碳酸盐岩风化成土过程中REE超常富集及Ce强烈亏损的地球化学机理  

Formation mechanism of the superhigh concentration of REE and the strong negative Ce anomalies in the carbonate rock weathering profiles in Guizhou Province, China

作　　者：孙承兴[1] 王世杰[1] 季宏兵[1] 

机构地区：[1]中国科学院地球化学研究所环境地球化学国家重点实验室,贵州贵阳550002

出　　处：《地球化学》2002年第2期119-128,共10页Geochimica

基　　金：国家自然科学基金(49833002);国家攀登计划(95-预-39)

摘　　要：碳酸盐岩的REE含量极低,但最近在贵州一些碳酸盐岩风化壳土层底部首次发现了REE的超常富集层,REE总量最高可达近31000μg/g;Ce强烈亏损,δCe值最低达0.007。这种低背景、高富集、强分异的REE富集现象在贵州碳酸盐岩风化壳中具有一定的普遍性。选取发育在下三叠统花溪灰岩和平坝白云岩之上的两个较具代表性的碳酸盐岩风化壳进行研究,结果表明,这种富集现象与碳酸盐岩风化成土的两阶段性密切相关。(1)在残积土形成阶段(风化早期),碳酸盐的快速溶解导致风化前缘形成一个垂直方向相对狭窄、突变的碱性障(pH值为8左右)。此障既能有效地将碳酸盐岩分解释放的REE以及下渗水携带的REE分解沉淀和以吸附于粘土矿物上的方式富集,也容易使Ce3+氧化成Ce4+,并与HCO-3形成稳定的可溶性络合物随下渗雨水流走,从而使Ce进一步亏损。(2)残积土演化阶段,轻、重稀土发生明显分异,Ce3+氧化成Ce4+并发生水解沉淀,致使下渗水富重稀土而贫Ce,最终使REE在剖面上显示出一般风化壳共有的分异特征。另外,根据对碳酸盐岩中的酸不溶物、可溶性稀土的提取以及质量平衡计算,碳酸盐岩能够提供足够的REE物源;以可溶态为主的赋存状态有利于REE的淋滤(活化)。As well known, the REE concentration is very low in carbonate rocks. However, superhigh concentration beds of REE, where the REE concentration can be up to about 31 000 μg/g, are found at the weathering front of carbonate rock weathering profiles in Guizhou, China. The strong negative Ce anomalies are also found in those REE enrichment beds, whose δCe can be low to 0.007. The phenomena of low background, strong enrichment and strong fractionation of REE have some certain universality in the carbonate rock weathering profiles in Guizhou. According to the results of mass balance calculation, it is considered that carbonate rocks can supply enough REE sources. The soluble existing states of REE in the parent rocks and enrichment beds, which are found by chemical leaching experiments, are in favor of activation and transportation. A sudden change and narrow alkali barrier, which is considered that it is the result of rapid solution of carbonate minerals, is also found according to the variation of the pH values of samples along the weathering profiles. The alkali barrier may be the most important factor that causes the REE enrichment. During the early stage of carbonate rocks weathering, the alkali barrier can preserve the REE effectively, which is released by weathering. And the barrier can also cause the complexes and/or ions of REE that were carried from the upper part of the profile to weathering front in infiltration water to precipitate and adsorb on clay. The strong negative anomalies of Ce in the studied profiles suggest that significant fractionation has been occurred between cerium and other rare earth elements during weathering processes. The soluble Ce3+ is oxidized to Ce4+ and precipitates from solution as very insoluble CeO2. Consequently, the solution shows a negative Ce anomaly. However, there is not a weathering profile or a groundwater sample in weathering profile having such strong negative Ce anomaly as our studied profiles. And this imply that the fractionation between cerium and other r

关 键 词：碳酸盐岩 风化前缘 REE超常富集 Ce强烈亏损 形成机理 贵州 铈 风化成土 赋存状态 

分 类 号：P588.245[天文地球—岩石学] P595[天文地球—地质学]