The effect of fluorine on reaction-rim growth dynamics in the ternary CaO-MgO-SiO2 system

Growth of reaction rims is mainly controlled by a change in physical parameters such as pressure and temperature, a change in the chemical composition of the system, and/or by the presence of volatiles. In particular, the effect of volatiles other than water on reaction-rim growth remains poorly understood. To accurately model metamorphic and metasomatic processes, a quantification of the efect of volatiles on reaction-rim growth dynamics is necessary but hitherto missing. In this study, reaction rims were experimentally grown in a series of piston-cylinder experiments in the ternary CaO-MgO-SiO2 system at 1000 degrees C and 1.5 GPa with 0-10 wt% F for 20 min. In the fluorine-free system, a rim sequence of wollastonite (Wo) vertical bar merwinite (Mer) vertical bar diopside (Di)vertical bar forsterite (Fo)vertical bar periclase (Per) formed, complying with the stable phase configuration at water-saturated conditions. As soon as 0.1 wt% F was introduced into the system, humite group minerals (HGMs) and monticellite (Mtc) appeared, resulting in the multilayer rim sequence Wo vertical bar Mer vertical bar Mtc vertical bar Fo + HGMs vertical bar Per. In experiments with fluorine concentrations >= 0.5 wt%, cuspidine (Csp) appears in the layer sequence and represents the major fluorine sink. Our data show that the addition of fluorine may stabilize the fluorine-bearing phases cuspidine and HGMs to higher temperatures, which is in agreement with previous studies (Grutzner et al. 2017). However, the appearance of the nominally anhydrous minerals (NAMs) monticellite and akermanite (Ak) at this P-T condition suggests that the addition of fluorine may also afect the stability of nominally fluorine-free minerals. This may be explained by the efect of fluorine on the Gibbs free energies of fluorine-bearing phases, which in turn afects the relative Gibbs free energies and thus the stabilities of all phases. An increase in absolute rim thickness from 11.8(21) to 105.6(22) mu m (1 sigma standard deviations in parentheses) in fluorine free and 10 wt% F experiments, respectively, suggests that fluorine enhances absolute component mobilities and thus results in faster rim growth rates. Additionally, due to the presence of fluorine, a change in relative component mobilities results in microstructural changes such as a phase segregation of diopside and cuspidine at high-fluorine (>= 3 wt% F) concentrations. These results not only imply that reaction rims may be used as a tool to infer the amount of fluorine present during metamorphic reactions but also that we need to consider the role of fluorine for a correct interpretation of the P-T-t history of metamorphic and metasomatic rocks.

Automated summary

The growth of reaction rims is influenced by physical parameters, chemical composition, and the presence of volatiles. This study investigates the effect of fluorine on the growth dynamics of reaction rims in the CaO-MgO-SiO2 system. The addition of fluorine not only affects the sequence of minerals in the reaction rim but also enhances component mobilities and leads to faster rim growth rates. These findings have implications for understanding the role of fluorine in metamorphic and metasomatic processes.