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

Abstract

AbstractGrowth 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 effect 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 °C and 1.5 GPa with 0–10 wt% F for 20 min. In the fluorine-free system, a rim sequence of wollastonite (Wo) | merwinite (Mer) | diopside (Di) | forsterite (Fo) | 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 | Mer | Mtc | Fo + HGMs | 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 (Grützner et al. 2017). However, the appearance of the nominally anhydrous minerals (NAMs) monticellite and åkermanite (Ak) at this P-T condition suggests that the addition of fluorine may also affect the stability of nominally fluorine-free minerals. This may be explained by the effect of fluorine on the Gibbs free energies of fluorine-bearing phases, which in turn affects 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) µm (1σ 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.