Geochemical Characteristics Of Sr-Nd-Pb Isotopic And Tracer Study Of Low Temperature Hydrothermal Sediments From The Southwest Indian Ridge

Submarine hydrothermal systems play an important role in the material and energy cycle between the solid earth and the ocean.In this paper,mineral assemblage and geochemical characteristics are used to determine whether the sediments in the southwestern Indian Ocean ridge are low-temperature hydrothermal sediments.Sr-Nd-Pb isotopes are used to trace provenances.Mineralization mechanism of sediments is discussed.Geochemical characteristics of sediments and BIF are compared and the relationship between them is discussed.In this study,modern hydrothermal Fe-Si sediments were collected from the vicinity of three hydrothermal vent systems of the ultra-slow spreading ridge of the Southwest Indian Ocean.Mineralogy(XRD,scanning electron microscopy,etc.),elemental geochemistry(major,trace and rare earth elements composition)and three stable isotopes(Sr,Nd and PB)were comprehensively analyzed.Firstly,this study systematically shows that both ancient and modern sediments related to hydrothermal activity have good affinity in mineralogy,geochemistry(including REE)and isotope geochemical composition,and can be regarded as their corresponding substances.The main minerals of sediment samples are nontronite,amorphous opal,2-line ferrihydrite,goethite and birnessite.nontronite and amorphous opal shows smooth filamentous structure and granular aggregation scale structure under scanning electron microscope respectively.The whole rock analysis shows that the sediments are rich in Fe,Si,Mn and other major elements while the contents of Al,Ca,Mg,Na and K are very low.The contents of Cu,Zn,Pb,Sr,Co,V and Mo are relatively high,while the contents of Hf,Cs,Th,Be,Hg and A are relatively low.The distribution curves of rare earth elements show that most of the samples have negative Ce anomalies and positive Eu anomalies,which have typical characteristics of hydrothermal source.According to the ⁸⁷Sr/⁸⁶Sr ratio,it can be determined that sediments are fully mixed with seawater before sedimentation.The sediments have a ⁸⁷Sr/⁸⁶Sr ratio close to seawater,indicating that they are the source of low-temperature hydrothermal dispersion rather than high-temperature hydrothermal plume.The ¹⁴³Nd/¹⁴⁴Nd of SWIR is relatively low,which represents the product of hydrothermal dispersion and seawater mixing.It may also be the product of mantle source,mid-ocean ridge basalt and seawater mixing.The similar mineralogical composition of modern hydrothermal Fe-Si-rich sediments compared with banded iron formation(BIF)as long as similar Fe-Si banded structures found in modern hydrothermal solutions,the characteristics of Al,Mg,K and Ca rich in Si and Fe but poor in Al,Mg,K,Ca,the distribution patterns of rare earth elements,the oxidation potential show that they are related to hydrothermal activities closely.