Quantitative research into the thermodyamics of silicate magmatic systems has traditionally been heavily biased toward measurements intended to represent conditions of equilibrium. Considerable progress has been made toward understanding and predicting equilibrium states at a wide range of temperatures and pressures over wide compositional ranges. However the corresponding study of the irreversible processes through which perturbed systems attain equilibrium has lagged far behind. Until now, no experimental study has successfully permitted a quantitative thermodynamic treatment of chemical diffusion in a multicomponent melt of direct geological relevance.
We have described chemical diffusion processes in a five-component base composition near to the 2 kbar water-saturated eutectic melt composition in the haplogranite system, at temperatures of 1300°C and 1600°C. We observe no effects in our experiments that cannot be accounted for by the standard treatment of multicomponent diffusion in the context of irreversible thermodynamics. Salient observations we have made are a) that the eigenvectors of the diffusion coefficient matrix are constant over wide ranges of temperature and water content, and b) that the eigenvalues of the diffusion coefficient matrix show a very regular dependence on viscosity. These two observations together suggest that it should be possible to describe multicomponent diffusion in a very wide range of melt types under a similarly wide range of conditions knowing only the viscosity of the melt in question.