扬子地台西缘下寒武统黑色页岩土壤元素地球化学特征

Elemental Geochemical Characteristics of Soils Derived from the Lower Cambrian Black Shales in the Western Yangtze Platform,China

本文分析扬子地台西缘黑色页岩土壤(BSS)及其成土新鲜黑色页岩(FB)的主、微量元素的地球化学特征,探讨黑色页岩土壤成土地球化学作用机制。结果表明,黑色页岩土壤明显亏损MgO、CaO、Na2O、P2O5和LOI(有机质),而富集Al2O3和Fe2O3等主量组分;且明显亏损Ba、V、Ni、Cu、Zn和Sr等微量元素,但其他微量元素则不然。与成土母岩相比,黑色页岩土壤的化学组成相对均一。主量组分对黑色页岩与黑色页岩土壤之间化学组成有不同的影响,这种影响与CaO、Na2O和MgO等碱质组分、LOI所代表的有机质等的淋滤释出,及Fe2O3和Al2O3等氧化物的次生富集有关,其影响程度顺序为:CaO>Na2O>LOI>MgO>Al2O3>Fe2O3>MnO>K2O>SiO2>P2O5。推断黑色页岩风化-成土过程中有机质、黄铁矿、方解石、钠长石、斜绿泥石等不稳定矿物的化学蚀变分解和长石等硅酸盐矿物的水解导致CaO、LOI、Na2O、MgO、K2O和SiO2等主量组分,以及V、Cr、Ni、Cu、Zn、Ge、Ba、Sr等微量元素的淋滤释出;而黏土矿物、铁-锰氧化物等次生矿物的次生富集,导致Fe2O3、MnO和Al2O3等主量组分,以及Pb、Cs、Co、Th、Sc、Ga、Rb、REE等微量元素在土壤中形成次生富集。故而黑色页岩可为环境重金属污染源,而黑色页岩土壤本身也可存在重金属污染。

This paper studied geochemical characteristics of major and trace elements in soils,which derived from the Lower Cambrian black shales and,thereafter,were named as black shale soils(BSS),and their parent rocks,the fresh black shales(FB)in the western Yangtze platform,China.To understand relevant geochemical processes of the soil formation,contents of major elements in BSS and FB and their variations were revealed by using geochemical statistical methods.The results showed that the major elements,MgO,CaO,Na2O,P2O5 and LOI,were significant depleted and the major elements,Al2O3 and Fe2O3,were enriched in BSS.The trace elements,such as Ba,V,Ni,Cu,Zn and Sr,were also significantly depleted in BSS,while other trace elements showed similar distributions in both BSS and FB.Compared to those of FB,chemical compositions of BSS were much more homogeneous.The chemical composition variations betweenBSS and their parent FB correlated differently with different major components,which might be resulted from the differentgeochemical behaviors of the mobile components,CaO,Na2O,MgO and LOI,and the stationary components,Fe2O3,MnO,and Al2O3.The contributions of major elements to the elemental compositions variation in BSS and in FB followedan order of CaO>Na2O>LOI>MgO>Al2O3>Fe2O3>MnO>K2O>SiO2>P2O5.Therefore,there might be two major geochemicalprocesses during the soil formation:(1)The dissolution of labile minerals,such as organic matter,pyrite,calcite,albite and clinochlore,and the decomposition of silicate minerals,such as feldspar,all of which led the leaching and releasingof major components,including CaO,LOI,Na2O,MgO,K2O,SiO2,and trace elements,including V,Cr,Ni,Cu,Zn,Sr,and Ba.The leaching of elements from FB might be a source of the environmental contamination of heavymetals.(2)The secondary enrichment of minerals,such as clay minerals and Fe-Mn oxides,resulted in the secondary enrichmentof major components,including Fe2O3,MnO and Al2O3,and trace elements,including Pb,Cs,Co,Th,Sc,Ga,Rb and REE,in BSS,leading the heavy metal contamination of BSS in FB distribution areas.