1:45 PM - 3:15 PM
[O08-P56] Microtextural study of Tetsuganseki, a brownish nodule in Setogawa accretionary prism
Keywords:Tetsuganseki
Introduction: Tetsuganseki is a type of carbonate nodule found in the accretionary prism in the Shimanto Belt. Previous studies suggest that carbonate nodules are related to trace fossils derived from organisms living in sediments. Tetsuganseki consists mainly of siderite(FeCO3) and quartz and sometimes contains pyrite. In addition, calcite has been reported from Tetsuganseki from Muroto, Kochi. However, the microstructure and elemental distribution within the Tetsuganseki have not yet been investigated in detail. These are expected to lead to a better understanding of the formation process of Tetsuganseki. In this study, microstructural observation and elemental mapping were done on Tetsuganseki collected from the Abegawa basin in Shizuoka, Japan, to investigate the formation process.
Method: A spherical sample of Tetsuganseki with a diameter of about 6 cm was collected from a riverbed in the Abegawa basin. The sample was processed into polished thin sections, and microstructural observations were done using the optical microscope and the field emission scanning electron microscope (FESEM: JEOL JSM-IT700HR at Shizuoka University). In addition, element concentration mapping was performed using the FESEM and an EDS analyzer (Oxford instruments Ultim Max 170 at Shizuoka University).
Results: In the investigated Tetsuganseki sample, a flamboidal pyrite of ~1 cm in diameter was observed in the center of the sample. The framboidal pyrite was surrounded by the black spherical shell part of ~2.5cm thickness. The black part consists of mineral particles of several tens to 50 microns diameters embedded in a fine-grained matrix. Pyrites are found as framboidal aggregates or single crystals. Element mapping suggests that other mineral particles include siderite, quartz, and zeolite with a minor apatite.The siderite was mainly spherical, but ribbon- and crescent-shaped particles were also observed. The siderite and pyrite were often in contact with each other. Ca-rich carbonates such as calcite were not observed. The matrix filling the gaps between these mineral particles is composed of particles smaller than a few micrometers, and their mineral species have not been identified. The outer rim of Tetsuganseki is altered to brownish.
No concentric or bedding structures were observed in the interior of the investigated Tetsuganseki. In the matrix around the frlamboidal pyrite, Fe decreased and Si increased. The brownish rim with ~600 microns thickness is enriched in Fe and depleted in C compared to the black part and pyrite is almost absent. In addition, Tetsuganseki often had veins leading to the outside, and these veins were chiefly filled with quartz.
Discussion: The present result indicates that siderite and pyrite were in equilibrium during Tetsuganseki formation. The pe-pH diagram for the Fe-O-S-C system assuming 25 degree C seawater condition suggests that siderite and pyrite can be in equilibrium at the narrow conditions of pH ~ 4-7 and pe ~ -2-0 with elevated Fe activity. Therefore, Tetsuganseki is thought to be formed under Fe-rich, slightly acidic, and reduced condition such as submarine hydrothermal system. The increase in Fe and decrease in C and pyrite observed in the brownish rim may be due to the oxidation-induced decomposition of pyrite and siderite to form hematite.Since quartz veins cut the brownish rim, oxidation of the surface layer of Tetsuganseki is considered to have occurred prior to the formation of quartz veins.
Method: A spherical sample of Tetsuganseki with a diameter of about 6 cm was collected from a riverbed in the Abegawa basin. The sample was processed into polished thin sections, and microstructural observations were done using the optical microscope and the field emission scanning electron microscope (FESEM: JEOL JSM-IT700HR at Shizuoka University). In addition, element concentration mapping was performed using the FESEM and an EDS analyzer (Oxford instruments Ultim Max 170 at Shizuoka University).
Results: In the investigated Tetsuganseki sample, a flamboidal pyrite of ~1 cm in diameter was observed in the center of the sample. The framboidal pyrite was surrounded by the black spherical shell part of ~2.5cm thickness. The black part consists of mineral particles of several tens to 50 microns diameters embedded in a fine-grained matrix. Pyrites are found as framboidal aggregates or single crystals. Element mapping suggests that other mineral particles include siderite, quartz, and zeolite with a minor apatite.The siderite was mainly spherical, but ribbon- and crescent-shaped particles were also observed. The siderite and pyrite were often in contact with each other. Ca-rich carbonates such as calcite were not observed. The matrix filling the gaps between these mineral particles is composed of particles smaller than a few micrometers, and their mineral species have not been identified. The outer rim of Tetsuganseki is altered to brownish.
No concentric or bedding structures were observed in the interior of the investigated Tetsuganseki. In the matrix around the frlamboidal pyrite, Fe decreased and Si increased. The brownish rim with ~600 microns thickness is enriched in Fe and depleted in C compared to the black part and pyrite is almost absent. In addition, Tetsuganseki often had veins leading to the outside, and these veins were chiefly filled with quartz.
Discussion: The present result indicates that siderite and pyrite were in equilibrium during Tetsuganseki formation. The pe-pH diagram for the Fe-O-S-C system assuming 25 degree C seawater condition suggests that siderite and pyrite can be in equilibrium at the narrow conditions of pH ~ 4-7 and pe ~ -2-0 with elevated Fe activity. Therefore, Tetsuganseki is thought to be formed under Fe-rich, slightly acidic, and reduced condition such as submarine hydrothermal system. The increase in Fe and decrease in C and pyrite observed in the brownish rim may be due to the oxidation-induced decomposition of pyrite and siderite to form hematite.Since quartz veins cut the brownish rim, oxidation of the surface layer of Tetsuganseki is considered to have occurred prior to the formation of quartz veins.