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In this chapter, an overview of the findings to date from laboratory measurements of diffusion of cations in the accessory mineral phases zircon, monazite, apatite, titanite (sphene) and xenotime is presented. These minerals differ widely from one another in structure and composition, but share some common characteristics. While they are generally low in modal abundance, they tend to incorporate elements that are important tracers of geochemical processes (e.g., the rare earth and high-field strength elements) as well as U and/or Th, which make them valuable as geochronometers. Two other phases commonly considered “accessory,” rutile and baddeleyite, are discussed in other chapters (Farver 2010; Van Orman and Crispin 2010), and a third, allanite, is not considered in this review because no diffusion data exist. Since other chapters in this volume review diffusion of the noble gases (Baxter 2010, this volume) and oxygen (Farver 2010, this volume) in minerals, diffusion of these elements is not discussed in detail here.
Because of the importance of these phases in geochronometry and trace-element budgets in geologic systems, attempts have been made for nearly four decades to measure diffusion (especially of Pb) in these accessory minerals. While diffusivities of certain elements in some of these minerals can be relatively fast, in most cases diffusivities in these phases are comparatively slow, requiring analytical methods capable of high depth resolution. In the last two decades or so profiling techniques with better depth resolution have been employed, resulting in much new diffusion data for accessory minerals. These diffusion findings have important implications for isotopic dating, closure temperatures, geothermometry, and formation and preservation of primary chemical composition and zoning. Several of these mineral phases have also been proposed as materials for nuclear waste storage, where reliable diffusion data are critical in order to evaluate their long-term stability …