11:15 〜 11:30
[SIT17-09] Conduction mechanisms of olivine and wadsleyite deduced from electrical conductivity and Seebeck coefficient measurements
キーワード：マントル鉱物、電気伝導度、ゼーベック係数
Electrical and thermal properties of the mantle minerals are crucial issues to understand the electric structure, constituent composition and dynamic process of the Earth’s and planetary interiors. Electrical conductivity measurement is a powerful tool to investigate the essential characteristics such as charge carrier concentration and mobility of the mantle minerals. Meanwhile, the Seebeck Coefficient is also important parameter to construct a relationship between electrical and thermal properties. Determination of Seebeck Coefficients of mantle minerals can not only qualitatively specify the species of charge carriers by the sign of the obtained values but also quantitatively determine the charge carrier concentration and mobility of the target minerals combined with electrical conductivity data. Nominally anhydrous minerals (NAMs) such as olivine and wadsleyite are the dominant phases of the Earth’s mantle at different depths of the mantle. Measurements of electrical conductivity and Seebeck coefficient of these phases can clearly manifest the contributions of different conduction mechanisms to the total and give some indications to the thermodynamic processes of the deep planetary interior.
Seebeck Coefficient and electrical conductivity measurements on olivine and garnet were conducted at 3 and 5 GPa up to 1400 K. Sintered aggregates of dry olivine with various Fe content (Fo50, Fo70, Fo80, and Fo90) were prepared as the starting materials. It shows systematically higher Seebeck Coefficient and electrical conductivity of olivine with higher Fe content. Simultaneous modelling of electrical conductivity and Seebeck coefficient of olivine indicates small polarons dominate the total conduction within the measuring temperature range and Mg vacancies make more contributions to the total with increasing temperature. Small polaron conduction is efficiently enhanced by increasing the total iron content. The ferric iron contents () of Fo50, Fo70 and Fo80 are similar while Fo90 shows a much smaller value. Because Fo50, Fo70and Fo80 are synthesized under QFM buffer whereas Fo90 is under IW buffer. The derived diffusion coefficients of small polarons and Mg vacancies increase with temperature and show an consistency with previous studies.
Seebeck coefficient measurements of Fe-free wadsleyite with 2800 ppm H2O and Fe-bearing wadsleyite with 50 and 500 ppm H2O were conducted at 15 GPa up to 1000 K. Fe-free wadsleyite shows an always negative Seebeck coefficient with temperature. The conduction mechanism is interpreted by the joint contributions of protons and Mg vacancies. Both types of Fe-bearing wadsleyite show a slight increase and then decrease of Seebeck coefficient with temperature. Meanwhile, less hydrous one shows a relatively larger value of Seebeck coefficient. This is triggered by the enhancement of ionic conduction with more water in the lattice. The conduction mechanism of Fe-bearing wadsleyite is explained by the joint contributions of proton vacancies, small polarons and Mg vacancies. The distinct diffusion mechanism of hydrogen in Fe-free and Fe-bearing wadsleyite can be the cause of the opposite charge for hydrogen in them.
Seebeck Coefficient and electrical conductivity measurements on olivine and garnet were conducted at 3 and 5 GPa up to 1400 K. Sintered aggregates of dry olivine with various Fe content (Fo50, Fo70, Fo80, and Fo90) were prepared as the starting materials. It shows systematically higher Seebeck Coefficient and electrical conductivity of olivine with higher Fe content. Simultaneous modelling of electrical conductivity and Seebeck coefficient of olivine indicates small polarons dominate the total conduction within the measuring temperature range and Mg vacancies make more contributions to the total with increasing temperature. Small polaron conduction is efficiently enhanced by increasing the total iron content. The ferric iron contents () of Fo50, Fo70 and Fo80 are similar while Fo90 shows a much smaller value. Because Fo50, Fo70and Fo80 are synthesized under QFM buffer whereas Fo90 is under IW buffer. The derived diffusion coefficients of small polarons and Mg vacancies increase with temperature and show an consistency with previous studies.
Seebeck coefficient measurements of Fe-free wadsleyite with 2800 ppm H2O and Fe-bearing wadsleyite with 50 and 500 ppm H2O were conducted at 15 GPa up to 1000 K. Fe-free wadsleyite shows an always negative Seebeck coefficient with temperature. The conduction mechanism is interpreted by the joint contributions of protons and Mg vacancies. Both types of Fe-bearing wadsleyite show a slight increase and then decrease of Seebeck coefficient with temperature. Meanwhile, less hydrous one shows a relatively larger value of Seebeck coefficient. This is triggered by the enhancement of ionic conduction with more water in the lattice. The conduction mechanism of Fe-bearing wadsleyite is explained by the joint contributions of proton vacancies, small polarons and Mg vacancies. The distinct diffusion mechanism of hydrogen in Fe-free and Fe-bearing wadsleyite can be the cause of the opposite charge for hydrogen in them.