Effect of Fluorine on the Stability of Dense Hydrous Mg Silicates With Implications for the Deep Water Cycle

Water is believed to be transported, at least locally, into the Earth's deep mantle via hydrous minerals. Fluorine not only tends to interact with water but also influences the stability of hydrous minerals. Thus, knowledge of its stability could be important in understanding the deep-water cycle. In this study, we conducted high-pressure experiments to investigate the stability of dense hydrous Mg silicates in the presence of fluorine at pressures ranging from 9 to 23 GPa. In the presence of even small amounts of fluorine, clinohumite or phase E survives up to approximately 17 GPa, allowing it to exist without dehydration at temperatures by about 200°C higher than a fluorine-free system. At pressures above 18 GPa, superhydrous phase B stabilizes as a major fluorine host. Fluorine partitions very little into phase D, leaving its stability field unchanged, but the stability field of superhydrous phase B extends to temperatures above 1400°C, thereby enabling the hydrous phase in the dragged wedge mantle just above the slab to have transport potential into the lower mantle.