IOCAS-IR
东太平洋海隆13°N地区含金属沉积物中蒙皂石成因
荣坤波
学位类型博士
导师曾志刚
2018-05-28
学位授予单位中国科学院大学
学位授予地点中国科学院海洋研究所
学位专业海洋地质
关键词含金属沉积物,元素赋存状态,蒙皂石成因,硅同位素,稀土元素
摘要

对东太平洋海隆13°N洋中脊西侧含金属沉积物(站号:E271)进行矿物学和地球化学分析,并与前人在洋中脊轴部地区的热液沉积物研究结果对比,分析相同热液区远端含金属沉积物和近喷口含金属沉积物的不同成因,及其对元素赋存状态造成的差异,通过对粘土粒级组分(< 2μm)的研究,探究自生蒙皂石的成因,探讨自生作用过程中Si的物质来源,以及Si同位素和稀土元素的地球化学行为。

E271站位沉积物是远端含金属沉积物,由非浮力热液羽状流中颗粒物沉降所形成的;而17A-EPR-TVG1站位沉积物是近喷口含金属沉积物,由黑烟囱或者热液硫化物丘状体崩塌、堆积,或者由热液羽状流中Fe-Mn氧化物和硫化物快速沉淀而形成的,近喷口沉积物比远端沉积物更富集FeCuZn等元素。元素在两种含金属沉积物中的赋存状态基本相同,除了FeCuZnMoREE等元素在远端沉积物中主要存在于Fe-Mn氧化物相,在近喷口沉积物中主要在残留相中。远端沉积物中REE页岩标准化配分模式与海水相似,表明REE主要来自海水,而近喷口沉积物中REE配分模式与热液流体相似,说明REE以高温热液流体来源为主。

受海底热液活动和靠近陆地影响,E271站位含金属沉积物中粘土矿物来源复杂,伊利石、绿泥石和高岭石可能来自北美洲或中美洲,通过河流或风搬运而来,而蒙皂石极可能是在含金属沉积物中通过热液来源Fe羟基氧化物和二氧化硅以及海水发生反应形成。二氧化硅可能来自硅质微体生物碎屑(硅藻或者放射虫)、热液流体或碎屑矿物,其δ30Si值都高于自生蒙皂石,因为自生作用过程中Fe羟基氧化物选择性吸附较轻的Si同位素而导致硅同位素分馏。E271粘土粒级组分中Sm/Fe比值低于含金属沉积物、远端热液羽状流颗粒物和沉积物Fe-Mn氧化物相中的Sm/Fe比值,表明蒙皂石自生作用过程中Fe羟基氧化物吸附的稀土元素发生迁移,极有可能是因为REE离子半径过大不能类质同象替换蒙皂石的四面体或八面体位。

其他摘要

Mineralogy and element geochemistry of a 43 cm long E271 sediment core collected from the East Pacific Rise (EPR) near 13°N were studied to better understand the differences in the orgin and element occurrence states between near-vent and distal metalliferous sediments in the same hydrothermal fields by comparison with the hydrothermal sediments from the EPR axis. The clay minerals (< 2μm) in the metalliferous sediments were analyzed to investigate the smectite formation, and to discuss the origin of Si and the geochemical behavior of the Si isotope and REE during the authigenic process.

Sediment samples collected from E271 site, along the ridge flank on west sides of the EPR are distal metalliferous sediments that result from hydrothermal non-buoyant plume fallout. However, samples (Site 17A-EPR-TVG1) from the slope of a hydrothermal mound on the EPR axis are near-vent metalliferous sediments that are formed by mass-wasting and erosion of sulfide edifices, and hydrothermal plume fallout of sulfides. Near-vent metalliferous sediments are richer in Fe, Cu, and Zn than distal metalliferous sediments. In terms of sequential leaching, although most of element occurrence states are basically similar in the two kinds of metalliferous sediments, Fe, Cu, Zn, Mo, Y, and REE are mainly present in the residual fraction of near-vent sediments, they mainly occur in the Fe-Mn oxide fraction of distal sediments. The shale-normalized REE distribution pattern of the sediments shows a peculiar positive Eu anomaly similar to those of hydrothermal fluids, suggesting a hydrothermal origin, but those of distal sediments show Ce depletion, characteristic of seawater.

Due to the influence of seafloor hydrothermal activity and close to continent, the sources of clay minerals are complex. Illite, chlorite, and kaolinite are suggested to be transported from either North or Central America by rivers or winds, but smectite is authigenic. Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments. The SiO2 that participates in this reaction may be derived from siliceous microfossils (diatoms or radiolarians), hydrothermal fluids, or detrital mineral phases. And their δ30Si values are higher than those of authigenic smectites, which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Fe-oxyhydroxides. Sm/Fe mass ratios (a proxy for overall REE/Fe ratio) in E271 clay minerals are lower than those in metalliferous sediments, as well as distal hydrothermal plume particles and the Fe-Mn oxide fraction of the E271 sediments. This result suggests that some REE are lost during the smectite formation, perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure.

学科领域海洋地质学
学科门类理学::海洋科学
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/154453
专题中国科学院海洋研究所
第一作者单位中国科学院海洋研究所
推荐引用方式
GB/T 7714
荣坤波. 东太平洋海隆13°N地区含金属沉积物中蒙皂石成因[D]. 中国科学院海洋研究所. 中国科学院大学,2018.
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