Elemental distribution pattern and forming mechanism of the two types of phosphates in ferromanganese crust in Western Pacific Ocean and their implications
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摘要: 太平洋铁锰结壳在中新世以前大多经历了磷酸盐化，一般认为其形成代表了高生产力时期富磷组分对先期已形成结壳的交代作用，即磷酸盐化事件的产物。然而，前人对铁锰结壳磷酸盐化的机制研究多基于点-线分析元素或同位素，对铁锰结壳生长结构及二维元素分布特征研究较少。本研究以西太平洋水成铁锰结壳MDD53为研究对象，利用电子探针（EPMA）进行高分辨率元素定量分析，并结合微区X射线荧光光谱（μXRF）面扫得到结壳元素二维分布图。电子探针和μXRF结果揭示了该结壳中下部发生了磷酸盐化。μXRF结果显示该结壳存在两种不同类型的磷酸盐富集特征：第一类是结壳顶部中出现零星的Ca、P富集，同时伴随着强烈的后期改造特征，如结壳呈破碎状结构，Fe丢失、Mn相对富集。电子探针数据进一步表明，在成岩改造中，Co、Ni等微量元素发生了相对富集，而Pb则发生了丢失，这一现象反映了这些元素与Fe、Mn不同的亲和力，与大量有机质在结壳海水界面分解导致的成岩扰动特征相符。第二类是结壳底部连续出现的纹层状Ca、P富集特征，且无后期扰动现象，暗示 MDD53结壳底部磷酸盐是同沉积/早成岩作用的产物，而不是反映后期磷酸盐化事件成岩改造的影响。这类未报道过的同沉积磷酸盐可能表明结壳形成早期，其水深相对较浅时，海水中大量磷酸盐与铁锰氧化物胶体共沉淀而产生的磷酸盐富集现象，因而为重建晚白垩世-早新生代高时间分辨率古海洋演化提供了可靠载体。Abstract: Most of the Pacific ferromanganese crusts have experienced phosphatization before Miocene, presumably by impregnation of phosphorus-rich component into the old crust section during the period of high productivity, that is called the product of phosphatization events. Most of the previous studies on phosphatization were based on point/line analysis of element contents or isotopes, while few studies have been devoted to the growth structure and 2-dimentional element distribution. In this study, a hydrogenetic ferromanganese crust (MDD53) sampled from the western Pacific Ocean was analyzed by electron probe X-ray microanalysis (EPMA) and micro–X-ray fluorescence scanning (μXRF), generating high resolution quantitative data on element concentrations and 2-dimensional element maps, respectively. The results of EPMA and μXRF reveal that the middle to lower part of the crust was phosphatized. The μXRF map shows that there are two types of phosphates. The first type shows sporadic enrichment of Ca and P in the top part of the crust, accompanied by strong post-depositional alterations, such as broken structure, Fe loss, and relatively Mn enrichment. The EPMA data further shows that trace element enrichment of Co and Ni as well as depletion of Pb in the phosphatized area. This observation reflects different affinity of these elements with Fe and Mn oxides, which is consistent with the hypothetic phosphatization mechanism of organic matter degradation at the crust-seawater interface and the resulting alteration of the preformed crust. The second type of phosphate is characterized by an unreported structure of continuous Ca and P enriched laminae located at the bottom of the crust. The lack of post-depositional alteration of the growth structure suggests that this type of phosphate is of syn-depositional or early diagenetic in origin, which is distinctly different from post-diagenetic alteration. The syn-depositional phosphate may indicate an early stage coprecipitation of phosphate and ferromanganese oxide colloids in a relatively shallow water depth, making it a reliable paleoceanographic indicator for high temporal resolution studies of the Late Cretaceous-Early Cenozoic period.
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Keywords:
- ferromanganese crust /
- EPMA /
- μXRF /
- phosphate
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图 2 MDD53顶部和底部的微区XRF元素含量分布图
亮色代表含量相对高而暗色代表含量相对低；下图为Si、P、Ca元素局部区域放大图，图中白色方框对应放大位置。
Figure 2. Micro–X-ray fluorescence images of the top to bottom section of MDD53
Light colors represent relatively high content and dark colors represent relatively low content. The bottom of figure are enlarged the parts of Si, P, Ca Micro-X-ray fluorescence images, the white box in the figure corresponds to the zoom in position.
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图 3 铁锰结壳MDD53电子探针元素分布
Figure 3. Element composition of MDD53 analyzed by electron probe
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图 4 MDD53 EPMA分析元素（a） P和Ca、（b） Co和Mn、（c） Ni和Mn、（d） Pb和Mn、（e） Si和Fe、（f） Pb和Fe相关图
Figure 4. Relation between （a） P and Ca、（b） Co and Mn、（c） Ni and Mn、（d） Pb and Mn、（e） Si and Fe、（f） Pb and Fe in MDD53 analysed by EPMA
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图 5 MDD53磷酸盐化部分与未磷酸盐化部分之间的元素平均值相对差异
a. 结壳底部元素均一化结果，b. 结壳顶部2元素均一化结果，c. 结壳底部1元素均一化结果，d. 结壳底部2元素均一化结果。
Figure 5. Relative differences in average elemental composition of phosphatized parts to the non-phosphatized parts of MDD53
a. The normalized result of the bottom of the crust, b. the normalized result of top part 2, c. the normalized result of bottom part 1, d. the normalized result of bottom part 2.
表 1 MDD53各部分电子探针元素分布的均值
Table 1 The average elemental composition of different parts of MDD53 analyzed by electron probe
% 取样位置/mm Co Sr Pb Fe Mn Ni Si Al P Ca 顶部（0～63） 0.69 0.08 0.13 17.2 42.7 0.65 3.37 0.82 0.45 3.79 底部（63～119） 0.27 0.08 0.21 20.8 32.4 0.26 3.37 1.03 4.18 10.8 顶部1（ 0～7） 0.45 0.11 0.16 26.5 29.0 0.28 7.83 1.41 0.79 2.99 顶部2 （7～63） 0.71 0.08 0.13 16.0 44.3 0.69 2.83 0.75 0.41 3.89 底部1 （63～93） 0.30 0.08 0.22 20.6 34.0 0.29 3.39 1.00 3.40 9.27 底部2 （93～119） 0.23 0.08 0.19 20.1 30.4 0.22 3.33 1.06 5.22 12.8 
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