Argon diffusion and its use to determine thermal histories

A full exposition of diffusion theory and diffusion mechanisms is beyond the scope of this chapter but the reader is referred to McDougall and Harrison (1999). Many observations show that argon diffusion rates in natural and laboratory experiments follow an Arrhenius relationship. There are however, important departures where fast track diffusion dominates in nature (Kramar et al. 2001 Reddy et al. 2001b) and laboratory analysis of hydrous minerals (Gaber et al. 1988 Lee 1993 Lo et al. 2000). Even in natural cases which might earlier have been identified as volume diffusion effects, careful compositional control shows that phase mixing can mimic argon loss profiles (e g., Onstott and Peacock 1987 Wartho 1995). In such cases the data can not easily be inverted to produce thermal histories. 

Having considered cases in which lattice or volume diffusion does not explain the data, it should be noted that lattice diffusion seems to dominate many natural systems, in argon and other noble gases (cf Farley 2002, this volume). Lattice diffusion follows a second order diffusion mechanism (McDougall and Harrison 1999) with an Arrhenius relationship given by the equation  

The discussion above considered only simple whole grain or sub-grain diffusion when lattice diffusion was the transporting mechanism. In recent years, the advent of laser analysis has demonstrated just how variable the actual within grain patterns can be (Fig. 5) (e.g., Hodges et al. 1994 Kelley and Turner 1991 Kramar et al. 2001 Lee et al. 

Lee (1995) has proposed a model for fast-track diffusion to explain the effects of combined lattice diffusion and diffusion along fast diffusion pathways through the lattice such as defects. Lee proposed that the combined diffusion could be modeled as two parallel diffusion mechanisms with argon atoms partitioning between the two. The mathematical model produces realistic release patterns, but does not currently take account of the distances between fast track pathways and the time taken for atoms to reach one (cf. Arnaud and Kelley 1995). Future development of the fast track model may provide very fruitful avenues for research. 

---