Characterstics and formation mechanism of pyrite tubes in sediments from Xisha Trough in northern South China Sea
Shi Si-Si1,2, Wu Chao-Dong1,2, Liang Jin-Qiang3, Wang Yi-Zhe1,2, Ye Yun-Tao4, Fang Yun-Xin3, Ma Jian1,2, Zhai Li-Na5
1 Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education,School of Earth and Space Sciences, Peking University,Beijing 100871,China; 2 Institute of Oil & Gas,Peking University,Beijing 100871,China; 3 Guangzhou Marine Geological Survey, Guangzhou,510760,China; 4 Research Institute of Exploration & Development,PetroChina,Beijing 100083,China; 5 Key Laboratory of Marine Geology and Environment,Institute of Oceanology,Chinese Academy of Sciences, Shandong Qingdao 266071,China
Abstract：Authigenic pyrite is widely developed in sediments of core column at S1 in the Xisha Trough in northern South China Sea. It is mainly tubular in form and has inner hollow circle structures. In order to study the origin of tubular shape and circle structure,SEM,LA-ICP-MS,SIMS and other testing methods are used. The results show that: (1)Pyrite tubes have an inner hollow circle structure. The inner layer(Ipy) is composed of pyrite framboids which are densely packed as pyritohedron,while the outer layer(Opy) is composed of octahedron pyrite with better crystalline shape and larger grain size,mixed with sedimentary debris and calcareous biological shell. (2)The inner and outer layers are characterized by poor S-rich Fe and rich S-poor Fe respectively. It mainly resulted from the reduing environment caused by methane leakage,the lattice vacancy of pyrite which caused by the entry of As,and absorbing Ni,Co,Cu,Zn and other elements with similar radius and charge of Fe. (3)Pyrite tubes underwent distillation of sulfur isotopes,with an average of-37.8‰ for the inner layer and-29.3‰ for the outer layer. It is concluded that pyrite tubes served as a channel for methane leakage,the growth mechanism of which can be divided into three stages: (1)formation stage of gas-water channel: upward migration of methane fluids gradually forms gas-water channel in sediment pore;(2)formation stage of Opy: anaerobic oxidation of methane occurs when upward migration of methane and sulfate meet,the Opy with larger and better crystalline shape is gradually formed;(3)formation stage of Ipy: with the concentration of methane decreases gradually and the action of microorganisms in the air-water channel,the remaining methane reacts with downward sulfate to form the Ipy. Therefore,a large number of tubular pyrites in mudstones in the northern South China Sea are often related to the presence of methane hydrates in the formation.
Shi Si-Si,Wu Chao-Dong,Liang Jin-Qiang et al. Characterstics and formation mechanism of pyrite tubes in sediments from Xisha Trough in northern South China Sea[J]. JOPC, 2020, 22(1): 175-192.
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