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Elemental and isotopic separation by diffusion in geological liquids


Chemical speciation in molten silicates is of broad interest for understanding thermodynamic and transport properties. However, it is difficult to elicit the nature of dissolved species in silicate liquids, let alone the role that speciation plays in the diffusion process. This dissertation is about chemical diffusion in silicate melts; it combines experiments with isotopic measurements to infer the physical mechanisms of cation diffusion and the role that diffusion plays in producing isotope effects in nature.

Diffusion of calcium (Ca) between natural volcanic liquids in diffusion-couple experiments leads to 44Ca/40Ca variations of ca. 5‰ due to a mass dependence of Ca diffusion coefficients. The efficiency of Ca isotope separation by diffusion varies with bulk liquid composition and depends also on the magnitude and direction of aluminum (Al) gradients. Some Ca isotopic fractionations seem to arise solely from gradients in Al. These observations indicate that isotopic discrimination by diffusion reflects the mass difference between isotopically-substituted molecular species, and there is evidence for at least one Ca-bearing and one Ca-Al-bearing diffusing species in the volcanic liquids that were studied. The inferred existence of a Ca-Al complex is consistent with the additional observation that Ca diffuses slowly since it is inferred that Ca atoms interact strongly with their nearby Si and Al "solvent" molecules in the liquid.

The third part of this thesis describes Ca diffusion between silicate liquids of simplified chemical composition. Results from these experiments indicate that the efficiency of diffusive separation of Ca isotopes is systematically related to the normalized diffusivity - the ratio of the mobility of the cation DCa to the mobility of the liquid matrix DSi. A similar dependence is observed for Fe, Li, and Mg isotopes, and in aqueous solutions. This empirical result provides a predictive tool that can be used to understand diffusive isotopic effects in a wide variety of geologic environments and a basis for a more comprehensive theory of isotope separation in liquid solutions. A conceptual model is presented for the relationship between diffusivity and liquid structure that is consistent with available data.

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