formation and preservation of primary chemical composition and zoning in zircon.
HISTORY—A BRIEF REVIEW OF BULK-RELEASE AND EARLY LOWER-RESOLUTION DIFFUSION MEASUREMENTS
Efforts have been made for some time to quantify and characterize diffusion in zircon, most notably of Pb, in deference to its significance in interpreting Pb isotopic signatures and refining understanding of thermal histories. As is evident from …