The effect of water on diffusion-controlled reaction rim growth (TP3)

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Our recent high temperature - high pressure experiments and modelling of simple and complex reaction rim growth in the two component MgO-SiO2 (MS) and the three component CaO-MgO-SiO2 (CMS) systems have shown that rim growth is strongly affected by minute amounts of water present in the system. Traces of water may either be inherited from the reactants, particularly when mineral powders are used, or introduced via water diffusion from the pressure medium through the noble metal capsule into the reacting system in hydrothermal, piston-cylinder, and multianvil devices. Traces of water do not only affect the absolute growth rates, which can be faster by up to several orders of magnitude in „wet“ experiments than in “dry” experiments, but also the relative mobilities of the diffusing species/components, so that in the CMS system, for example, different rim sequences are produced for “wet” and “dry” setups. The driest conditions are achieved in IHPV single crystal experiments, whereas several “levels” of humidity were found in using piston cylinder experiments with different pressure media. We now have some qualitative understanding about the effect of water; quantitative information is, however, missing. We suggest a series of experiments with minute and clearly defined amounts of water present in both the MS and CMS systems to evaluate the dependence of the kinetic parameters such as nucleation, grain growth, diffusivity patterns, diffusivities, etc., on water fugacities. This will be done by loading the reactants with OH-defects and subsequently reacting them to grow rims in IHPV and piston-cylinder apparatuses. This issue calls for more detailed investigation because the effect of traces of water is of paramount importance for reaction kinetics in natural rocks. Whereas a large amount of recent work deals with the storage capacity and incorporation mechanisms of water into nominally anhydrous minerals including work on OH-dependent transformation kinetics of mantle phases, there is very little information about the effect of OH in NAMs on net-transfer reactions generally, and on reaction rim formation specifically. The project fills the space between the multitude of available data on OH incorporation into NAMs and their implications for reaction kinetics.