Importance of grain boundary diffusion : an experimental study
Hiscock, Matthew John
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This research is concerned with the mechanisms of diffusion in the Earth and the implications of such an understanding. Specifically, this work is concerned with one particular aspect of diffusion: Grain Boundary Diffusion (GBD). An experimental investigation of GBD has been conducted by considering three specific scenarios; GBD of H in stoichiometric Mg-spinel, GBD of Ti in Quartz and GBD of Li in olivine. By considering the GBD of three very different elements it has been possible to synthesise an understanding of some of the mechanisms involved in the process. GBD is potentially a very important process within the Earth with wide ranging implications. Grain boundaries may provide fast pathways for transportation of a range of compatible and incompatible diffusing species in the Earth’s interior – potentially acting as storage locations and also as efficient pathways between different geological reservoirs. It is also potentially very important in the application of a number of techniques including dating and geothermometry and geobarometry. Here, an experimental study of the GBD of H has been carried out with the overall finding that GBD appears to occur at slightly greater yet broadly similar rates to lattice diffusion. This finding is considered in terms of the mantle properties which are affected by the presence and transport of H. A follow up series of experiments was conducted looking at Li diffusion. Li was chosen due to its volatile nature and larger atomic radius as compared to H. As such, it provided a useful test of the hypothesis that the radius of a diffusant might affect its chosen method of diffusion. A third set of experiments were carried out to investigate the GBD of Ti in quartz with particular reference to the TitaniQ geothermo(baro)meter. This set of experiments provided a very useful comparison to the data which had previously been obtained from lighter elements. This investigation has found that a combination of factors including charge, diffusant diameter and the specific mineralogical characteristics of the host phase will define the dominant diffusive mechanism and the size of the contribution made by that mechanism towards observed bulk diffusivities. A characterisation of the temperature dependency of diffusion within each setting has also been completed. As such, it also makes a useful contribution to the current dataset for GBD.