Discussion On Quantitative Description Of Content Of Compatible Elements In Different Rock Types And Weathering Degrees

It is of great significance for the delineation and interpretation of geochemical anomalies to explore the changes of trace elements in the process from rocks to weathered rocks,soils to stream sediments.On the basis of previous studies,this paper collects the data of weathering profiles and rock samples from all kinds of lithologies in China.By analyzing the weathering profiles and rock samples of different lithologies,and considering the influence of lithologies and weathering,a new empirical equation is tried to quantitatively characterize the compatible elements V,Cr,Co,Ni:C?10[a*]lg(Al2O3/Ti)+b]+[10(c*WIG+d)-10(c*WIGp+d)]In the formula,C is the element content of V,Cr,Cr and Ni,the unit is ?g/g,a,b,c and d are fitting parameters,in which the lithology is from ultrabasic to acid,the values of C and D are different at different lithology,the value range of Al2O3/Ti is 0.0011?0.0348,the value range of wig is 43.87?118.2,and the value of WIGp is the average value of wig of the fresh rock of the sample lithology.The former and the latter two parts of the empirical equation respectively represent the influence of different influencing factors on the element behavior in the weathering process.The former part represents the element content of the parent rock of the research object,and the latter part represents the change value of the weathering products of different weathering stages or the element content of the rock relative to the element content of the parent rock.This equation can be used to predict the background values of rocks and their weathering products from ultrabasic and basic rocks to intermediate basic rocks,neutral rocks,intermediate acid rocks,acid rocks and various lithology.Based on the form of this empirical equation,it is found that there are great differences in the change rate of elements with weathering index WIG in different lithology.It is guessed that the weathering index WIG is too sensitive,which leads to the limitation that WIG is only suitable for igneous rocks.Therefore,a new weathering index WII(Weathering Index of Igneous rock)is proposed to characterize the weathering process of igneous rocks.WII=(Na2Om+K2Om)/(Fe2O3m+Al2O3m)*100,in which the mass of oxide is molar content.The weathering index WII of igneous rocks shows a good exponential relationship among Cu,Pb,Zn,Cd,V,Cr,Co,Ni,Cu,Pb,Zn,Cd,Zr,Nb,Th,U and other elements in the process of weathering.When fitting the empirical equation for predicting the background value of elements,WII can effectively contract due to the difference of the variation rate of element content with WIG due to different lithology of the samples.After analyzing that there is an exponential relationship between WII and Al2O3/Ti,the average value of WII corresponding to different lithology of each sample can be estimated by the value of Al2O3/Ti,so the empirical equation can be simplified as follows:C=10[a*lg(x)+b]+[10(e*y+f)-10[e*(49.874*lgx+171.782)+f].where x is Al2O3/Ti;y is WII;a,b,e,and f are the fitting parameters respectively;and the value range of WII is 20?80.The range of Al2O3/Ti value is 0.0011?0.0348.It is proved that the fitting result has good statistical significance.The improved empirical equation does not need the data of parent rocks and is suitable for most igneous rock samples.This equation represents the geochemical background values of trace elements in samples due to different lithology and weathering degree.