To explore the influence of fluorine (F) on mantle minerals and its behaviors during subducting, we investigated the compressibility of F-bearing superhydrous phase B (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 presence of F can largely 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 addition of F enhances the density (~1.3-1.7%) and the bulk velocity (~1.0-2.4%) of Shy-B relative to OH end-member at uppermost lower mantle conditions. The decomposition of F-bearing Shy-B would lead to an abrupt increase in density (~8.9-10.5%) and a small increase in bulk velocity (~0.7-1.8%). The combined results could provide constraints for modeling the geodynamic process related to subduction and transportation of F and OH into lower mantle.

Redox input by subducting slab into deep mantle is of vital importance for deep cycle and isotopic evolution of volatile elements, whose chemically stable forms are controlled by redox state. Lithospheric mantle is crucial in redefining redox state of the Earth’s deep mantle. To constrain to which extent subducted slab can modify redox state of the upper mantle and how much oxygen slab can carry into deep Earth, we investigated redox kinetics of olivine adopting diffusion couple methods at 1 GPa and 1373-1573 K in a piston cylinder apparatus. It is found that redox process in olivine is diffusion-controlled, and diffusing on the order of 10-12 m2/s at 1473 K. The oxidation process in initially reduced olivine is oxygen fugacity (fO2)-independent with activation enthalpy of 235±56 kJ/mol, while the reduction process in initially oxidized olivine is fO2-dependent with an exponent of 2/5. Diffusion profile analysis reveals that redox state of starting material plays decisive role in determining redox mechanism. Below ΔFMQ+1, redox process in olivine is controlled by oxygen grain boundary diffusion, while above ΔFMQ+1, it is rate-limited by faster diffusion species which might be hydrogen related Mg vacancy. The extremely slow redox rate limits the homogenization of the slab and its surrounding mantle as redox state of the upper mantle remains unchanged for over past 3.5 Gyrs. The subducted slab has the ability to efficiently transport oxidized components to the region deeper than the mantle transition zone. A highly underestimated oxygen reservoir may have formed in the deep Earth.

The Mercury’s magnetic fields are known to be weaker than that predicted by conventional dynamo models. In order to explain the Mercury’s weak magnetic field, several models are proposed (Stanley and Glatzmaier, 2010). One of them is the thermoelectric dynamo, which drive the dynamo via the thermoelectric force (Stevenson, 1987). The field strength is proportional to the relative Seebeck coefficient between the core and the mantle. Because the Seebeck coefficient of insulators is more than one order larger than that of metals, the Seebeck coefficient of Mercury’s mantle is the central parameter. Therefore, we investigated the Seebeck coefficient of mantle minerals from the first-principles calculations. The structure relaxation and band structure calculations were conducted by using the Quantum ESPRESSO package. The bandgap energy was calibrated by means of the quasiparticles self-consistent GW (QSGW) approximation adopted in the ecalj package. The Seebeck coefficient was calculated via the Boltzmann equation implemented in the BoltzTraP package. The results indicate that the Seebeck coefficient of forsterite with a small amount of dopant exhibit comparable to that previously thought (|S| ~ 1000 μV/K). This value may constrain the upper limit. The Mercury’s mantle may contain ~3wt% FeO (Robinson and Taylor, 2001). The Fe substitution and O vacancy act as donor, which is predicted to reduce the Seebeck coefficient, significantly. The field strength also depends on the electrical conductivity of the mangle. Recent high pressure experiments suggest that the electrical conductivity of the Earth’s mantle is ~ 10^-2 Sm. Considering the both of the Seebeck coefficient and the electrical conductivity of mantle material, the field strength is calculated to be ~ 0.1 nT, which is significantly weaker than the observed value of 300 nT. Therefore, we conclude that the thermoelectric dynamo cannot generate the Mercury’s magnetic fields.