网纹红土稀土元素地球化学特征的初步研究

Preliminary Study on Geochemical Behavior of Rare Earth Element of Plinthitic Red Earth in South China

稀土元素（REE）在揭示沉积环境,研究沉积物物源及成因方面具有重要的示踪意义。江西、湖南、浙江等地更新世加积型红土REE含量和分布特征的研究结果表明：①除结核外,红土各类样品的REE总量变化于158.2～224.0μg.g-1之间。由大至小排序为：无网纹棕黄色土〉无网纹均质红土〉网纹红土。网纹层内白网纹REE含量高于红网纹。结核样品REE异常富集,是网纹红土母质的3～6倍,Ce富集为主要贡献者。②来自中亚热带地区不同红土沉积剖面的各类网纹红土样品,其稀土分布模式相似,与上地壳（UCC）稀土组成相近。比较研究发现,网纹红土母质与黄土高原黄土、镇江下蜀黄土、西峰第三纪红粘土的稀土配分模式非常一致,而与碳酸盐红土、花岗岩红土存在明显差异。这些特征暗示网纹红土母质与北方黄土红粘土间存在物源联系,网纹红土物质组成均匀,经历了高度混合,显示风成特性。③网纹红土稀土元素特征一方面继承了原始物质的特性,另一方面体现了南方湿热气候条件下,化学风化导致的再分馏特征。化学风化中,Eu未出现明显的再分馏,而Ce的再分馏显著。通常情况下,风化淋溶指数越小,Ce富集程度越高。然而,网纹化过程中Ce分馏特征显示出复杂性。

Rare earth elements（REE） have important tracing significance on studies about sedimentation environment, source and forming of sediments. We have studied the REE composition for plinthitic red earth and their distribution characteristics in Jiangxi, Hunan, Zhejiang provinces. The findings show that （1）∑ REE of all kinds of plinthitic red earth samples except for nodule samples vary in range from 158.2 to 224.0 μg·g^-1. Compared with the other samples, nodule samples are much higher in REE absorbed abundance, from 385.5 to 1 641.6 μg·g^-1, especially the Cerium element absorbed abundance. The REE abundance of un-vermiculate samples is higher than that of vermiculate samples, and the REE abundance of white vermiculate samples is higher than that of red vermiculate samples. （2）Though these samples come from different places or ages, their REE distribution patterns are almost identical, which displays uniformity of plinthitic red earth in material composition. They were quite similar to the upper continent crust （UCC） in REE distribution pattern, showing ＂rich LREEs and Europium-depletion＂ characteristics. By further comparison with the loess in Loess Plateau, Xiashu loess in Zhenjiang, red clay in Xifeng, carbonite red clay in Guizhou and granite red clay in Guangdong, we find that the plinthitc red earth in REE distribution pattern and characteristic parameters are more closed to loess-red clay in northern China, which implies their material origin are identical. So, the materials of plinthitic red earth might get highly mixed as aeolian dust during the process of transportation. （3）The REE characteristics of plinthitic red earth are inherited from its source materials, and also lead to REE re-fractionation characteristics because of stronger chemical weathering and soil-forming process. On the whole, the fractionation between LREE and HREE for plinthitic red earth is significantly apparent, Europium （Eu） fractionation is not any apparent, Cerium （Ce） fractionation is remarkable during weathering. Normally, Ce absorbed abundance is negatively related to ba （weathering-eluviating index） value, meanwhile, Ce-depletion become more complicated during vermiculate process.