1:45 PM - 2:00 PM
[SCG45-01] Evolution of Thermoelectric performance in sulfide chimneys of submarine hydrothermal vents: an example from Myojinsho Caldera
Keywords:sulfide chimney, thermoelectromotive force, zonal structure
Submarine hydrothermal vents act as electrical power plants, which supply electrical energy to deep-sea ecosystems. The most part of such energy is thought to generated from redox conditions, because a potential difference of several hundred mV exists between reducing hydrothermal water and oxidizing seawater, and the walls of the chimney are made of a mixture of sulfide and sulfate minerals that conduct electricity and release electrons into the seawater. Besides, it has long been known that sulfide minerals are semiconductors, but complex sulfide materials with complex structures have attracted attention as materials with high thermoelectric conversion performance. Nevertheless, as the chimney shows variously zoned and its mineralogy and microstructure change during the hydrothermal activities; and it is unclear whether thus when and how extent of thermoelectromotive force (EMF) is generated in the life of a chimney.
In this study, we analyzed the microstructures and electrical properties of the sulfide chimneys collected from the hydrothermal fields in the Myojinsho Caldera, Izu-Ogasawara region. The most of the chimneys shows porous structures, and mainly composed of a mixture of fined-grained sphalerite (Sp) and galena (Gn) with barite (Brt). Spherical pyrite rarely occurs in some chimney samples. One dead chimney sample (HPD#2182R01 sample) shows yellowish metallic walls around the vents, which is composed of coarse-grained galena and chalcopyrite (Cp) replacing sphalerite, whereas the outer parts show similar mineralogy and texture of other samples. These systematic zoning suggest that initial formation of sulfate minerals (stage 1), the growth of chimney as formation of fine-grained sphalerite particles and reprecipitation of barite at the outer region (stage 2), and developments of galena and chalcopyrite growth at the inner wall (stage 3).
To measure the EMF, the sample was placed bridged on between two Peltier devices, a temperature difference was applied, and the potential difference between the two electrodes was measured. Based on the analyses of monomineralic samples, we found that sphalerite, galena, and chalcopyrite are n-type semiconductor, whereas pyrite shows both n-type and p-type even in single samples. The absolute value of the Seebeck coefficient, Sw (= E(mV) / ΔT (K)), of Py, Cp and Gn are commonly less than 1 mV/K, except for sphalerite, which has a wide-band gap with Sw>100 mV/K. In addition, the resistivity of Py, Cp, and Gln are low (10^-2 – 10^1 Ω cm), whereas Sph is extremely high (>10^7 Ω cm). For the most of the chimney samples from the Myojinsho Caldera, the Seebeck coefficient cannot be measured due high resistivity of porous structure and complicated texture of Sp+Brt mixture. In contrast, the inner wall of the chimney composed of Cp+Gn exhibits n-type semiconductor characteristics, with the Seebeck coefficients ranging from -0.01 to -0.2.
The thermoelectric performance of materials is evaluated as a factor of ZT = Sw^2 x (electrical conductivity) x (temperature)/(thermal conductivity). Our results suggest that evolution of thermoelectric performance changes with the evolution of chimney. At the early stage, young chimney, which is composed of sulfate minerals + sphalerite particles with porous structure, cannot transport electrons due to high resistance, although Sw of Sp is high. After the inner wall of Cp+Gn has been developed, the resistance becomes significantly lower and thermoelectrical performances appears. Assuming the temperature gradient of 300 degreeC between hydrothermal fluids and seawater, and the EMF is up to ~500 mV, which is comparative to those measured as the redox potential between the two fluids. This suggests that power generation at the submarine hydrothermal systems was significantly enhanced at the specific timing of chimney growth, which could affect the energy supply for the microorganism.
In this study, we analyzed the microstructures and electrical properties of the sulfide chimneys collected from the hydrothermal fields in the Myojinsho Caldera, Izu-Ogasawara region. The most of the chimneys shows porous structures, and mainly composed of a mixture of fined-grained sphalerite (Sp) and galena (Gn) with barite (Brt). Spherical pyrite rarely occurs in some chimney samples. One dead chimney sample (HPD#2182R01 sample) shows yellowish metallic walls around the vents, which is composed of coarse-grained galena and chalcopyrite (Cp) replacing sphalerite, whereas the outer parts show similar mineralogy and texture of other samples. These systematic zoning suggest that initial formation of sulfate minerals (stage 1), the growth of chimney as formation of fine-grained sphalerite particles and reprecipitation of barite at the outer region (stage 2), and developments of galena and chalcopyrite growth at the inner wall (stage 3).
To measure the EMF, the sample was placed bridged on between two Peltier devices, a temperature difference was applied, and the potential difference between the two electrodes was measured. Based on the analyses of monomineralic samples, we found that sphalerite, galena, and chalcopyrite are n-type semiconductor, whereas pyrite shows both n-type and p-type even in single samples. The absolute value of the Seebeck coefficient, Sw (= E(mV) / ΔT (K)), of Py, Cp and Gn are commonly less than 1 mV/K, except for sphalerite, which has a wide-band gap with Sw>100 mV/K. In addition, the resistivity of Py, Cp, and Gln are low (10^-2 – 10^1 Ω cm), whereas Sph is extremely high (>10^7 Ω cm). For the most of the chimney samples from the Myojinsho Caldera, the Seebeck coefficient cannot be measured due high resistivity of porous structure and complicated texture of Sp+Brt mixture. In contrast, the inner wall of the chimney composed of Cp+Gn exhibits n-type semiconductor characteristics, with the Seebeck coefficients ranging from -0.01 to -0.2.
The thermoelectric performance of materials is evaluated as a factor of ZT = Sw^2 x (electrical conductivity) x (temperature)/(thermal conductivity). Our results suggest that evolution of thermoelectric performance changes with the evolution of chimney. At the early stage, young chimney, which is composed of sulfate minerals + sphalerite particles with porous structure, cannot transport electrons due to high resistance, although Sw of Sp is high. After the inner wall of Cp+Gn has been developed, the resistance becomes significantly lower and thermoelectrical performances appears. Assuming the temperature gradient of 300 degreeC between hydrothermal fluids and seawater, and the EMF is up to ~500 mV, which is comparative to those measured as the redox potential between the two fluids. This suggests that power generation at the submarine hydrothermal systems was significantly enhanced at the specific timing of chimney growth, which could affect the energy supply for the microorganism.