Geochemistry Of Major And Trace Elements In Soils Derived From The Lower-cambrian Black Shales In The Western Yangtze Platform,China

This research is based on the the Lower-Cambrian black shales and the soils?defined as black shale soil in this paper?derived from the black shales in western Yangtze Platform.Major elements and trace elements in fresh black shales and the soils derived from the fresh black shales are analyzed by analytical techniques such as X-ray fluorescence spectrometer?XRF?and Plasma mass Spectrometer?ICP-MS?.By using the method of elemental statistical analysis,the difference of chemical composition between fresh black shales and black shale soil as well as the reason of the difference of chemical composition are discussed.On this basis,the geochemical mechanism of weathering-soil-forming of black shale is discussed.The following research results and understandings have been achieved:?1?The Lower-Cambrian black shales in western Yangtze Platform generally has similar major element composition to that of the upper crust,and it is enriched with trace elements such as Ba,V,Cr,Ni,Cu,Zn,Pb,U,Cs,while depleted of trace elements such as Sr,Zr,Hf.The total composition of REE changes greatly,and the overall performance shows that the composition of REE near to consistent with the average shale and upper crust,and the LREE are relatively enriched,HREE are relatively depleted,as well as shows weak Eu negative anomaly?the average value of Eu/Eu*is0.586?.?2?The composition of major elements in black shale soil is relatively homogeneous.Compared to the upper crust,on the one hand,the black shale soil is obviously enriched with major elements such as TiO₂,Al₂O₃,TFe₂O₃,P₂O₅ and LOI,while depleted of major elements such as MgO,Ca O and Na₂O;on the other hand,the black shale soil enriched with trace elements such as Ba,V,Cr,Ni,Cu,Zn,Pb,U,Cs,while depleted of trace elements such as Mn and Sr.The total composition of REE changes small,showing the same REE distribution model as that of black shale,and the total composition of REE is slightly high,and LREE is enriched,while the HREE is depleted,as well as shows weak Eu negative anomaly?the average value of Eu/Eu*is0.567?.?3?There are obvious differences in chemical composition between black shale and black shale soil.Compared to the fresh black shale,on the one hand,the black shale soil is obviously depleted of major elements such as MgO,CaO,Na₂O,P₂O₅,LOI and enriched with Al₂O₃,Fe₂O₃;on the other hand,the black shale soil is obviously depleted of trace elements such as Ba,V,Ni,Cu,Zn,Sr,and enriched with Th,while other trace elements in the soils show similar distribution patterns with that in the parent black shales.More importantly,the chemical composition of the soils is relatively homogeneous,whereas this of the fresh black shales is distinctly heterogeneous.?4?The obvious difference of chemical composition between the black shale and the black shale soil is caused by the change of the major elements composition.ANOVA analysis of variance shows that the order of the influence of the major elements on the difference is:CaO>Na₂O>LOI>MgO>Al₂O₃>Fe₂O₃>MnO>K₂O>SiO₂>P₂O₅,which results in leaching and secondary enrichment of elements in the process of weathering-soil-forming.?5?The difference of chemical composition between black shale and black shale soil is the result of weathering-soil-forming reaction.In the process of weathering-soil-forming reaction,the dissolution of labile minerals such as organic matter,pyrite,calcite,albite,and clinochlore,and the decomposition of silicate minerals such as feldspar may lead to the leaching and release of major components including CaO,LOI,Na₂O,MgO,K₂O,SiO₂ and trace elements including V,Cr,Ni,Cu,Zn,Sr,Ba;the secondary generation of minerals such as clay minerals and Fe-Mn oxides results in secondary enrichment of major components including Fe₂O₃,MnO,Al₂O₃ and trace elements including Pb,Cs,Co,Th,Sc,Ga,Rb,REE in the soil.These chemical processes lead to the leaching and secondary enrichment of black shale chemical components in the process of weathering-soil-forming,which results in obvious differences between fresh parent rock and soil.?6?Chemical differentiation and chemical homogenization occurred in the process of black shale weathering-soil-forming reaction.The process of chemical differentiation mainly includes:oxidation and decomposition of sulfide minerals?organic carbon;chemical alteration and decomposition of labile minerals such as clinochlore,calcite and phosphorus bearing minerals;hydrolysis of silicate minerals such as feldspar;the secondary generation of minerals corresponds to the alteration reaction of minerals mentioned above,on the one hand,the hydrolysis of silicate minerals such as feldspar provides material source for the formation of clay minerals,on the other hand,the oxidation and decomposition of pyrite and other sulfide minerals lead to the change of the valence state of Fe in the process of weathering-soil-forming reaction,forming secondary Fe-Mn oxides.The chemical homogenization is mainly manifested as:elements redistributed and migrated laterally during the process of mineral alteration and decomposition,which made the content of elements in the soil formed by weathering of the black shale with high element content is relatively reduced,while on the contrary,it is relatively increased,which results in the difference of elements in the black shale soil is significantly smaller than that in the parent rock,which made the content of elements in the black shale soil tend to be homogeneous.