- © The Mineralogical Society Of America
The rates of mass transfer and ultimately the attainment of thermodynamic equilibrium in magmatic systems fundamentally depend on the intrinsic mobility and activity—composition relations for a magma’s constituents. The time scales on which chemical potential and isotopic differences are eliminated can be accounted for using various expressions/forms of the diffusion coefficients that are governed by the identity of the constituents and the nature of the gradients driving mass transfer.
In broadest terms we can distinguish between two categories of diffusion—chemical diffusion and self-diffusion. Chemical diffusion is the local and directional transfer of mass in response to spatial differences in chemical potential, whether arising from differences in the concentration of the constituent itself and/or in other components of the system. [Chemical diffusion may, and usually does, involve diffusion of other components required locally to conserve mass, volume, moles, charge balance, or density depending on one’s choice of reference frame (see Brady 1975 for discussion of reference frames).] Non-ideal mixing exerts direct control on chemical diffusion behavior that can lead to diffusion towards higher, rather than lower, concentration. “Uphill diffusion” can also occur in ideal solutions with more than two components due to diffusive coupling effects.
Self-diffusion, the chief subject of this chapter, sensu stricto is a measure of the random walk of constituents in the absence of chemical gradients. In principle this applies to any random motion of constituents leading to permanent displacement of individual ions or molecular species. The path of random walk for a given species cannot be measured directly. Rather, in practice one tracks the net displacement of species by monitoring changes in the distribution of “tagged” constituents, often one or more isotopes for the element of interest, within an otherwise homogeneous substance. We often approach this task in the laboratory in one of two …