Multicomponent diffusion in silicate melts: SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O System

Abstract

Nine successful diffusion couple experiments were carried out in a 7-component haplobasaltic silicate melt SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O system to study multicomponent diffusion at ∼1500°C and 1GPa, typically with compositional gradients in only two components in each experiment. At least two concentration traverses were measured for each experiment. Effective binary diffusion coefficients (EBDC) for monotonic profiles were obtained by an error function fit, and the EBDC of a given component is dependent on its counter diffusing component, especially for SiO2. The EBDC’s of SiO2 vary from 15.7μm2/s when diffusing against Al2O3, to 102.9μm2/s when diffusing against K2O. Furthermore, the multicomponent diffusion matrix was obtained by simultaneously fitting profiles of all oxides in all experiments. Most features in the diffusion profiles, for example uphill diffusion, are captured well by this 6×6 diffusion matrix. The slowest diffusing eigenvector is largely due to the exchange between Si and Al, and the fastest diffusing eigenvector is the exchange of Na with all other components. An anorthite dissolution experiment was also conducted to test whether the diffusion matrix can be applied to mineral dissolution experiments. The calculated diffusion profiles in the melt during anorthite dissolution roughly match the measured profiles, demonstrating the validity and utility of the diffusion matrix in this FeO-free aluminosilicate melt system.