Thermal Stability of F‐Rich Phlogopite and K‐Richterite During Partial Melting of Metasomatized Mantle Peridotite With Implications for Deep Earth Volatile Cycles

Phlogopite and K‐richterite constitute important carrier phases for H and F in Earth's lithosphere and mantle. The relative importance depends on their stabilities at high pressure and temperature, which in turn depends on bulk composition. Most previous experimental studies focused on the thermal stability of phlogopite and K‐richterite were conducted using simplified chemical compositions. Here, partial melting experiments on metasomatized and carbonated, OH ± F‐bearing near‐natural peridotite were performed at high pressures (2 and 5 GPa) and temperatures (1,100–1,350°C) to assess the thermal stability of F‐free versus F‐bearing phlogopite and K‐richterite. Experimental results demonstrate that the thermal stability of F‐bearing phlogopite is increased by >55°C/wt.% F, relative to F‐free phlogopite, whereas K‐richterite is absent in all experiments with significant degrees of melting (>2%). The thermal stability of phlogopite containing several wt.% F exceeds continental and oceanic geotherms within the upper 150 km. Fluorine‐rich phlogopite would therefore be stable in virtually all of the continental lithosphere, only to be decomposed during large, regional melting events such as continental break‐up, thereby acting as a major long‐term sink for F and/or H. This could even be the case for the oceanic asthenosphere, depending on the oceanic geotherm of the area of interest. Plain Language Summary Phlogopite and K‐richterite are important minerals for the fluorine and hydrogen cycles of the Earth's interior. The effects of fluorine on the thermal stability of phlogopite and K‐richterite was studied using a suite of high pressure, high temperature experiments, mimicking partial melting in Earth's upper mantle and lower crust. The experimental results show that F dramatically enhances the stability of phlogopite, implying phlogopite can retain F and H up to greater depths in the Earth than previously assumed. Fluorine does not significantly affect the thermal stability of K‐richterite. Given that F is more compatible in phlogopite compared to H, our results show that phlogopite becomes increasingly more stable through time as F is preferentially incorporated over OH during fluid alteration at high temperatures. Fluorine‐rich phlogopite may only be decomposed during large, regional melting events, such as continental break‐up, thereby acting as a long‐term carrier of F and to a lesser extent OH in the Earth's lower crust and/or upper mantle. Key Points Par