Abstract
We have measured multicomponent chemical diffusion coefficients in a melt near to the low pressure water-saturated eutectic granite composition in the system K 2 O-Na 2 O-Al 2 O 3 -SiO 2 -H 2 O at 1.0 GPa and temperatures of 1300 and 1600 degrees C. The measured diffusion profiles can be accounted for within the analytical error by diffusion coefficients, which are not dependent on composition within the range of compositions accessed by our experiments. The multicomponent diffusion coefficient matrix [D] has a highly degenerate set of real, positive eigenvalues that show a regular relation to melt viscosity on an Arrhenius diagram. The smallest eigenvalue is that associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated with Si-Na and Si-K exchange and are effectively degenerate, with the result that exchanges of alkalis for silica or for each other can proceed in pseudo-binary fashion without inducing fluxes of other components. The eigenvalue associated with H-Si exchange is smaller than the alkali-silica eigenvalues, but analytical uncertainties make it also effectively degenerate with the alkalis. Uphill diffusion, notably of water and alkalis, was observed in several experiments, and this would lead to transient partitioning of water and alkalis across diffusion interfaces showing large Al 2 O 3 concentration gradients. Such partitioning in natural systems would persist until Al concentration gradients were erased by continued, much slower Al-Si interdiffusion.
