海洋地质与环境重点实验室知识库

    本研究采用粒度分级、自生铁锰相提取、元素与同位素分析等方法，对冲绳海槽中段热液区7个站位表层沉积物开展的全样和不同粒级子样（<2μm、2~10μm、10~63μm、>63μm）开展研究，探讨海槽中段表层沉积物元素地球化学特征及其对物源和热液活动的指示。

    距热液喷口越近的表层沉积物，其Fe、Cu、Zn、Pb、Ba和As等元素越富集。其中，T4站位沉积物为典型含金属沉积物。区内沉积物化学风化程度中等偏上，反映温暖湿润的环境，其母岩成分近似花岗闪长岩。样品稀土配分曲线整体较为平坦，部分样品显示与热液流体相似的正铕异常。结合(La/Sm)_N、(La/Yb)_N比值的结果，初步判断研究区内表层沉积物的陆源组分主要来自于台湾岛与黄河。

    随着粒径增大，样品粒级子样中自生铁锰相的主、微量元素含量逐渐降低，即<2μm粒级组分内元素含量最高。此外，各粒级自生铁锰相都表现出中稀土富集的特征。<2μm和2~10μm粒级自生铁锰相明显富集热液来源的铝、钙等元素，且部分元素如P、V、Cu、Cr等与Fe密切相关。结合铁锰氧化物三元图的判别结果，确定绝大部分子样中自生铁锰相受热液影响，为热液成因，少数为水生成因。

    自生铁锰相在形成过程中，从热液流体和海水中清扫稀土元素，使其负铈异常减弱。但细粒级子样（<2μm和2~10μm）自生铁锰相的稀土元素总含量未随着与热液喷口间距离增加而升高。综合主量、微量元素和Sr、Pb同位素特征，确定热液活动对沉积物元素组成影响明显，热液物质随流体的运移方式主要为悬浮移动。<2μm粒级中，自生铁锰相所记录的热液活动信号最强，且区内岩浆、沉积物体系均为该粒级子样的自生铁锰相的提供部分的Pb。各粒级子样自生铁锰相的稀土以及Sr、Pb同位素指标显示，随子样粒度的减小，其自生铁锰相记录的热液信号逐渐增强，具体表现为：>63μm子样地球化学指标信号指示作用不明显；10~63μm子样中主要表现为黄河物质的信号；2~10μm子样中则为台湾河流和黄河的陆源物质信号；<2μm子样中可识别出冲绳海槽中段岩浆、热液信号和台湾陆源物质信号。

Through grain size classification, authigenic Fe-Mn phase extraction, and other methods, the samples and their authigenic Fe-Mn phase (<2μm, 2~10μm, 10~63μm, >63μm) of 7 stations in the hydrothermal area of ​​the middle Okinawa Trough were fully analyzed. To explore the elemental geochemical characteristics of the samples and their indications for provenance and hydrothermal activity.

The closer the sediments in the area are to the hydrothermal vents, the more elements such as Fe, Cu, Zn, Pb, Ba, and As are enriched. The sediment at station T4 is typical metalliferous sediments. The chemical weathering degree of the sediments in the area is moderate. It reflects a warm and humid environment and its parent rock composition is close to granodiorite. The standardization curve of REE by North American Shale is relatively flat. Some samples show positive europium anomalies similar to hydrothermal fluids. This pattern and ratios of (La/Sm)_N and (La/Yb)_N indicate that the terrigenous component of the samples in the middle Okinawa Trough comes mainly from the Yellow River and Taiwan River.

As the grain size increases, the element content in the authigenic Fe-Mn phase gradually decreases. The <2μm component has the highest element content. In addition, the authigenic Fe-Mn phases of each grain size all show the characteristics of enrichment of middle REE. The authigenic Fe-Mn phases of <2μm and 2~10μm components are obviously enriched in trace elements such as Al and Ca from hydrothermal fluids. In addition, some elements such as P, V, Cu, and Cr are closely related to Fe. Combined with the Fe-Mn oxide ternary diagram, it is determined that the authigenic Fe-Mn phase in most of the sub-samples (<2μm, 2~10μm, 10~63μm, >63μm) is caused by hydrothermal formation, and a few are caused by hydrogenous formation.

During the formation process of the authigenic Fe-Mn phase of the sub-samples, the REE are removed from the hydrothermal fluid and seawater, so that the negative cerium is abnormally weakened. However, in the fine-grained sub-samples (<2μm and 2~10μm), the REE content of the authigenic Fe-Mn phase did not increase with the increase of the distance from the hydrothermal vent. Synthesizing their elements and Sr, Pb isotopic characteristics, it is determined that hydrothermal activity has a significant impact on the elemental composition of sediments and that suspension is the main transportation approach of hydrothermal material. In the <2μm component, the authigenic Fe-Mn phase recorded the strongest signal of hydrothermal activity. Also, the magma and sediment system in the area provided part of the Pb for the authigenic Fe-Mn phase. The REE and Sr-Pb isotopic indexes of the sub-samples from the Fe-Mn phase show that the hydrothermal signal recorded by the authigenic Fe-Mn phase gradually increases with decreasing grain size. The geochemical indicator signal of >63μm component is not obvious; the signal of Yellow River sediments can be identified in 10~63μm component; the signals of Taiwan and Yellow River terrestrial materials can be identified in 2~10μm component; the signals of magma and hydrothermal fluids in the area and signal of terrestrial materials from Taiwan River can be identified in <2μm component.