- © The Mineralogical Society Of America
The data summarized in this RiMG volume reflect the growing interest among geoscientists in the characterization of atomic transport in natural materials. Interest in diffusion dates back to the early 20th century (e.g., Bowen 1921), but the need for constraints on mobility of atoms became especially important in the 1950s and 60s with our increasing reliance upon radioactive decay for determination of mineral and rock ages: the question of decay-product retention in relevant phases became vitally important. With a few notable exceptions, however, experimental geoscientists did not take up the cause of diffusion in solids until the 1970s and 80s. By that time, the motivation for acquiring diffusion data had spread to many areas of petrology, mineralogy and geochemistry, and experimental techniques began to blossom. The result is very much in evidence in this volume.
All experiments aimed at measuring a diffusion coefficient have two things in common: a technique to introduce the diffusant of interest to the sample, and a method to determine the extent of diffusion. Traditionally, diffusion experiments are designed to take advantage of an existing solution to the non-steady state diffusion equation, and experiments are conducted in such a way as to reproduce as well as possible the boundary conditions specified for the solution. For ease of analysis and interpretation, most experiments are set up to limit diffusion to just one dimension. Most researchers have further settled on one of three general experiment designs: 1) the interdiffusion couple; 2) the “thin-film” geometry; or 3) the “constant surface concentration” design (Fig. 1⇓). Noble-gas geochemists (including those interested in K/Ar, Ar/Ar and U-Th/He dating methods) have exploited 3-D variants of this last approach by conducting bulk degassing studies of minerals in which radiogenic noble gases have accumulated over time in the natural environment (Baxter 2010, …