Geochemical Characteristics and Their Causative Mechanism of Polymetallic Nodules from the Northwest Continental Margin of the South China Sea
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摘要: 对取自南海西北陆缘海域的大型多金属结核进行了电子探针、X射线粉晶衍射(XRD)、等离子质谱仪(ICP-MS) 和等离子光谱仪(ICP-AES) 等方面的分析.结核核心部位的主要矿物组成为石英、伊利石、钠长石和绿泥石, 壳层的主要矿物为δMnO2等.铁、锰组分呈现Fe含量高、Mn含量低和Mn/Fe低的特征.Si含量高, Cu、Co、Ni含量低; 稀土元素(REE) 含量高, 平均为1472.30×10-6, 轻稀土与重稀土的比值(LREE/HREE) 达19.54, 并且存在较强的Ce正异常.元素含量的变化显示: 从结核内壳层到外壳层, Fe、Mn、Cu、Co等元素含量呈不规律变化, 具有典型的边缘海特征, 该特征反映结核在形成过程中受到边缘海沉积环境波动变化的影响, 陆源物质供应量的增加对Fe、REE、Si等元素的富集起到了促进作用, 而对Mn、Ca等元素的富集则产生明显的稀释作用.多金属结核Mn/Fe比及Mn-Fe- (Cu+Ni) ×10三组分图解显示, 南海北部陆缘多金属结核为水成成因, 该成因与结核所赋存的边缘海环境密切相关, 反映了结核成长发育的过程中, 南海典型的边缘海沉积条件和多变的古海洋环境因素对其产生了重要影响.Abstract: This study employs various observation and analytical techniques including electron microprobe analysis, X-ray difraction (XRD) and ICP-MS (AES) on polymetallic nodules collected from the northwest continental margin of the South China Sea. The analyses indicate that the cores of the samples are made up of quartz, illite, albite and chlorite, whereas the shells are made up of δMnO2. The samples are rich in Fe, Si, and ∑REE, and they are poor in Mn, Cu, Co and Ni. These samples show lower ratios of Mn/Fe. The characteristics of the REE (rare earth elements) show higher contents with more than 1472.30×10-6 in average. Heavy REE (HREE) depletes relative to the light REE (LREE) very obviously that the latter is 19.54 times than that of the former, and shows distinct enrichment of δCe. The elements of Fe, Mn, Cu and Co change obviously in different ways from the inner crust to the outer. These characteristics show that the growth of the nodules has been affected by the environmental fluctuations and the change of terrigenous sediments. In this condition, Fe, REE and Si are enrichment, but Mn and Ca are dilutedness. Elements correlation of Mn-Fe- (Cu+Ni) ×10 suggests that the origin of the sample may be hydrogenic. This study comes to the conclusion that these nodules are dominative due to the special environment of the marginal sea which includes the geographical condition and the oceanic environmental factors.
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图 1 样品的外部形态及内部结构
Fig. 1. Morphological features and inner structures of the polymetallic nodules
图 2 南海多金属结核X射线衍射
δ.水羟锰矿; Q.石英; M.云母; An.长石; C.绿泥石
Fig. 2. X-ray diffraction analysis results of the polymetallic nodules
图 3 南海多金属结核样品北美页岩标准化配分曲线
Fig. 3. REE distribution pattern for normalized shale of the polymetallic nodules
表 1 南海多金属结核矿物组成及其组分含量
Table 1. Minerals and major components of the polymetallic nodules
表 2 南海多金属结核主要组分含量(%)
Table 2. Major components of the polymetallic nodules
表 3 南海多金属结核样品稀土元素含量(10-6)
Table 3. REE contents of the polymetallic nodules
表 4 南海多金属结核微量元素含量(10-6)
Table 4. Trace elements of the polymetallic nodules
表 5 南海北部陆缘多金属结核部分元素相关系数矩阵
Table 5. Some elements relationship of the polymetallic nodules
表 6 南海多金属结核与大洋结核的主要组分含量比较(%)
Table 6. Comparison of main elements between the different samples
表 7 南海多金属结核与大洋结核几种微量元素含量比较(10-6)
Table 7. Comparison of trace elements between the different samples
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