2:00 PM - 2:15 PM
[SCG48-08] The linkage of zonal structure of sulfide chimney and its semiconducting properties at the seafloor hydrothermal vents
Keywords:submarine hydrothermal vent, chimney, Kuroko, submarine hydrothermal deposits
Submarine hydrothermal vents develop chimneys by precipitating sulfide minerals, when the hydrothermal fluids are mixed with seawater and cooled rapidly. Sulfide minerals often show semiconducting properties and show high electrical conductivity. Recently, it has been proposed that sulfide chimneys act as a catalyst between hydrothermal fluids and seawater, releasing electrons into the ocean through chemical reactions (Yamamoto. et al, 2018). However, it is still unclear whether chimneys, that is a complex aggregate of sulfides, acts as p-type or n-type semiconductors. The Kuroko deposits also show concentric zonal structures, which are thought to be chimneys at the ancient hydrothermal vents. In this study, in order to clarify the possible relationship between power generation and the structures of sulfide chimneys, we conducted the measurement of the electromotive force and analyzed the microstructures of chimneys of the Kuroko sample and the chimney samples.
The analyzed samples are a Kuroko sample from the Hanaoka Mine in NE Japan and a dead chimney from the Myojinsho caldera in the south of the Izu Islands were analyzed.
Chimney samples were cut to be 75 mm long, 75 mm wide, and 10 mm thick, and Kuroko was 50 mm long, 75 mm wide, and 10 mm thick, respectively. The Kuroko sample mainly consists of pyrite and chalcopyrite at the outer parts and of a mixture of barite, quartz, galena, sphalerite, and pyrite that filled the inner conduit. The chimney sample consists of barite and sphalerite with several µm of pyrite, galena, and pores.
The distance between the probes was set to 1 mm, and local and wide-area potentials were measured across the zonal structures. The measurements at room temperature showed that monomineralic parts with high conductivity had a small potential of less than 100 µV, but multiple mineral aggregates had a large potential, exceeding 500 mV. In the measurement with a temperature gradient, the sample was placed on two Peltier devices. The maximum temperature is 120℃, with a temperature difference of 1℃ over a distance of 1 mm between the probes. For the Kuroko sample showed that the potential of semiconducting minerals such as chalcopyrite was several hundred µV greater, that result is consistent with previous studies showing that the Seebeck coefficient of chalcopyrite is approximately -400 µV/K at 100°C or less (Tsujii and Mori, 2017). The electromotive force of pyrite showed both positive and negative values depending on the measurement points. This is probably due to the trace impurities within the crystals. In the inner part of the chimney, which mainly consists of barite, the potential could not be measured. In general, the young chimney is initially composed of anhydrite, wurtzite, and pyrite, then, with growing the chimney walls, the temperature rose to precipitate chalcopyrite and galena.
When the inner layer of chalcopyrite was formed, the rapid temperature gradient created a thermoelectromotive force. Furthermore, anhydrite is dissolved due to lower external temperatures. It is thought that sphalerite and barite form at the later stage that after the hydrothermal fluids ceases and the temperature decreases as a dead chimney. Chimney samples observed in this study were matched to dead chimney structures. The analyzed Kuroko sample showed a low porosity and high density by the later stage crystal growth of pyrite and barite, as the chimney was buried in the seafloor for a long period of time.
These results suggest that the thermoelectromotive forces can be generated in chimneys at the seafloor with very high temperature gradients and that they can be generated at specific times in response to the changes in semiconducting properties and the temperature environment during the textural development of the Kuroko deposits.
Reference : Yamamoto. et al., 2018, Chem Electron Chem, 1u6, 2162-2166. Tsujii and Mori., 2017, Journal of the Japan Society of Powder and Powder Metallurgy. 64, 173-179.
The analyzed samples are a Kuroko sample from the Hanaoka Mine in NE Japan and a dead chimney from the Myojinsho caldera in the south of the Izu Islands were analyzed.
Chimney samples were cut to be 75 mm long, 75 mm wide, and 10 mm thick, and Kuroko was 50 mm long, 75 mm wide, and 10 mm thick, respectively. The Kuroko sample mainly consists of pyrite and chalcopyrite at the outer parts and of a mixture of barite, quartz, galena, sphalerite, and pyrite that filled the inner conduit. The chimney sample consists of barite and sphalerite with several µm of pyrite, galena, and pores.
The distance between the probes was set to 1 mm, and local and wide-area potentials were measured across the zonal structures. The measurements at room temperature showed that monomineralic parts with high conductivity had a small potential of less than 100 µV, but multiple mineral aggregates had a large potential, exceeding 500 mV. In the measurement with a temperature gradient, the sample was placed on two Peltier devices. The maximum temperature is 120℃, with a temperature difference of 1℃ over a distance of 1 mm between the probes. For the Kuroko sample showed that the potential of semiconducting minerals such as chalcopyrite was several hundred µV greater, that result is consistent with previous studies showing that the Seebeck coefficient of chalcopyrite is approximately -400 µV/K at 100°C or less (Tsujii and Mori, 2017). The electromotive force of pyrite showed both positive and negative values depending on the measurement points. This is probably due to the trace impurities within the crystals. In the inner part of the chimney, which mainly consists of barite, the potential could not be measured. In general, the young chimney is initially composed of anhydrite, wurtzite, and pyrite, then, with growing the chimney walls, the temperature rose to precipitate chalcopyrite and galena.
When the inner layer of chalcopyrite was formed, the rapid temperature gradient created a thermoelectromotive force. Furthermore, anhydrite is dissolved due to lower external temperatures. It is thought that sphalerite and barite form at the later stage that after the hydrothermal fluids ceases and the temperature decreases as a dead chimney. Chimney samples observed in this study were matched to dead chimney structures. The analyzed Kuroko sample showed a low porosity and high density by the later stage crystal growth of pyrite and barite, as the chimney was buried in the seafloor for a long period of time.
These results suggest that the thermoelectromotive forces can be generated in chimneys at the seafloor with very high temperature gradients and that they can be generated at specific times in response to the changes in semiconducting properties and the temperature environment during the textural development of the Kuroko deposits.
Reference : Yamamoto. et al., 2018, Chem Electron Chem, 1u6, 2162-2166. Tsujii and Mori., 2017, Journal of the Japan Society of Powder and Powder Metallurgy. 64, 173-179.