1:45 PM - 2:00 PM
[SCG53-01] Mineral replacement of sulfide minerals and its impact one thermoelectric conversion at seafloor hydrothermal vents
Keywords:seafllor Hydrothermal Vents, Sulfide Chimney, Mineral replacement reactions, Thermoelectric conversion performance
Seafloor hydrothermal sites are distributed along mid-ocean ridge, island arc, back-arc basins, and rifting zone where the mixing of hydrothermal fluids containing sulfur and dissolved metal elements derived from magma and volcanic rock with ambient cold seawater leads to the formation of chimney-shaped sulfide mineral structures. At these sites, electron supply to the deep seafloor has been reported due to the redox potential gradient between hydrothermal fluid and seawater, as well as the conductivity of sulfide minerals. Understanding the process of electron transfer to seawater via sulfide chimneys is crucial for elucidating the formation of seafloor ecosystems and the origin of life (e.g., Yamamoto et al., 2018). However, no studies have focused on the mineral textures of chimneys as electron transfer pathways, leaving the electron transport processes within chimney walls as a black box. Additionally, since sulfide minerals have semiconductor properties and thermoelectric effects, they are suggested to convert the thermal energy into an electric potential comparable to redox potential differences. However, no research has yet investigated the thermoelectric conversion performance of constituent minerals in chimneys. Therefore, this study aims to experimentally investigate the evolution of chimneys and the associated changes in their electrical properties, ultimately examining the mechanisms of power generation in hydrothermal systems. As an initial approach, we conducted mineralogical observations and measurements of the Seebeck coefficient S [mV K-1] and electrical conductivity σ [S cm-1] of chimney samples collected during cruises KS-22-12, KS-24-3, and KS-24-14. Furthermore, we performed experiments by reacting synthetic hydrothermal fluids with individual sulfide minerals-sphalerite (ZnS), pyrite (FeS2), and galena (PbS)-as well as composite samples composed of these minerals, attempting to reproduce mineral replacement reactions involved in the evolution process of chimney zoning structure. By measuring the electrical properties of the products, we evaluated changes in power generation performance induced by mineral replacement reactions, aiming to elucidate the relationship between mineralogical evolution of chimney and power generation phenomena.
Sulfide chimneys can be classified into following three types: (i) Porous barite-sphalerite-rich samples (early stage), (ii) Sphalerite-rich with spherical pyrite (mid-stage), and (iii) Sphalerite-pyrite-rich samples whose voids were filled with galena and chalcopyrite (late stage). Conductivity σ [S cm-1] was on the order of 10-12 for (i) and (ii), whereas galena-rich (iii) exhibited 100 [S cm-1]. The power factor S2σ (hereafter referred as PF), an indicator of thermoelectric performance, was 106 order higher in (iii) than those of (i) and (ii), suggesting a linkage between mineralogical zoning in chimney and thermoelectric power generation.
Experiments were conducted at 200C and 300C under saturated vapor pressure used NaCl (1 M) and, in some cases, Na2SO3 (1.1 M) as a reducing agent. After 12 days, sphalerite and galena reacted with Cu2+ to form Cu-S layers having an oxidation state between Cu2S and CuS, while reducing conditions produced Cu-S phases and chalcopyrite. Pyrite underwent systematic changes, forming Cu-S phases at the surface and chalcopyrite within cracks, likely reflecting chemical potential and oxygen fugacity gradients. PF calculations showed that replacing sphalerite with Cu-S phases and chalcopyrite increased S2σ by up to 106 orders of magnitude.
Early-stage chimneys, mainly composed of barite and sphalerite, have low conductivity, limiting electron mobility and thermoelectric power. Over time, conductive layers of Cu-S phases, chalcopyrite, and galena are formed, creating high S2σ networks that facilitate electron transport. Additionally, temperature gradients within the chimney wall may enhance thermoelectric conversion, promoting electron transfer.
Sulfide chimneys can be classified into following three types: (i) Porous barite-sphalerite-rich samples (early stage), (ii) Sphalerite-rich with spherical pyrite (mid-stage), and (iii) Sphalerite-pyrite-rich samples whose voids were filled with galena and chalcopyrite (late stage). Conductivity σ [S cm-1] was on the order of 10-12 for (i) and (ii), whereas galena-rich (iii) exhibited 100 [S cm-1]. The power factor S2σ (hereafter referred as PF), an indicator of thermoelectric performance, was 106 order higher in (iii) than those of (i) and (ii), suggesting a linkage between mineralogical zoning in chimney and thermoelectric power generation.
Experiments were conducted at 200C and 300C under saturated vapor pressure used NaCl (1 M) and, in some cases, Na2SO3 (1.1 M) as a reducing agent. After 12 days, sphalerite and galena reacted with Cu2+ to form Cu-S layers having an oxidation state between Cu2S and CuS, while reducing conditions produced Cu-S phases and chalcopyrite. Pyrite underwent systematic changes, forming Cu-S phases at the surface and chalcopyrite within cracks, likely reflecting chemical potential and oxygen fugacity gradients. PF calculations showed that replacing sphalerite with Cu-S phases and chalcopyrite increased S2σ by up to 106 orders of magnitude.
Early-stage chimneys, mainly composed of barite and sphalerite, have low conductivity, limiting electron mobility and thermoelectric power. Over time, conductive layers of Cu-S phases, chalcopyrite, and galena are formed, creating high S2σ networks that facilitate electron transport. Additionally, temperature gradients within the chimney wall may enhance thermoelectric conversion, promoting electron transfer.