| The fluid circulation in the deep ocean ridge triggers the exchange of material and energy between lithosphere and seawater,promotes the formation of various altered minerals and the precipitation of polymetallic sulfides.Hence,it has an important impact on the ocean's geochemical reservoirs.In this thesis,detailed petrologic,mineralogical and geochemical studies have been conducted on the altered abyssal peridotite that collected from the Tianxiu hydrothermal field of the slow-spreading Carlsberg Ridge,Northwest Indian Ocean,to reveal the indication of alteration processes and the element migration to mineralization by the geochemical and mass balance calculation methods.Based on the analyses of primary and secondary minerals' texture and composition,the protolith of the studied peridotite is harzburgite,which is consist of olivine,orthopyroxene,minor clinopyroxene and spinel with average vol% of 73: 24: 2: 1.The studied samples have low Al2O3/Si O2(0.01-0.04)and Mg O/Si O2(0.91-1.00)ratios and the content of clinopyroxene is very low.Besides,the samples are also characterized by relative enrichment of Light Rare Earth Element(LREE)to Medium Rare Earth Element(MREE)and Heavy Rare Earth Element(HREE).The positive correlation between LREE and LILE and the decoupling between Pb and U with High Field Strength Element(HFSE)and LREE are also identified.The above results indicate that the samples have undergone not only a high degree of partial melting,but also extensive hydrothermal alteration and weathering.The Water/Rock(W/R)ratios calculated from whole-rock Sr isotope data were used to quantify the contribution of hydrothermal fluid to the enrichment of REE and Fluid Mobile Element(FME)in the samples.The results show that the hydrothermal fluid contributes significantly to the enrichment of LREE and FME in the rock.The characteristics of trace elements in secondary minerals and veins reflect the properties of hydrothermal fluids.The concentrations of As,Ba,Cs,Li,U,Pb,Sr and LREE in bastite-serpentine-serpentine veins change regularly.Calcite veins also show the enrichment of U,Pb,Sr and Ba.These suggest that the fluid components are similar during alteration and the differences are caused by different mineral types.We built geochemical reaction models to predict the precipitation of secondary minerals during alteration based on the petrological and chemical analysis.The results show that the decomposition of orthopyroxene into bastite is favored at temperature > 340 ?,followed by the alteration of olivine into serpentine as temperature decreases,which contribute to the formation of brucite,magnetite and the release of H2(g)into the fluid as well.When the system changes from the rockdominated system to the fluid-dominated system,serpentine veins and calcite veins fill the fractures in the rock.Based on in-situ analyses,geochemical models and mass balance calculation,we propose the characteristics of element migration during the alteration of abyssal peridotite in the Tianxiu hydrothermal field.During the formation of bastite,elements like As,Ba,Cs,Li,U,Pb,V,Cr,Cu,B,Rb,Zr,Sr and LREE in the fluid migrate into bastite,while Co,Zn,Ni are released into the fluid.As the alteration continues,more As,Ba,Cs,Li,U,Pb,Cu,B,Rb,Zr,Sr and LREE migrate into serpentine from the fluid,while V,Cr,Co,Ni,Zn migrate from serpentine.With the increase of W/R ratios,serpentine veins and calcite veins that enriched in As,Ba,Cs,U,Pb,Cu,B,Sr and LREE,but depleted in V,Cr,Co,Zn and Ni precipitate from the fluid.The depletion of these elements suggests they may be involved in the precipitation of sulfide,while those enriched in secondary minerals and veins indicate high concentrations in the fluid.Especially,the enrichment of Cu,Pb and As indicates that the Tianxiu hydrothermal field has the metallogenic potential to form Cu-dominated sulfide,followed by Co,Zn,Ni-bearing sulfide. |
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