Geochemical Characteristics And Provenance Indication Of The Surface Sediments Near Hydrothermal Fields,Mid-Okinawa Trough

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.