Revisiting the Mg isotopic systematics of siliciclastic components of sediments and sedimentary rocks:A new geochemical proxy of continental weathering in Earth's history  

作　　者：Tianzheng HUANG Bing SHEN Kangjun HUANG Meng NING Chao LI Jinzhuang XUE Yuanlin SUN Baoqi HUANG 

机构地区：[1]Key Laboratory of Orogenic Belt and Crustal Evolution,MOE,School of Earth and Space Sciences,Peking University,Beijing,100871,China [2]State Key Laboratory of Continental Dynamics,Shaanxi Key Laboratory of Early Life and Environments,Department of Geology,Northwest University,Xi’an,710069,China [3]Institute of Sedimentary Geology,Chengdu University of Technology,Chengdu,610059,China [4]Key Laboratory of Rhenium-Osmium Isotopic Geochemistry,Chinese Academy of Geological Sciences,Beijing,100037,China

出　　处：《Science China Earth Sciences》2024年第2期620-633,共14页中国科学（地球科学英文版）

基　　金：supported by the National Key Research and Development Program(Grant Nos.2022YFF0800200 and 2021YFA0718200);the National Natural Science Foundation of China(Grant No.42225304)。

摘　　要：The Mg isotopic compositions of siliciclastic components(δ^(26)Mg_(sili))of sediments and sedimentary rocks have been commonly used to constrain the intensity of continental weathering,based on observations of,(1)an upward enrichment of26Mg in modern weathering profiles,(2)preferential uptake of26Mg in soil clays,and(3)general positive correlations between weathering intensities andδ^(26)Mg_(sili)values.However,not all weathering profiles display an upward increase ofδ^(26)Mg_(sili),and not all soil clays enrich in26Mg,complicating the interpretations ofδ^(26)Mg_(sili)data of sediments and sedimentary rocks.To further explore the controls ofδ^(26)Mg_(sili)of sediments and sedimentary rocks,here we measuredδ^(26)Mg_(sili),mineralogical compositions and major element compositions of deep sea sediments from South China Sea(SCS)and carbonate rock samples from two late Paleozoic sections in South China.Carbonate samples show decreasing trends of both normalized Mg concentration(Mgnorm)andδ^(26)Mg_(sili)values with the increase of weathering intensity,while SCS sediments samples do not show any correlation between Mgnormorδ^(26)Mg_(sili)values and weathering intensity.A three-stages weathering model is developed to explain the observed Mgnorm andδ^(26)Mg_(sili)data.In the new model,weathering can be divided into three stages,(1)the preliminary weathering stage characterized by dissolution of primary minerals and formation of secondary clay minerals in saprolite,(2)the transitional stage with massive vermiculite and chlorite formation on the bottom of soil,and(3)the advanced stage showing dissolution of vermiculite and chlorite in the top of soil.The new model explains anδ^(26)Mg_(sili)increase and Mgnormdecrease with an increase weathering intensity in preliminary weathering stage and decrease of Mgnormandδ^(26)Mg_(sili)with increase weathering intensity in advanced weathering stage.The revised weathering model warrants the application of Mg isotope in the study of continental weathering in deep time.

关 键 词：Clay mineral Chemical weathering CIA Soil Weathering model 

分 类 号：P512.2[天文地球—地质学] P597