Diffusion of fluids through anhydrous minerals (quartz)

Knowledge of diffusion rates of water-related chemical and isotopic species in quartz is of importance because they can be used to reconstruct fluid-rock interactions and P-T-t histories of rock (fluid inclusion research). Moreover, quartz is a common mineral in the Earth`s crust and a relative stable mineral phase in a variety of geological environments. Diffusion of water and water-related species, such as D2 O, H2 18O, H2 , O2 through anhydrous quartz will be experimentally investigated in this study by analyzing the properties of fluid inclusions. Fluid inclusions with known fluid composition and density will be synthesized in anhydrous natural quartz. The fluid inclusions are synthesized in cylindrical cores that can be drilled in specific crystallographic orientations from the starting material. Micro-cracks are formed in those cores by rapid cooling after heating up to about 400 ?C. The fluid inclusions are synthesized by crack-healing at high experimental temperature and pressure in autoclaves with a confining hydrostatic pressure (argon). The experimental conditions can be selected within the a-quartz or ß- quartz stability field (max. 700&supo;C and 1 GPa). Those inclusions will be characterized by a detailed petrographical study (distribution and morphology), microthermometry and Raman spectroscopy. Subsequently, those inclusions will be re-equilibrated at high temperature and pressures in a fluid environment that differs from the included fluid. Re-equilibration conditions are selected at similar temperature-pressure conditions to minimize the effect of pressure gradients that could induce additional mechanisms of fluid inclusion alteration. The imposed gradients in chemical potential (or concentration) between fluid inclusions and the outside of the crystal will provoke diffusion of the specific fluid components. Any change in the properties of the original fluid inclusions will be detected and quantified mainly by microthermometry and Raman spectroscopy. A relationship between the length of the diffusion path (depth of inclusion below the crystal surface) and fluid inclusion properties will be expressed in a "concentration" profile along the crystal, that will be used to determine a three dimensional diffusion model and corresponding diffusion coefficients. A mathematical diffusion model will be developed according to the boundary conditions of the experiments as a specific solution of the Fick`s laws. Diffusion of fluid components in anhydrous minerals may occur through the "bulk"-crystal and it may be enhanced along crystal defects, such as dislocations, stacking faults and nano-cracks. The presence of those defects will be confirmed with transmission electron microscopy and will be correlated with specific anomalies in the estimated concentration profiles along the crystals. Additionally, re-equilibration experiments will also be performed with well-defined natural fluid inclusions from Alpine vein-quartz. The results of this study will contribute to the knowledge about the reliability of fluid inclusions as rigid containers that may conserve its fluid content throughout the geological history of the rock, one of the basic assumptions for the application of fluid inclusions studies. In addition, this study will provide new ideas and data about diffusion of fluid components through anhydrous minerals.

Diffusion of fluid components such as H2O through single crystals of quartz, i.e. a non-porous solid medium is difficult to imagine, but is a common process in rock at higher temperatures and pressures. Moreover, it is difficult to imagine that a piece of rock contains small amounts of fluids that is not present in pore space between crystals, but within crystals that are part of this rock. These entrapped fluids are called fluid inclusions. In some rock, the fluid content may be as large as 1 volume%, and it has a great impact on the properties of the rock. Fluid inclusions provide information about the condition and the environment of rock formation. Each type of rock that is found and sampled on the earth surface has a geological history, and fluid inclusions help to understand and to unravel this history. For this purpose, we need to analyse the fluid content of inclusions, i.e. the fluid composition and density, and these parameters help us to calculate temperature and pressure condition of formation of the rock. An important assumption is that fluid inclusions are rigid and inert objects, in other words, they do not modify their composition and density after they are formed at specific geological conditions. Modifications may lead to incorrect conclusions about the original trapping conditions of the fluid. You can easily imagine that a high fluid pressure inside inclusions may result in an explosion, where the surrounding quartz violently breaks, and the fluid content is completely lost. Diffusion of fluid through the crystal is another way that may modify the fluid inclusion properties. These processes do not play an important role in shallow rock, such as sedimentary rock, but have a large impact on fluid inclusions in deeper rock, such as metamorphic rock and plutonites. Fluid inclusions are in general composed of H2O, gases (CO2, CH4, N2, etc.) and salts (NaCl, KCl), and we have experimentally investigated if one or more of these components is able to move through quartz at higher temperatures and pressures. The experiments were performed at about 600 ?C and 336 MPa with synthetic and natural fluid inclusions. We applied a gradient in H2O concentration (fugacity) between the inclusions and the pore space outside the crystal at constant pressure, and we observed that H2O can diffuse through solid quartz crystals. We could determine a diffusion constant as obtained from concentration profile of inclusions at variable distance from the edge of the crystal. We also observed that under specific conditions CO2 can be mobile, whereas NaCl remains within the inclusions. The lower temperature limit of H2O diffusion was estimated at about 450 ?C at 336 MPa. Knowledge about the modification possibilities of fluid inclusions can be used to improve our understanding of natural fluid inclusions in metamorphic rock.