【作者】 李艳丽；

【导师】 王世杰； 张殿发；

【作者基本信息】 中国科学院研究生院（地球化学研究所） ， 环境地球化学， 2004， 硕士

【摘要】 近期对贵州碳酸盐岩红色风化壳研究发现，碳酸盐岩基岩中稀土含量一般为几十个μg/g，而在岩-土界面附近REE显示超常富集，REE总量最高可达31000μg/g以上，远高于碳酸盐岩基岩和一般风化壳REE含量。同时，稀土富集的部位是剖面底部，而在其它的风化壳中稀土往往在剖面中部即全风化层底部富集。REE超常富集的样品同时伴有Ce的强烈亏损，δCe值最低为0.007，这也是目前报道过的风化壳中的最低值。另外，风化前缘稀土相对于PAAS标准化后显示MREE富集的特征。弄清碳酸盐岩红色风化壳中稀土富集和分异的机理，有助于丰富稀土表生地球化学知识，同时也为喀斯特地区风化成土地球化学过程和土壤物质来源的示踪提供了重要依据。本文通过对稀土赋存状态的实验研究，结合稀土元素质量迁移系数，系统的分析了稀土元素在碳酸盐岩风化壳中的迁移、转化行为，探讨了稀土发生富集和分异的机理。本次研究取得了如下认识： 1、通过对白云岩和灰岩风化壳稀土赋存状态的对比，发现在碳酸盐岩红色风化壳中稀土具有相同的赋存和转化规律。稀土富集层中稀土主要以专性吸附态和残渣态的形式存在，其次是有机质结合态。稀土的赋存状态说明粘土矿物对稀土的吸附是导致稀土在底部超常富集的主要原因。其次，次生稀土磷酸盐矿物，主要是La、Nd等的轻稀土磷酸盐矿物也是稀土富集的主要载体。此外，有机质和铁锰氧化物对稀土的迁移、富集也发挥着重要作用。非富集层主要以残渣态和铁锰氧化物结合态的形式存在，表明随着风化作用的增强，稀土越易进入稳定的相态中。 2、元素质量迁移系数真实的反应了元素的迁入迁出情况。碳酸盐岩的非等体积风化使得稳定的微量元素相对基岩富集，也为风化壳稀土提供了基本的物质来源，但不是导致稀土超常富集的主要原因。稀土元素在风化壳上部表现出很强的活化迁移能力，几乎全部迁出，并且在底部显示明显的带入富集，上部淋漓的稀土为底部稀土的超常富集提供了稳定而充足的物质来源。 3、风化壳底部狭窄的碱性障是稀土超常富集的主要制约因素。碳酸盐岩的酸不溶物含量低，碳酸盐矿物可溶性强，发育程度较高的风化壳剖面能够在垂向上很窄的范围内形成碱性障。7-9的pH值增大了粘土对稀土的吸附量，同时还提高了稀土与各种络阴离子络合的稳定性，有利于稀土沉淀。对于发育完整的碳酸盐岩红色风化壳，介质条件的改变是缓慢的，不能在剖面底部形成碱性障，因此，稀土在剖面上部迁出，下部迁入富集，富集层位往往和其它岩类风化壳中国科学院研究生院硕士学位论文一样在全风化层的底部。4、经页岩标准化的稀土分布模式在风化前缘显示MREE富集。导致这一现象的原因主要是风化壳中MREE的迁出程度大于HREE和LREE，风化流体显示MREE富集。在底部碱性障条件下，富MREE的风化流体沉淀、富集。风化流体向下渗透，水岩反应的结果是白云岩也显示MREE富集，灰岩由于岩一土界面的阻挡，水岩反应深度不大，基岩往往无明显的稀土分异。5、在碳酸盐岩红色风化壳上部Ce显示正异常，底部富集层显示强烈负异常。剖面仁部，ce“+氧化水解，氧化产物主要与铁锰氧化物共沉淀，少量的与粘土形成氢键或者与有机物络合沉淀，导致剖面上部正异常。Ce在剖面中的迁出量很少，在底部稀土富集层，沉淀富集时主要是La、Pr等LREE， Ce相对贫乏，因此，富集层;无显示强烈的负异常。与碳酸(氢)根或者磷酸根等络合物形式迁走的Ce很少，不是导致Ce强烈负异常的主要原因。6、基岩性质和良好的水热条件导致碳酸盐岩风化前缘形成特殊的微环境—碱性障，这是稀土在碳酸盐岩风化壳底部超常富集和分异的主要原因，也是碳酸盐岩红色风化壳稀上行为不同于其它岩类风化壳的主要原因。稀土元素在风化作用过程中发生明显的重组分配，其分布特征不能用以指示物源信息，只能反应风化作用过程信息。更多还原

