The effect of volatiles on reaction rim growth dynamics (2019-2023, FWF project P31787)

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.


Figure 1: Effect of fluorine on the reaction rim thickness, phase sequence and microstructure in two experiments performed at identical P-T-t-conditions.

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 simplified experiments and complex natural systems, we will determine 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.


Franke MG, Joachim-Mrosko B (2022) The effect of fluorine on reaction rim growth dynamics in the ternary CaO-MgO-SiO2 system. American Mineralogist, DOI:

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