南海北部西沙海槽沉积物管状黄铁矿特征及其形成机理

Characterstics and formation mechanism of pyrite tubes in sediments from Xisha Trough in northern South China Sea

南海北部西沙海槽S1站位的岩心柱沉积物中广泛发育自生矿物黄铁矿,其形态以管状为主,且具有内部中空的圈层结构。使用扫描电镜、电子探针、LA-ICP-MS、SIMS等测试方法研究了管状黄铁矿的形态及圈层结构,结果显示:(1)管状黄铁矿发育内部中空的圈层结构,其中内圈层(Ipy)由莓球状黄铁矿呈五角十二面体紧密堆积组成,外圈层(Opy)由晶形较好晶粒较大的八面体黄铁矿组成,并混有沉积碎屑及钙质生物壳体;(2)内圈层和外圈层分别呈现出贫S富Fe和富S贫Fe的特征,其成因是甲烷渗漏造成的局部还原环境使得As进入黄铁矿中导致晶格空缺或被扭曲,从而促进Ni、Co、Cu、Zn、Pb等微量元素的掺入;(3)内圈层、外圈层发生了明显的硫同位素分馏现象,内圈层中δ34S平均为-37.8‰,外圈层中δ34S平均为-29.3‰。研究认为,管状黄铁矿作为曾经甲烷渗漏的通道,其生长机制可分为3个阶段:(1)气水通道形成阶段:向上运移的甲烷流体在沉积物孔隙中逐渐形成气水通道;(2)外圈层形成阶段:当向上运移的甲烷与硫酸盐发生甲烷厌氧氧化时,逐渐形成晶体较大、晶形较好的八面体黄铁矿外圈层;(3)内圈层形成阶段:随着甲烷浓度逐渐降低,在气水通道中的微生物作用下,剩余甲烷与向下运移的硫酸盐继续反应形成莓球状黄铁矿内圈层。因此,南海北部的泥岩中大量发育的管状黄铁矿常常与地层中甲烷水合物的存在有关。

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.