14:45 〜 15:00
[SIT16-05] Self-redox of subducted slab by Seebeck effect
キーワード：酸化還元状態、マントル、沈み込むスラブ、ゼーベック効果、熱電特性
Subduction of the cold oceanic slab not only disturbs temperature distribution of mantle and but also plays a key role in deep cycle of volatile components such as H2O and CO2 into the deep Earth’s interior. The oceanic plate oxidized at the ocean floor also affects the redox state of the deep mantle, which controls stability of the volatile species. Thermal conduction of the subducted slab itself and its surrounding mantle would govern the heat flux to the descending slab through the thermal boundary layer adjacent to the slab. In this thermal boundary layer, there should exist a huge thermal gradient, which may form an electric field due to thermoelectric effect driven by the temperature difference. The Seebeck effect is a phenomenon in which a temperature difference produces a voltage difference along thermal gradient. The Seebeck coefficient S is defined by S = V/K and may have different signs for materials, negative for negatively charged carriers and positive for positively charged carriers. P-type semiconductor will transport positive electric charge to the colder side, while N-type semiconductor will transfer negative charge carrier to the colder side. The cold subducted slab would be oxidized or reduced by thermal gradient dependent on thermoelectric properties of both the slab and its surrounding materials. Consequently, transport of electric charge by a temperature gradient can change redox state of the subducted slab. However, we have little knowledge of how the electric field affects the redox state.
In this study, we investigate the change of redox state of olivine aggregates, known as the most abundant mineral in the Earth’s upper mantle, in electric potential field. The high-pressure experiments were conducted in a Kawai-type multi-anvil press at 5 GPa and temperature ranging from 1073 to 1473 K. After annealing a few hours for oxygen fugacity equilibrium, we applied DC voltage up to 2 V to the both ends of the sample. Oxygen fugacity profiles were obtained using Pt oxygen sensor mixed with olivine aggregates, and show that the reduction occurs at anode side, while the oxidation occurs at cathode side. The region showing an oxygen fugacity different from the initial state increases as the temperature increases and the annealing duration increases. The present result demonstrates that the redox state of the subducted slab can be affected by the Seebeck effect. The Seebeck coefficient of olivine changes positive to negative at a certain temperature as dominant electric conduction mechanism changes from small polaron conduction (electron hole hopping or proton hopping) to ionic conduction (Mg vacancy). For the cold slab, transport of positive charge carriers to the cols slab induces the self-oxidation of the subducted slab. For the hot slab, the self-reduction is likely to occur. Such a phenomenon suggests that it has become possible to carry more oxygen into the deep mantle as the mantle potential temperature decreases throughout the Earth’s history.
In this study, we investigate the change of redox state of olivine aggregates, known as the most abundant mineral in the Earth’s upper mantle, in electric potential field. The high-pressure experiments were conducted in a Kawai-type multi-anvil press at 5 GPa and temperature ranging from 1073 to 1473 K. After annealing a few hours for oxygen fugacity equilibrium, we applied DC voltage up to 2 V to the both ends of the sample. Oxygen fugacity profiles were obtained using Pt oxygen sensor mixed with olivine aggregates, and show that the reduction occurs at anode side, while the oxidation occurs at cathode side. The region showing an oxygen fugacity different from the initial state increases as the temperature increases and the annealing duration increases. The present result demonstrates that the redox state of the subducted slab can be affected by the Seebeck effect. The Seebeck coefficient of olivine changes positive to negative at a certain temperature as dominant electric conduction mechanism changes from small polaron conduction (electron hole hopping or proton hopping) to ionic conduction (Mg vacancy). For the cold slab, transport of positive charge carriers to the cols slab induces the self-oxidation of the subducted slab. For the hot slab, the self-reduction is likely to occur. Such a phenomenon suggests that it has become possible to carry more oxygen into the deep mantle as the mantle potential temperature decreases throughout the Earth’s history.