碳酸盐岩红色风化壳Ce异常特征及形成机理

THE CHARATERISTICS OF Ce ANOMALIES IN WEATHERING CRUSTS OF CARBONATE ROCKS AND ITS FORMATION MECHANISM

Ce在贵州碳酸盐岩红色风化壳剖面上部表现为正异常,在底部稀土超常富集层显示强烈亏损。通过对Ce的质量迁移系数的计算,结合相态分析和水化学分析结果,对碳酸盐岩红色风化壳中Ce的异常行为进行系统的分析后发现:在剖面上部,Ce3+氧化水解,水解产物主要与铁锰氧化物共沉淀,少量的与粘土矿物结合,使得Ce保留而相对显示正异常;在剖面底部,Ce负异常一方面来源于对基岩负异常的继承;更重要的是,Ce较其他轻稀土稳定,发生迁移富集的量远远小于La,Nd等轻稀土元素,导致Ce在底部富集时相对于其他轻稀土元素发生强烈亏损。而Ce与HCO3-络合迁出并不是导致碳酸盐岩红色风化壳中强烈Ce负异常的主要原因。

In this paper,Ce geochemical behavior has been studied systematically through REE mass transport coefficient,REE combining states and water chemistry of cave drip waters. In the upper part of weathering profiles,Ce^3＋ is oxidized into Ce^4＋ and hydrated,then mainly co-precipitated with Fe-Mn oxides and partly adsorbed by clay minerals ,showing positive anomalies ,whereas at the rock-soil boundary,Ce shows intensively negative anomalies. Two reasons are considered to be responsible for this phenomenon. Firstly,soil samples inherited Ce negative anomalies from the basement carbonate rocks. Secondly,more important reason is that in the weathering process Ce is more stable than the other LREEs and the Ce content in the downward leaching solution is much less than the other LREE contents such as La and Nd,resulting in intensively negative Ce anomalies when LREE are super-enriched at the rock-soil boundary. Consequently,the authors excluded the possibility of Ce migrating downward by the preferential formation of complexes with HCO3^- ligand.