Element geochemistry and genesis of cobalt-rich crust on the Line Seamount of the Central Pacific
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摘要:
富钴结壳是一种经济价值高、开发前景好的海洋矿产资源，其外部形态、内部构造和地球化学特征记录了古海洋环境的演变信息。本次研究借助富钴结壳细致分层的显微构造及元素地球化学的差异分析，探讨中太平洋莱恩海山富钴结壳成因及形成过程。富钴结壳样品由上至下可分为5层，顶部较致密Ⅰ层黑褐色，表面光滑，杂质较少，具有雪松状叠层石构造，反映了沉积水动力较强；较致密Ⅱ层与Ⅰ层宏观特征类似，具有柱状和胡萝卜状叠层石构造，沉积水动力最强；疏松Ⅲ层和疏松Ⅳ层黄褐色，内含沉积物杂质，多见斑杂状构造，沉积水动力比较弱；致密Ⅴ层为磷酸盐层，具有亮黑色的沥青光泽，富含有孔虫化石，底部Ⅴ层Sr、P、Ca等元素明显富集，表明了海洋生物的明显参与，沉积水动力最弱。莱恩海山Ce异常及高钴高锰低铁特征，表明结壳长期处于氧化环境；而Mn、Fe、Co等元素地球化学特征综合表明，海洋水动力及氧化性总体表现为由底部Ⅴ层至Ⅱ层氧化性逐渐增强，至顶部Ⅰ层有所减弱的趋势；依据元素分配系数、Ce正异常、Ho正异常、Y负异常及成因三角图，判定结壳为南极底流影响下的海水成因。综合结壳年代学数据，在元素剖面中记录了三期磷酸盐化事件，恢复了莱恩海山富钴结壳的生长过程。
Abstract:Cobalt rich crust is a kind of marine mineral resource with high economic value and significant development prospect. The external morphology, internal structure and geochemical characteristics of the crust recorded the evolution of paleoceanographic environment. In this paper, the genesis or forming process of the cobalt rich crusts on the Line Seamount of the Central Pacific Ocean is studied upon the basis of microstructure and element geochemistry. The cobalt rich crust sample is composed of five layers from top to bottom. The top layer is relatively dense, dark brown in color with smooth surface and little impurities. It has a cedar-like stromatolite structure, indicating a strong hydrodynamic environment; The second layer has similar macroscopic characteristics with the first, characterized by columnar and carrot shaped stromatolite structure, which indicates the strongest hydrodynamic deposition; The third and fourth layers are yellowish brown in color, with patchy structures containing certain amount of sediment impurities suggesting weak hydrodynamics; The fifth is a phosphatic layer with bright black asphalt luster, and rich in foraminifera fossils. Meanwhile, the fifth layer is rich in Sr, P, Ca and other elements, indicating obvious involvement of marine organisms and the weakest hydrodynamics. The geochemical characteristics of Ce anomaly and high Co, high Mn and low Fe indicate that the cobalt rich crust was formed in an oxidizing environment for a long time. The geochemical characteristics of Mn, Fe, Co and other elements reveal that the marine hydrodynamic and oxidizability of the environment gradually increases starting from the fifth on bottom layer up to the second layer, but decreases at the first or top layer. According to the distribution coefficients of the elements, positive anomaly of Ce, positive anomaly of Ho, negative anomaly of Y and the triangle genesis diagram, it is inferred that the cobalt-rich crust of the Line Seamount is formed in seawater environment under the influence of the Antarctic bottom current. Based on the geochronological data of the cobalt rich crusts, the growth process of the cobalt rich crusts is recovered in the paper, and three phosphorylation events are recognized on the element profile.
