Fluorine solubility in bridgmanite: A potential fluorine reservoir in the Earth's mantle

Abstract

Chondrite-normalized abundance of fluorine (F) in the bulk silicate Earth (BSE) is more abundant than the other heavy halogens. Recent studies suggested that the common nominally anhydrous minerals (NAMs) in the mantle can contribute to the bulk F content. In this study, the F solubility in bridgmanite was determined by high pressure experiments on the system MgO − SiO2 + MgF2 (±Al2O3 ± H2O) at 2073 K and 25 GPa to understand the major host for F in the lower mantle. Electron microprobe measurements showed that the F concentrations in Al-bearing bridgmanite occasionally exceed 1 wt%, while the F content in bridgmanite for the Al- and H2O-free system is below the detection limit (50 ppm). The F solubility in aluminous bridgmanite is higher than that in wadsleyite. The solubility of F in bridgmanite shows a strong compositional dependence, with a positive correlation between Al3+ and F−. In H2O-free system, the solubility of F in bridgmanite increases with increasing Al content, implying a coupled substitution of F− for O2− balanced by Al3+ for Si4+. Water enhances F solubility in bridgmanite, regardless of Al content, by adding a second substitution mechanism of F in bridgmanite. In water-bearing system, we observe a coupled substitution mechanism involving a coupled incorporation of F and OH to form [□Fx, OHy, O6−x−y]8− (x+y≤6) octahedrons associated with Si vacancies in a [SiO6]8− in bridgmanite. The F-storage capacity of aluminous bridgmanite (>0.8 wt%) is even higher than those of wadsleyite and ringwoodite. Al-bearing bridgmanite has sufficient capacity to store the amount of F in the lower mantle predicted from the oceanic island basalt source region. Therefore, bridgmanite can be considered as the main host of F in the lower mantle. The large storage capacity of bridgmanite for F must be taken into account when calculating the Earth's budget of halogens or global cycles of halogens in the deeper Earth.