Multicomponent Diffusion in Basaltic Melts

Abstract

This dissertation focuses on understanding multicomponent diffusion of major elements in basaltic melts. Diffusion in simpler 7-component FeO-free haplobasaltic melts was first investigated at a single temperature to establish the method and approach that a diffusion matrix can reproduce all features in multicomponent diffusion. After the success in the simpler melts, diffusion in realistic basaltic melts at various temperatures was studied and diffusion matrices were obtained and applied to predict mineral dissolution.

Nine successful diffusion couple experiments were carried out in a 7-component SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O system at ~1500 ºC and 1 GPa, to study multicomponent diffusion in haplobasaltic melts, 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 of SiO2 varies from 15.7 µm2/s when diffusing against Al2O3, to 102.9 µm2/s when diffusing against K2O. Furthermore, the multicomponent diffusion matrix was obtained by simultaneously fitting all diffusion profiles in all experiments. All 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 SiO2 and Al2O3, and the fastest diffusing eigenvector is the exchange of Na2O 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.

Twenty seven successful diffusion couple experiments were conducted in an 8-component SiO2–TiO2–Al2O3–FeO–MgO–CaO–Na2O–K2O system at ~1260 ºC and 0.5 GPa, at ~1350 ºC and 1 GPa and at ~1500 ºC and at 1 GPa, to study multicomponent diffusion in basaltic melts. At least 3 concentration traverses were measured to obtain diffusion profiles for each experiment. Multicomponent diffusion matrices at 1260, 1350 and 1500 ºC were obtained by simultaneously fitting diffusion profiles of diffusion couple experiments. Furthermore, in order to better constrain the diffusion matrix and reconcile mineral dissolution data, mineral dissolution experiments in the literature, in addition to diffusion couple experiments from this study, were also fit to obtain a new diffusion matrix. All features of diffusion profiles in both diffusion couple and mineral dissolution experiments were well reproduced by this new diffusion matrix. Diffusion mechanism at each temperature is inferred from eigenvectors of diffusion matrix, and it shows that both eigenvectors of diffusion matrix and inferred diffusion mechanism in basaltic melts are insensitive to temperature. The diffusive exchange between network-formers SiO2 and Al2O3 is the slowest and the diffusive exchange of Na2O with all other components is the fastest, which are consistent with those for simpler systems in most literature. Temperature dependence of diffusion matrix is examined by assuming eigenvectors to be independent of temperature and eigenvalues to follow Arrhenius relation. Diffusion matrix at other temperatures can be calculated, and is successfully applied to predict diffusion profiles during olivine and anorthite dissolution in basaltic melts at ~1400 ºC.