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Key words:
- cobalt-rich crusts /
- paleoceanography /
- geochemistry /
- Line Seamount
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表 1 莱恩海山电子探针元素分析
Table 1. Electron probe element analysis of the Line Seamount
% 测点 层位 Na2O FeO P2O5 MgO MnO CaO CoO K2O ZnO SO3 CuO 总量 D1 Ⅴ 1.05 0.07 30.95 0.27 — 51.67 — 0.03 0.09 1.19 — 89.07 D2 1.15 0.04 31.14 0.34 — 47.36 — 0.03 0.04 1.21 — 85.11 D3 1.02 0.06 31.18 0.27 — 47.55 — 0.04 — 1.43 — 84.99 D4 Ⅳ 1.41 24.81 1.05 2.31 20.79 2.09 0.45 0.79 0.09 0.38 0.04 54.21 D5 Ⅲ 2.32 20.86 0.83 1.79 29.91 3.10 0.63 0.47 0.03 0.75 0.11 60.80 D6 Ⅱ 1.26 20.53 0.85 2.11 31.47 3.59 0.84 0.90 — 0.50 0.12 62.17 D7 2.74 11.44 0.48 2.56 39.09 3.32 1.06 0.78 0.04 0.51 0.19 62.21 D8 Ⅰ 2.29 15.09 0.56 1.94 38.63 3.58 1.13 0.53 0.09 0.79 0.09 64.72 D9 2.95 12.87 0.59 2.27 36.20 3.41 1.30 0.61 0.11 0.59 0.09 60.99 注：测点D1 —D9见图3h-l，D4—D9见图7投点。 
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表 2 莱恩海山结壳样品分层微量元素含量
Table 2. Trace elements of layered cobalt rich crusts on the Line Seamount
μg/g 样品编号 分层 Co Ni Cu Zn V Cr Sr Mo Ba W Pb Th U Th/U Ni/Co Q1 Ⅰ 较
致
密
层10 444 3 038 392 497 612 7.77 1 560 492 1 012 93.2 1 717 15.8 12.0 1.32 0.29 Q2 Ⅱ 10 311 3 120 500 520 563 9.16 1 626 449 1 115 99.8 1 545 8.8 11.9 0.74 0.24 Q3 Ⅲ 疏
松
层9 924 3 561 618 502 536 8.86 1 432 464 1 198 99.9 1 554 7.84 11.2 0.70 0.36 Q4 Ⅳ 6 673 3 373 790 559 521 13.4 1 558 366 1 315 79.2 1 333 9.78 10.2 0.96 0.51 Q5 Ⅴ 致
密
层10 425 6 003 965 714 599 1.19 1 662 624 1 723 127.0 1 349 2.51 11.9 0.21 0.58 平均值 9 555 3 819 653 588 566 8.08 1 568 479 1 273 99.8 1 500 8.95 11.4 0.79 0.39
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表 3 莱恩海山富钴结壳样品稀土元素
Table 3. Analysis table of rare earth elements of cobalt rich crust samples from the Line Seamount
样品
编号分层 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y LREE/
HREE(La/Sm)N (Gd/Yb)N Y/Ho δCe δEu δGd δY Q1 Ⅰ 233 682 43.6 189 38.4 9.88 40.3 7.12 41.8 9.27 27.9 4.48 27.1 4.17 185 7.38 0.83 0.89 19.96 1.46 1.10 0.95 0.85 Q2 Ⅱ 202 657 29.7 149 28.8 7.49 33.2 6 32.9 8.31 23.4 4.06 26.7 4.01 159 7.75 0.73 0.74 19.13 1.79 1.06 0.97 0.86 Q3 Ⅲ 171 786 28.9 130 26.4 6.81 28.7 5.16 27.5 6.54 19.6 3.56 22 3.53 133 9.86 0.75 0.78 20.34 2.39 1.08 0.96 0.89 Q4 Ⅳ 213 761 39.3 182 35.7 8.99 39.1 6.85 36.2 8.52 25.5 3.95 24.7 3.96 190 8.33 0.84 0.94 22.30 1.79 1.05 0.98 0.98 Q5 Ⅴ 232 775 38.3 167 32.2 7.12 33.0 6.17 33.2 8.13 24.8 3.94 25.5 4.00 218 9.02 0.88 0.77 26.81 1.76 0.96 0.98 1.19 注：稀土元素单位为μg/g，δCe=2*CeN/(LaN+PrN)，δY=2*YN/(DyN+HoN)，δEu、δGd的计算方法类似，N为经北美页岩标准化后的数据，北美页岩数据取自文献[31]。
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