Geochemical Characteristics Of Sediments In Typical Hydrothermal Areas Of Southwest Indian Ocean And Their Indicative Significance

The study of submarine hydrothermal activity plays an important role in the development of submarine resources.As a typical ultra-slow expanding ridge,the hydrothermal properties of the southwest Indian Ridge are more unique from other hydrothermal areas,and the research on its hydrothermal activities is still relatively lagging behind.Sediments in hydrothermal areas usually contain information about hydrothermal activities,and their geochemical characteristics can indicate the source of materials and hydrothermal activities,which are precious materials for the study of deep oceans.In this paper,we analyzed the mineral compositions and geochemical characteristics of major and trace elements in the sediments of typical hydrothermal areas in the southwest Indian Ocean,and explored the elemental characteristics,material sources,and hydrothermal activities.In this paper,a total of 12 sets of samples from four stations in the Longqi hydrothermal field（ROV03,ROV04）and Tiancheng hydrothermal field（ROV06,ROV09）of the southwest Indian Ocean were used for geochemical major and trace element,rare earth element and mineralogy tests.Combined with 23 sets of previously published data,the material sources of sediments and hydrothermal contributions were discussed by means of element content characteristics,characteristic element triangle diagram,rare earth element standardization diagram,and elemental hydrothermal contribution calculation.The main results and discussions of this study are as follows:（1）The sediment samples in the Longqi hydrothermal area are mainly calcareous,debris and hydrothermal sediments（containing hydrothermal components）.The sediment samples in the Tianchang hydrothermal area are mainly detrital and hydrothermal derived sediments.In terms of geochemical characteristics,the contents of Fe,Mn,P and Ti O₂in non-biogenic sediments are higher than those in calcareous biogenic sediments.The vertical distribution of element contents in the columnar sediments of ROV03 station in Longqi hydrothermal area and ROV09 station in Tiancheng hydrothermal area is not consistent,and the element contents decrease from the top to the bottom.The element contents of columnar sediments at ROV03 station in Longqi hydrothermal area vary greatly from 4 cm to 6 cm,showing obvious stratification.The contents of Fe,Mn,Cu,and S decrease significantly from the top to the bottom,while the content of Ca increase significantly.Compared with the upper continental crust,the non-biogenic sedimentary samples in the study area are rich in clastic diagenetic elements and metal elements,while the hydrothermal deposits are rich in metal elements compared with the clastic sediments.The total amount of rare earth elements in the study area is generally low,which is characterized by enrichment of light rare earth elements and negative Ce anomalies.The Eu anomalies vary with different stations.（2）In terms of material sources,the sediments in the study area are composed of calcareous biogenic,clastic components,hydrothermal components,and chemical elements formed by seawater interaction.The calcareous biogenic deposits are mainly pelagic sediments,which are affected by calcareous biogenic deposition.The material sources of detritus include basaltic detritus,ultramafic detritus,biodetritus and terrigenous detritus.The sediments in the Longqi hydrothermal area can be divided into three categories:pelagic sediments mainly derived from biological debris,clastic sediments affected by basalt debris and hydrothermal sediments affected by hydrothermal activities.The samples are generally affected by seawater.The sediments at ROV06 station in the Tiancheng hydrothermal area are affected by the mixed input of terrigenous clastic and basalt clastic,while the sediments at ROV09 station are mainly affected by hydrothermal activity and ultramafic debris input.（3）In terms of hydrothermal contributions,the influence of hydrothermal activity on the sediment samples at ROV04,ROV09,ROV03 and ms-1 stations in the study area decreases successively.The sediment samples at ROV04 and ROV09 are mainly derived from hydrothermal input,while the sediment samples at ROV03 and ms-1 are affected by hydrothermal input and other material sources.The content of hydrothermal Mn and Cu in the sediments of Longqi hydrothermal area is relatively high,while in Tiancheng hydrothermal area,the contents of hydrothermal Co and Mo are higher in the sediments.Fe,Mn and Cu are the main elements affecting the sediments in Longqi hydrothermal area.The hydrothermal sources of the sediments in the study area are all fine colloids of iron and manganese oxide from non-buoyant plumes.The hydrothermal sediments in the study area are different from other hydrothermal areas in terms of ultramafic components,and the hydrothermal sediments in Longqi hydrothermal area are more Mn and Cu enriched than those in other hydrothermal areas.