Elemental Migration Characteristics and Chemical Weathering Degree of Black Shale in Northeast Chongqing,China
Wu Xiyong
Ling Sixiang
Ren Yong
Liao Xin
Zhao Siyuan
Li Xiaoning
Abstract：Black shales are special sedimentary rocks that contain significant amounts of organic matter and sulfide minerals;whereas, less is known about the elemental mobility and weathering mechanism in black shales during chemical weathering.For understanding the chemical weathering processes of black shale in different geographic locations,a number of Shuij ingtuo Formation black shale sam-ples were collected from three weathering profiles,namely,profile A (mid-ridge),profile B (near mountaintop),and profile C (val-ley)at Chengkou County,northeastern Chongqing Province in this study.The major element concentration,mineral composition,and bulk density of each sample were analyzed by XRF,XRD and chemical analysis methods.The results of geochemical analysis indicate that these weathering profiles were significantly depleted of Ca,Mg and Na in all samples but enriched in Al on the surface weathering zone by calculated mass transfer coefficient (τ)and mass transfer flux (Mj,flux )in all samples.From the perspective of mineralogy,the calcite,dolomite and plagioclase were dissolved in acid solution produced from pyrite and organic matter,and the illite,gypsum and Fe-oxides were formed during chemical weathering process.From the graphical representations of Na∕K-CIA,K∕Ca﹡-Al∕Na,A-CN-K, and A-CNK-FM show that profile A is located in the weak chemical weathering stage of Ca removal.Profile B has reached the weak to moderate chemical weathering stage and being initial stage of Ca and Na removal.Profile C has undergone the early stage of Ca and Na removal,and reached the Si removal (desilication)stage of moderate to strong chemical weathering.Different chemical weathering inde-xes of CIA,CIW,PIA,and MWPI were calculated in three profiles,showing that the chemical weathering sequence in all profiles is C>B>A.
Keywords：black shalechemical weatheringelement migrationmineral transformationchemical weathering indexgeo-chemistrypetrology
Publication Date：2016-01-01
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：16( 218-233 )