MULTICOMPONENT DIFFUSION IN THE MOLTEN SYSTEM K2O-NA2O-AL2O3-SIO2-H2O

We have measured multicomponent chemical diffusion coefficients in a m elt near to the low pressure water-saturated eutectic granite composit ion in the system K2O-Na2O-Al2O3-SiO2-H2O at 1.0 GPa and temperatures of 1300 and 1600 degrees C. The measured diffusion profiles can be acc ounted for within the analytical error by diffusion coefficients, whic h are not dependent on composition within the range of compositions ac cessed by our experiments. The multicomponent diffusion coefficient ma trix [D] has a highly degenerate set of real, positive eigenvalues tha t show a regular relation to melt viscosity on an Arrhenius diagram. T he smallest eigenvalue is that associated predominantly with Si-Al exc hange. The larger two eigenvalues are those associated with Si-Na and Si-K exchange and are effectively degenerate, with the result that exc hanges of alkalis for silica or for each other can proceed in pseudo-b inary fashion without inducing fluxes of other components. The eigenva lue associated with H-Si exchange is smaller than the alkali-silica ei genvalues, but analytical uncertainties make it also effectively degen erate with the alkalis, Uphill diffusion, notably of water and alkalis , was observed in several experiments, and this would lead to transien t partitioning of water and alkalis across diffusion interfaces showin g large Al2O3 concentration gradients. Such partitioning in natural sy stems would persist until Al concentration gradients were erased by co ntinued, much slower AI-Si interdiffusion.