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...

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