Depth Profiling of Geological Materials

Determining diffusion rates is critically importance for both metamorphic and igneous geochemistry. The ion probe, operated in depth profiling mode, is one of the most precise methods for experimentally determining such values. Typically, the mineral phase of interest will be polished and coated with film and the sample will then be heated under controlled conditions for a know length of time. Since the temperature and time (and possibly other parameters) are known from the experiment, then all that is required is that the diffusion distance of the migrating species be determined. Using a rastered primary ion beam the SIMS can determine relative variations in selected elements / isotopes as a function of time. By converting the time of analysis into depth of sputtering it is then possible to determine the relative variations of the target species as a function of depth. This conversion of time of sputtering into depth of the crater is achieved using a profilometer (see picture below). After an analysis is completed the sample is removed from the SIMS and the final depth of the sputter crater is determined. By assuming that the rate of erosion of the sample by the ion beam was constant, the cycle-by-cycle data from the ion probe can be translated into secondary ion intensity vs. depth below the sample's original surface. In theory, this technology will deliver a depth resolution of better than 5nm, though in practice this is seldom achieved with geological materials. The limiting factor in terms of depth resolution is the quality of the sample's surface polish. With surface roughness often on the order of 50 or 100nm it is necessary that the diffusion profile must extend into the sample no less than ~200nm.