| 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...
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