【作者】 蔡毅华；

【导师】 黄奕普；

【作者基本信息】 厦门大学 ， 海洋化学， 2002， 博士

【摘要】 本文对中太平洋海山区、西太平洋麦哲伦海山区富钴结壳的生长年代学、地球化学进行了系统、深入的研究，探讨了富钴结壳的生长对太平洋古海洋演化的响应，富钴结壳的成因和元素富集机理。主要研究结果如下： 1．太平洋富钴结壳的起始生长年代与生长速率 本文研究结壳中²³⁴U相对于²³⁸U亏损或平衡，这可能是结壳中²³⁴U优先于²³⁸U被海水淋滤造成的。所研究结壳的Th／U)_C.R为0.94-4.13，平均为2.32。采用²³⁰Th_ex法、²³⁰Th/²³²Th法得到太平洋富钴结壳的生长速率为1.93-4.97mm／Ma，与Co地层学的结果十分吻合。同时利用Co地层学得到结壳CAD15与MHD59的起始生长年代分别为～27Ma B.P.和～46Ma B.P.。 2．富钴结壳中元素的赋存形态与聚类分析 利用连续浸取法研究了未磷酸盐化结壳与磷酸盐化结壳中元素分别在碳酸盐及可交换相、Mn氧化物相、Fe氧化物相和碎屑相中的赋存状况。与未磷酸盐化结壳相比，磷酸盐化结壳的化学组成有显著的差别，最显著变化是所有元素在细晶碎屑相中的含量比例增大。对结壳进行Q型聚类分析，得到结壳层段的划分结果与样品的宏观构造和地球化学变化一致。同时，R型聚类分析结果表明结壳中的元素可分为Fe氧化物组分、碎屑组分、Mn氧化物组分、CFA组分及生源组分。 3．磷酸盐化事件对富钴结壳REE地球化学特征的影响 磷酸盐化事件对富钴结壳REE地球化学特征具有显著的影响：（1）导致结壳中稀土含量的增加。（2）对草酸相和HF相的REE分布模式有较大影响，表现为造成轻重稀土之间的分馏，以及Y异常的差异。（3）导致结壳Y／Ho比值增大，其中结 ＃4壳MHD59的磷酸盐化层段Y／H。重量比都大于28，而结壳CAD15的磷酸盐化层段YMO比值并未全部大于 28，反映了结壳 MHD59磷酸盐化的强度大于结壳 CAD。 4．未磷酸盐化结壳中稀上元素地球化学参数的时间演化及古海洋学意义 结壳CAD和MHD59的未磷酸盐化层段中SCe及LREEMREE比值都随时间逐渐减小，并夹杂有短期的增大。反映了两个海区～17Ma B卫以来的氧化性条件及结壳水成作用呈现波浪式的减小；结壳CAD15的6Ce总体上大于结壳MHD59，反映了中太平洋海山区底层水的氧化性强于麦哲伦海山区，这与AABW的流动方向一致。两个结壳TREE的时间演化图象类似，也反映了两个结壳所处环境的变化相似。 5．富钻结壳对古海洋演化的生长响应 全球变冷、碳酸盐溶解速率增加、底层流活动加强、火山活动频繁发生等古海洋学事件的发生，使结壳生长要求的条件得到满足，从而启动了结壳的初始生长及未磷酸盐化层段的重新生长。新生代太平洋发生的磷酸盐化事件则抑制了结壳的生长，并明显影响了结壳的矿物学组成和地球化学特征。结壳生长时随海山经过东北信风带，接受了大量来自大陆的风尘输入，使中部疏松层得到发育，而该层发育时间的不同则反映了风尘来源的不同及海山移动轨迹的差异。 6．宫钻结壳的成因与元素宫集机制 根据本研究的浸取实验结果，进一步证实结壳的成因主要是胶体化学成因。结壳的生长可用一个双阶段胶体一化学吸附模型来说明：第一阶段，水柱中形成MnO。／Fe（OH）。，Fe（OH）。H（OH）4和Fe（OH）。／AI（OH）。／SIO。等混合胶体，这些胶体根据各白的表面物理化学特征吸附和富集不同的元素；第二阶段是混合胶体在海山基岩上以氧化物或氢氧化物的形式沉积，吸附的元素结合进矿物点阵。然后通过自催化或自反馈的方式生长。生物活动在富钻结壳生长中可能起着重要作用。更多还原

