| 摘要 | 该论文在贵州及其相邻湖南吉首地区选取了十多个有代表性的碳酸盐红色风化壳,在野外仔细观察的基础上,运用矿物学、土壤物理学及地球化学等研究方法,首次较为系统地阐述了它们的物质来源及成因、矿物学及地球演化特征、稀土元素的超常富集及分异、稀土元素赋存状态等.取得了如下几点认识：1.系统论述了贵州岩溶区红色风化壳的主要物质来源,证实了碳酸盐岩上覆红色风化壳主要物质来源于碳酸盐岩中的酸不溶物,是碳酸盐岩风化后酸不溶物原地残余堆积的结果.2.系统阐述了碳酸盐岩风化壳形成过程中的矿物学与主量元素的演化特征,发现风化作用主要集中于岩-土界面附近进行.3.分析了导致剖面风化强度均匀的原因.4.提出了碳酸盐岩风化壳正反层序的判别方法.5.明确了碳酸盐岩风化壳发育的两阶段模式.6.详细研究了碳酸盐岩风化前缘地球化学碱性障的形成机理.7.初步揭示了岩-土界面稀土元素超常富集和Ce强烈亏损的地球化学机理.8.查明了稀土超常富集层中及剖面中上部稀土元素的赋存状态.9.风化前缘MREE富集现象及其形成机理. |
| 其他摘要 | Guizhou Province is located in the central of southwestern karst regions of China, which is the largest continuous distribution area of karst in the world. The red clay weathering crusts, which belongs an important part of the red weathering crusts in south of China, distributed abroad in trie karst areas of this province. Rock desertification and other environmental disasters are prone to form in karst areas because of its eco-environmental frangibility. The distribution areas of rock desertification become larger and larger recent years due to some natural processes and human activities. And some scientists hope to find and recover the palaeo-environmental change with the rise; of Qingzang Plateau by the development and evolvement processes of red clay weathering crusts in these areas. But up to now, the formation mechanism and origin of the weathering crusts overlying carbonate rocks in this area cannot be understood clearly. Especially, it is almost a blank for the formative and geochemical processes of the weathering crusts which be opened out. Some representative red clay weathering crusts overlying the carbonate rocks in karst areas of Guizhou province and the western area of Hunan province were chosen as the objects of study in this dissei-tation, On the base of geological field investigation, mineralogical, geochemical and soil physical studies, the soil material source, fonnation processes, and rare earth elements chemistry were studied comprehensively and some new ideas and conclusions have been obtained. 1. The Origin of red clay weathering crusts in karst areas of Guizhou province The significant difference of mineralogical and geochemical features occurring in different weathering profiles can exclude that dust and/or volcanic ash are the main source materials of soil overlying carbonate rocks. Very low content of quartz in some weathering profiles suggests that feldspar-quartz sandstones are not the main source because the sandstone contains much quartz. In addition to sandstone, clay rock and shale are two kinds of common clastic rock in investigated areas, so these were considered as the important potential source of soil overlying carbonate rocks. However, some weathering profiles have characteristics with enrichment of middle rare earth elements compared with shale (PAAS), and this indicate that they are not the results of weathering from clay rock and/or shale. The differences mineralogical and geochemical characteristics among the different profiles can be explained by the differences of insoluble residua of their parent rocks. So, insoluble residua of carbonate rocks was considered as the main soil material source of red clay weathering crusts in karst areas of Guizhou Province. 2. The mineralogical and major elements characteristics during the formation processes of carbonate rocks weathering crusts The mineral assemblage and contents do not change remarkably from fresh carbonate rocks to rock powders and new fonnation soils near the carbonate rock-soil interface. Correspondingly, the contents of major elements do not marked change also at this position. Those changes suggest that the residual soils near the interface can inherit the characteristics of the parent rocks. Upwards of several centimeters above the interface, the composition'of minerals and major elements change markedly and become a set of uniform residual soils. The weathering occur effectively near the interface was considered as the main reason to cause the special change of mineral and major elements. In addition, the mixture process of residual soils that formed before due to the collapse of weathering is another important reason that causes the soil layer uniform. For some cases, the characteristics of mineral and major elements can change with natural rule of weathering profiles if the contents of insoluble residua are very high in its parent rocks. 