Experimental Determination of Mechanisms and Rates of Fe-Mg Exchange Between Spinel Grains Mediated by a Fluid Phase

The overall mechanism and kinetics of mineral reactions results from a complex interaction of several processes such as surface reaction kinetics, volume diffusion and net transfer. In order to quantify the kinetics of reactions involving multiple phases in multicomponent systems, it is necessary to understand and characterize the nature and rates of each of these processes. Most laboratory experiments up to now have focused on kinetics of reactions where the reactants and products are in direct physical contact with each other. However, there is abundant textural evidence in rocks that reactions occurred between mineral grains that are physically separated from each other, frequently mediated by a fluid phase. We have devised an experimental setup to study the mechanism and kinetics of such reactions in the laboratory. Polished single crystals of two spinels (synthetic MgAl2O4 and a natural spinel with 44 mol% Hercynite component), 2mm on a side, were placed in a gold capsule (length: 2cm, diameter: 4mm) separated from each other by a 5mm long tube of Au or alumina. The capsule was welded shut after adding distilled water (80-100μl). Such capsules were annealed (2 Kbar, 700-750°C, up to 21 hours) in hydrothermal cold seal vessels. After annealing the crystals were cleaned in an ultrasonic bath in order to rinse them of possible quench products. The surfaces were examined optically and near surface chemistry was determined using Rutherford Backscattering Spectroscopy (RBS). We observe time dependent changes in the morphology as well as the chemistry of the crystals, as follows: After short times, the surface of the Mg spinel shows scattered etch pits while terraces form on the Fe spinel. After longer anneals, the etch pits disappear and the surface of the Mg spinels appear polished. Surface compositions are found to be different, depending on whether a Au or alumina separator was used in the experiments. The Fe rich spinel composition remains unchanged whereas the Mg spinel shows erratic gains in Fe (He1-30) when a Au separator is used. The results are more systematic with an alumina separator and the surfaces of both spinels show the same equilibrated compositions (He20-22). No concentration - depth profile could be resolved in the Fe rich spinel, whereas systematic diffusion profiles develop in the Mg rich spinel. Fits to these concentration profiles yield Fe-Mg diffusion coefficients of about 5e-19 m2/s (experiments with Au-separator) and 2e-18 m2/s (alumina separator), respectively, at 750°C. We interpret the difference in behavior between the Au-separator and the alumina-separator experiments to result from a more controlled activity of Al2O3 in the fluid in the latter experiments. Based on these observations we conclude: (i) The initial fluid that is out of equilibrium with both spinels attacks the surface of both but in different manners - forming etch pits in one case and terraces in the other. (ii) Once a steady state concentration distribution is attained in the fluid and it is in local equilibrium with each crystal, further element exchange appears to operate by diffusion in the Mg spinel and surface reaction in the Fe spinel. (iii) The diffusion profile that develops in the Mg spinel is a measure of reaction progress and allows characterization of the kinetics of the overall reaction. While more data are required to fully characterize this particular system, the success of this set up opens the possibility of exploring the kinetics and mechanism of various exchange and net transfer reactions.