Thermal equation of state of F-bearing superhydrous phase B (Mg₁₀Si₃O₁₄(OH,F)₄): Implications for the transportation of fluorine and water into the lower mantle

Superhydrous phase B (Shy–B), an important hydrous magnesium silicate, plays a key role in the transportation of water from upper to lower mantle via subduction slabs. Moreover, it may also be a potential carrier for another important volatile element: fluorine (F). To explore the influence of F on mantle minerals and its behaviors during subducting, we investigated the compressibility of F-bearing Shy-B using synchrotron-based single-crystal X-ray diffraction combined with diamond anvil cells up to 27 GPa and 750 K. Our results show that the substitution of OH by F can enhance the incompressibility of Shy–B. Based on the obtained thermal elastic parameters, density and velocity profiles are evaluated along cold and warm slabs. Our results demonstrate that the addition of F enhances the bulk velocity (~1.0–2.4%) of Shy-B relative to the OH end-member at uppermost lower mantle conditions. The decomposition of F-bearing Shy-B into bridgmanite and periclase would lead to a small increase in bulk velocity (~0.7–1.8%). Thus, the accumulation and decomposition of F-bearing Shy-B is hard to explain the velocity anomaly at the uppermost lower mantle. Our results provide constraints for modeling the geodynamic process related to subduction and transportation of F and H₂O into the lower mantle.