3. Two stage model of weathering and soil formation of carbonate rocks By the study of mineral and major elements, the two stages model of weathering and soil formation were demonstrated. The early stage is characterized by carbonate dissolution and insoluble residua large accumulation, and the later stage is similar to the weathering process of the crystalline rocks. It is definitely confirmed that the calcareous soil is the products of the early stage of weathering, and which is a transitional product between carbonate rocks and red clay weathering crusts. The red clay weathering crusts are derived from the calcareous soils. 4. Formation mechanism of the alkali barrier at the weathering front A sudden change and narrow alkali barrier that is the result of rapid solution of carbonate minerals was found at the weathering front. The barrier only appears near the carbonate rock-soil interface because carbonate mineral cannot preserve long time in acidic soils. The height of alkali barrier of dolostone weatliering profile is thicker than that of limestone weathering profile due to the residual time of dolomite longer than calcite in soil. 5. Geochemical mechanisms of the super high concentration of rare earth elements and the strong negative Ce anomalies at the weathering front . The results of mass balance calculation suggest that carbonate rocks can. supply enough sources of rare earth elements. The soluble existing states of rare earth elements in the parent rocks and enrichment beds are in favor of activation and transportation during the further weathering. The sudden change and narrow alkali barrier may be the most important factor that causes the rare earth elements enrichment. During the early stage of carbonate rocks weathering, the alkali barrier can preserve the rare earth elements effectively, which is released from carbonate rocks by weatliering. And the barrier can also cause the complexes and/or ions of rare earth elements that carried from the upper part of the profile to weathering front in infiltration water to precipitate and adsorb on clay minerals. The strong negative anomalies of Ce in studied profiles suggest that significant fractionation have been occurred between cerium and other rare earth elements during weathering processes. The soluble Ce~(3+) is oxidized to Ce~(4+) and precipitates from solution as very insoluble CeO_2 at the upper part of weathering profile. Consequently, in alkali solution of weathering front, Ce can be oxidized to Ce~(4+) easier, and it has greater mobilization (compared with other light rare earth elements) results from preferential formation of complexes with carbonate ligands. So the fractionation between cerium and otlier rare eartli elements is not only occur in the upper part of profile during the later stage of weathering, but also occur at weathering front during the early
stage of weathering. 6.The existing states of rare earth elements in weathering crusts The results of acid leaching experiments show that trie soluble existing state of rare earth elements is predominant in parent rocks. According to the results of sequential extraction experiments, the exchangeable phase, phase bond by iron and manganese oxides., and residual phase are most important phases of rare earth elements at the upper part of weathering crusts. And these constitute almost 90 percent of the rare earth elements gross of the soil samples at this part. But the proportion of rare earth elements phases occur a significant change at the weathering front. The carbonate and special absoiption phases and phosphate phase of rare earth elements can be up to 90 percent in Hie sample which borders upon the interface of carbonate rock and soil. Upwards, the organic phase becomes the largest proportion of rare earth elements, which exceeds 50 percent. All those main phases of rare earth elements are in favor of activation and transportation during the further weathering. 7.Geochemical formation mechanism of middle rare earth elements in the weathered rock and rock powder at the weathering front According to the results of mass balance calculation, the loss amount of middle rare earth elements are larger than that of light and heavy rare earth elements in the upper part of weathering crusts or during the later weathering stage of carbonate rocks, and this process can cause trie weathered rock and rock powder to enrich middle rare earth elements. At the weathering front, the clay minerals near the carbonate rock-soil interface can adsorb light rare earth elements preferentially and cause the weathering fluid to lack light rare earth elements and enrich middle rare earth elements and heavy rare earth elements. Meanwhile, heavy rare earth elements have great mobilization in virtue of formation of complexes with carbonate ligands. Those processes may be another reason that causes weathered rock and rock powder to enrich middle rare earth elements. |
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