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[SIT16-04] Redox kinetics of the subducted slabs indicated by Seebeck coefficient measurements
Keywords:Seebeck coefficient, Subducted slab, olivine, garnet , charge carrier, redox kinetics
The subduction zone is an important region to connect the Earth’s surface and deep interior. The subducted slabs can influence the redox state of the Earth’s interior because the descending slabs are always colder and more oxidized compared with the surrounding mantle. Since the thermal boundary layer is likely to develop around the cold slab, the heat conduction could produce the electrical potential gradient by the Seebeck effect. The Seebeck coefficient is a parameter to define the type of the dominant charge carrier inside the materials under thermal gradient. A positive value of the Seebeck coefficient means the charge carriers have positive valences such as electron holes (small polarons) and protons. When the value is negative, electrons or metal vacancy are potential charge carriers.
To determine the Seebeck coefficient under high pressure, we established the cell design with dual heating systems on the 6-axis apparatus. Two disk TiB2 heaters were placed in the octahedral cell to make two insulated heating systems. Temperature differences were produced on the two sides of the sample by dual heaters and the temperatures were measured by separated thermocouples on each side of the sample. In addition, the electromotive force (voltage difference) of the sample was measured by a high-resolution multimeter. Therefore, we could easily obtain the Seebeck coefficient from the formula, S=delta V/delta T.
To better understand the redox kinetics of the subduction zones, olivine, and garnet, the dominant phases in the upper mantle and the surface of subducted slabs, were investigated in this study. The Seebeck coefficients of the QFM buffered Fo90, Fo80, Fo70, Fo50, and Pyr70Alm30 aggregates were measured up to 5 GPa and 1100℃. The Seebeck coefficients are always positive and decrease with increasing temperature. The positive to negative transition of the charge carrier occurred above 1000℃. The absolute value of the Seebeck coefficient of olivine increases with decreasing Fe concentration from Fo90 to Fo50. Pyr70Alm30 shows a comparable Seebeck coefficient as Fo70. Therefore, olivine or garnet in the cold slab behaves as a P-type semiconductor, suggesting that the cold slab will be oxidized itself by the Seebeck effect possibly until the transition zone or lower mantle condition. This could also give an explanation to the superdeep diamonds which contain bridgmanite inclusions.
To determine the Seebeck coefficient under high pressure, we established the cell design with dual heating systems on the 6-axis apparatus. Two disk TiB2 heaters were placed in the octahedral cell to make two insulated heating systems. Temperature differences were produced on the two sides of the sample by dual heaters and the temperatures were measured by separated thermocouples on each side of the sample. In addition, the electromotive force (voltage difference) of the sample was measured by a high-resolution multimeter. Therefore, we could easily obtain the Seebeck coefficient from the formula, S=delta V/delta T.
To better understand the redox kinetics of the subduction zones, olivine, and garnet, the dominant phases in the upper mantle and the surface of subducted slabs, were investigated in this study. The Seebeck coefficients of the QFM buffered Fo90, Fo80, Fo70, Fo50, and Pyr70Alm30 aggregates were measured up to 5 GPa and 1100℃. The Seebeck coefficients are always positive and decrease with increasing temperature. The positive to negative transition of the charge carrier occurred above 1000℃. The absolute value of the Seebeck coefficient of olivine increases with decreasing Fe concentration from Fo90 to Fo50. Pyr70Alm30 shows a comparable Seebeck coefficient as Fo70. Therefore, olivine or garnet in the cold slab behaves as a P-type semiconductor, suggesting that the cold slab will be oxidized itself by the Seebeck effect possibly until the transition zone or lower mantle condition. This could also give an explanation to the superdeep diamonds which contain bridgmanite inclusions.