Experimental Studies of Diffusion in Olivine

Abstract

Knowledge of the rates and mechanisms of diffusion in olivine can be used, among other things, to elucidate timescales of magmatic processes, determine closure temperatures of radiogenic systems, quantify the robustness of melt inclusions and aid in development of point defect models. Firstly, however, the diffusion process itself must be well understood. This is only possible when diffusion studies are closely coupled with considerations of equilibrium thermodynamics. In this thesis, the diffusion of Mg2+, Be2+, Sc3+, Zr4+, Hf4+, Cr2+, Cr3+, Ti3+, Ti4+ and H+ in olivine are presented at various temperatures between 900-1600 °C, and pressures from 1 bar to 2.5 GPa under controlled oxygen fugacity, silica activity and crystal orientation. The interface concentration (at the crystal edge) of trace element diffusants should be at equilibrium with external conditions (such as silica activity, oxygen fugacity, temperature and pressure). This assumption must be satisfied in order to draw any conclusions regarding diffusion rates. The silica activity of a system affects both the concentration and diffusion rate of trace elements in olivine, with all cations (except H+ and Be2+) diffusing faster at high silica activity than when silica activity is low. Oxygen fugacity affects the diffusion rate and interface concentrations of Ti and Cr – these both (generally) diffuse faster and at higher concentrations in more reducing conditions, where Cr2+ and Ti3+ are more prevalent. The concentration of trivalent cations in olivine can affect their diffusion rate – the rate of Sc3+ diffusion is positively correlated with its interface concentration. The ionic radius of cations that substitute onto the M sites in olivine strongly affect the diffusive anisotropy (the difference in diffusion coefficients between the fastest and slowest axes), relating to preferential ordering onto the M1 sites, which are aligned into closely spaced chains along one crystallographic axis. There is no apparent causal relationship between ionic charge and diffusivity of M-site cations in olivine. More important are atomic mass, site preference and ionic radii. Taken together, the results from these studies show firstly that diffusion in olivine is more complex than previously thought, and secondly that the effect of thermodynamic variables must be considered when studying diffusion.