Chapter 2. New avenues in experimentation on diffusion-controlled mineral reactions
R. Milke, W. Heinrich, L. Götze and S. Schorr
Mineral coronas and reaction rims are frequent features in many metamorphic rocks commonly interpreted as having been controlled by solid-state diffusion or by diffusion in an undefined medium. In material science the term ‘interlayer growth’ is often used for such processes. However, the terms ‘reaction bands’, ‘corona structures’, or ‘rim structures’ are commonly used as descriptive terms in petrology and we will use these below.
The formation of reaction bands, coronas and reaction rims between incompatible phases requires that one or more chemical components are mobile. Knowledge of the transport mechanism and relative mobility of the distinct chemical components is of prime interest for the interpretation of reaction band sequences, growth rates, and subsequently forming fabrics and textures. Recent experimentation at high pressures and temperatures has shown that even minute traces of water are decisive in changing reaction mechanisms in silicate systems. In this chapter we present avenues of experimentation classified as water-rich, waterpoor and water-absent. All of them are important in order to gain deeper understanding of mineral-reaction kinetics in the Earth in different environments, and of the formation of texture and zoning patterns in rock assemblages of the Earth or Earth-like planets.
Experimental simulation and parameterization of variables relevant for mineral reaction kinetics and the development of microstructures and textures requires a specific experimental approach. This consists of (1) application of time (or temperature) series; (2) miniaturization of experimental setups; and (3) experiments with perfectly defined geometries of the phases involved. This chapter specifies experimental setups for investigating mineral-reaction kinetics at high P-T, including setups for time-resolved, real-time monitoring of mineral nucleation and growth between incompatible phases at high T. We address mainly the experimental strategies in elucidating the controls of mineral-reaction kinetics rather than responding to results of particular studies, which can be found in many other chapters in this volume and in previous review articles (e.g. Dohmen and Milke, 2010;Watson and Dohmen, 2010). This holds also for the applied analytical methods as many experimental setups are specifically designed for subsequent analytical procedures.
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