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 (similar to 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 (similar to 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.

Automated summary

This study explores the compressibility of F-bearing Superhydrous phase B (Shy-B) and its influence on mantle minerals during subduction. The findings show that the addition of fluorine enhances the bulk velocity of Shy-B, but the accumulation and decomposition of F-bearing Shy-B cannot explain the velocity anomaly at the uppermost lower mantle.