【Abstract】 Recently, many researches about the red residua on karst terrain of Guizhou province suggest that the superhigh concentration bed of REE, where the REE concentration can be up to about 31 000ug/gwhich more than the carbonate and other types of weathering profiles, are found at the weathering frontier of carbonate weathering profiles. At the same time, REE enriches at the bottom of the profile, not as the middle as in other type of weathering profile .The strong negative Ce anomalies are also found in those REE enrichment beds, whose 5Ce can be low to 0.0()7,which is lowest one of the values that have been reported. Moreover. PAAS-normalized REE distribution patterns for samples from the weathering frontier are characteristic of MREE enrichment. Studying the mechanism about the enrichment and fractionation of REE, will help to enrich the knowledge about the geochemistry and provide the important evidences about weathering and pedogenesis of carbonate rocks and the provenance of the red residue. This paper through the studies about the combining states of REE and the mass transport coefficient, analyses the behavior of REE systematically and provides the mechanism about the enrichment and fractionation of REE. Some important conclusions have been summarized as follows.1. Through comparing the REE combining states between the dolomite and the limestone weathering profiles, we find that REE has the similar combing states and transformation characters. At the superhigh enrichment bed, the main states are bound o carbonates and adsorbed (II) and residual (VI), then is bound to organics. It Illustrates that the clay minerals adsorbing the REE is the main reason that caused the superhigh enrichment. Secondly the secondary LREE-bearing phosphate such as is the predominant factor leading the enrichment. Moreover, Fe-Mn oxides and organics also play an important role in migration and enrichment of REE. In the upper of the profile REE has the main formations as residue and bound to Fe-Mn oxides, it suggests that REE transforms into stable states with the intense weathering.2. The mass transport coefficient can truly reflect the net loss or gain of REE. Local isovolumetric weathering of carbonate, which leads to the enrichment of stable elements, is also provides the fundamental substance, but not the main factor thatcauses the superhigh enrichment of REE. REE that loses mostly in the upper profile contribute to the enrichment at the bottom steadily and in full, consequently REE gains at the bottom.3. At the bottom of the profile the alkali barrier is the predominant factor that results the enrichment of REE. The carbonate is easily soluble and the concentration of insoluble residues is very small, so the alkali barrier can form in the vertical profile with higher weathering degree. The value of pH from 7 to 9 does goods to the precipitation of REE through increasing the adsorbing ability and the stability of complexes with several ligands. As for the weathering crust containing full structure of mother rock, the changes of medium conditions is so slow that cannot come into being the alkali barrier at the bottom of profile.4. PAAS-normalized REE distribution patterns for samples from the weathering frontier are characteristic of MREE enrichment. This phenomenon is mainly due to MREE preferentially transports downward in the regolith, as a result the weathering fluid, which characterizes MREE enrichment, precipitates and enriches at the alkali barrier. As the fluid crossing the rock the dolomite has MREE enrichment because of water-rock reaction, but the limestone stops the fluid from the rock that has no REE fractionation.5. Ce shows positive anomalies in the upper and intense negative anomalies at the bottom. In the upper Ce3+ is oxidized to Ce + and hydrated, as a result Ce shows positive anomalies owing to mainly precipitating with Fe-Mn oxides and secondary being adsorbed to clay minerals or precipitating with organics. Consequently, the net loss of Ce is very small and at th更多还原