The effect of volatiles on reaction rim growth dynamics

Reaction rims are the result of an incomplete reaction of two neighboring grains during a metamorphic reaction. Parameters that strongly affect the development of reaction rims are for example pressure, temperature, time or the chemistry of the system. This implies that we have the chance to determine the formation conditions of natural reaction rim structures, if we understand the effect of each parameter on reaction rim development. Recent research studies showed that the availability of volatiles is one of the most important controlling factors that affect reaction rim growth. For example, presence or absence of water may strongly affect growth rates, thickness ratios in multilayered rims or even the layer sequence. In this study we plan to quantify the effect of water on reaction rim growth dynamics. The addition of very small amounts of water (concentration: few parts per million) to experimental samples is a challenging task in laboratory experiments. We plan to do this by incorporating defined amounts of OH-defects in the crystal structure of synthetic starting materials. During a rim growth reaction, these starting materials will be consumed and water will be released. The longer the experimental duration lasts, the more water will be released. Experiments will be performed at constant pressure- temperature conditions in simplified, well-known chemical systems (MgO-SiO2 and CaO-MgO-SiO2). This will allow us to exclude all other parameters that might affect rim growth dynamics and quantify solely the effect of water on component mobilities, overall rim growth rates, relative thickness ratios in multilayer rim sequences and the layer sequences in a multilayered rim. Thus, based on the results of this project, we will evaluate the potential to use reaction rims as geohygrometer in metamorphic systems. As stated above, laboratory experiments are often performed using simplified setups to reduce the number of potential parameters that may affect a process such as reaction rim growth, which allows us to investigate the individual effect of each parameter separately. For that reason, pure water is used as fluid representative in the first part of this project. However, fluids in natural systems contain not only water but several other volatile elements such as nitrogen, carbon, fluorine, chlorine, bromine or iodine. All these can have a strong effect on reaction rim growth processes. To make a first step toward bridging the gap between experiments and natural systems, we will investigate in this project the effect of fluorine as additional volatile component on rim growth dynamics. This will also allow us to investigate, if reaction rims can also be used ad geo-fluido-meter that may allow us to determine the fluid composition during metamorphic events.

Metamorphic reaction rims form during the incomplete solid-solid reaction of two neighboring minerals. Parameters that significantly influence the formation of reaction rims are, for example, pressure, temperature, the duration of the reaction or the chemistry of the system. Consequently, naturally occurring reaction rims contain information about their respective formation conditions. However, in order to infer these, we need detailed information about the exact effect of each parameter on the reaction rim growth dynamics. Numerous scientific studies have already shown that small amounts of volatiles such as H2O can have a significant influence on the formation and growth of reaction rims. In this project, we investigated the effect of very small amounts of water on the formation of reaction rims. However, the addition of extremely small amounts of water (order of magnitude: 0.001 wt%) to high-pressure experiments posed a major experimental challenge, as a dried powder already adsorbs water on the surface of the grains in such a large amount that we need to refer to the system as "wet". Therefore, in order to add extremely small amounts of water in a controlled manner to an experiment, synthetic periclase single crystals experiments that were "doped" with minute amounts of OH-defects were used as starting materials for rim growth. During an experiment, these "doped" crystals release their structurally bound water, which can then affect as "active water" the growth of reaction rims. With this method, it could be shown experimentally that presence of ppm-amounts of water can be a necessary catalyst that is required to initiate rim growth. Furthermore, an existing model was revised, which now shows how smallest amounts of "active water" effect the growth rate and the thickness ratio of individual layers in a reaction rim. This implies that reaction rims have the potential to be used as geohygrometers, which means that they can provide information about the presence of minute amounts of water that were present long ago during a metamorphic reaction. However, natural fluids contain numerous other volatile elements, such as nitrogen, carbon, fluorine, chlorine, bromine or iodine. All these elements can also have a significant effect on the formation and growth dynamics of metamorphic reaction rims. In the second part of the project, we investigated experimentally the effect of fluorine on the formation and growth of reaction rims. Results showed that presence of fluorine may strongly increase the growth rate of reaction rims and can have an influence on the phase stability and thus the sequence and texture of the individual layers in a multilayered reaction rim sequence. This means that reaction rims can also provide information about the composition of complex fluids that were present during metamorphism and can consequently be used as geofluidometers.