,- University of Innsbruck, Institute of Mineralogy and Petrography, Innsbruck, Austria (Clara.Nikolaizig@student.uibk.ac.at)
An increase in fluorine concentration significantly enhances rim growth rates between periclase and wollastonite, as previously reported by Franke and Joachim-Mrosko (2022). However, the mechanisms responsible for this acceleration, as well as the quantitative relationship between volatile content and rim growth dynamics, remain only partially understood. A more detailed understanding of these processes is critical for our understanding of the role of volatiles in metamorphic and metasomatic reactions.
To address this, we conducted high-p-T experiments at 1000 °C and 1.5 GPa for 20 minutes, with fluorine contents ranging from 1 to 2 wt%. For example, at 1 wt% F, rim growth reached an overall thickness of approximately 50 µm. Our results indicate a stagnating reaction rate between 1-3 wt% F. This leads to rim growth dynamics being divided into three distinct regimes:
In regime 1 (0-1wt% F) fluorine gradually replaces OH at grain boundaries within the reaction rim leading to increasing reaction rates. This also leads to fluorine accumulating at the rim-wollastonite boundary. In regime 2 (1-3wt% F) fluorine has almost completely replaced OH leading to further fluorine addition not affecting reaction rates. In regime 3 (>3wt% F) increasing rim growth rates can be explained by microstructural changes from a mosaic to a lamellar structure, as well as the formation of a pore network.
This implies that the quantitative effect of fluorine on elemental mobility along grain and phase boundaries in a reaction rim is a more complicated relationship, whose rate and influence can be dependent on multiple parameters such as chemical composition, temperature and pressure. Thus, specifically designed experiments are required to use reaction rims in natural systems as geofluidometers.
References:
Franke, M. G., Joachim-Mrosko, B. (2022). The effect of fluorine on reaction-rim growth dynamics in the ternary CaO-MgO-SiO2 system. American Mineralogist, 107(8), 1477–1486
How to cite: Nikolaizig, C., Weber, L., and Joachim-Mrosko, B.: The effect of fluorine on rim growth dynamics in the ternary CaO-MgO-SiO₂ system, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3866, https://doi.org/10.5194/egusphere-egu26-3866, 2026.