【Abstract】 The geochronology and geochemistry of Co-rich crusts dredged respectively from the central Pacific seamounts and the Magellan seamounts were studied in this work. Moreover, the growth response of crusts to the Cenozoic evolution of Pacific Ocean, the genesis and elemental enrichment mechanisms of crusts were investigated too. The principal results are as follows:1. The growth rates and geochronology of Co-rich crusts from Pacific OceanThe activities of 234U are deficient or equilibrium relative to 238U in studied crusts. It may be caused by the preferentially leaching of 234U relative to 238U by seawater. The content ratios of Th/U of crusts are in the range of 0.94 to 4.13, with an average of 2.32. An average growth rate of 1.93-4.97mm/Ma for Pacific Co-rich crusts was obtained by 230TheX and 230Thex/232Th dating of outermost 1-2 millimeters of these crusts, which were consistent with the Co chronometer results. The growth of the crusts CAD 15 and MHD59 were started at 27Ma B.P. and 46Ma B.P., respectively, based on the Co chronologies.2. The speciation of elements and cluster analysis in Co-rich crustsThe speciation of elements in non-phosphatized and phosphatized crusts was studied using sequential leaching procedure. As a result, elements in crusts were distributed to exchangeable cations and Ca carbonate phase, manganese oxides phase, amorphous Feoxyhydroxides phase, and crystalline detrital phases (oxides, silica, aluminosilicates), respectively. Chemical composition of non-phosphatized and phosphatized crust layers occurred significant change that the proportions of elements content in detrital phases of phosphatized crusts increase obviously. The classification of layers in Co-rich crusts obtained by model Q Cluster Analysis is consistent with the variations of macrostructure and geochemistry in the crusts, while the elements in the crusts can be divided into several groups by model R Cluster Analysis, which represent Fe oxides, detrital, Mn oxides, carbonate fluorapatite and biogenic components, respectively.3. The effects of phosphatization on the REE geochemical features of Co-rich crustsThe REE geochemical features were altered significantly by phosphatization, including increase of REE content and Y/Ho weight ratio, change of REE distribution patterns of oxalic and HF phases, different Y anomalies and the fractionation between LREE and HREE. Y/Ho weight ratio of phosphatized layers of crust MHD59 all excess 28, while those of some phosphatized layers of crust CAD 15 were lower than 28. It means the episode of phosphatization imposed more influence on crust MHD59 than on crust CAD 15.4. The temporal variations of REE geochemical parameters in non-phosphatized crusts and its implication of paleoceanographyThe positive Ce anomalies in non-phosphatized layers of both crusts studied decrease from bottom to top with minor fluctuation, like LREE/HREE ratios, reflecting the decrease of paleoredox potential of seawater and hydrogenous intensity of crusts with minor fluctuation from 17Ma B.P. to present. The temporal variations of REE contents inboth non-phosphatized crusts are similar, indicating the similarity of these two environments in which crusts grew. The Ce anomalies in non-phosphatized layers of crust CAD 15 was more distinct than those of crust MHD59, reflecting the more oxic in the bottom water of central Pacific Seamounts than those of Magellan Seamounts. It is consistent with the flow path of AABW, which was flow from the central Pacific Seamounts to the Magellan Seamounts.5. Growth response of Co-rich to the evolution of Cenozoic PacificCo-rich crusts as well as the non-phosphatized layers started to grow when the marine conditions were suitable, for example, cool climate, high carbonate dissolution rate, strong bottom current and frequent volcanic activities, etc. However, Growth of crusts was inhibited by phosphatization, and the mineralogical and geochemical features of crusts were altered by phosphatization too. The formation of更多还原