| 其他摘要 | 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 31000|j,g/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 0.007,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 RISE. Local isovolumetric weathering of carbonate, which leads to the enrichment of stable elements, is also provides the fundamental substance, but not the main factor that causes 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. 3. 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 Ce~(3+) is oxidized to Ce~(4+) 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 the enrichment bed have relevantly opulent LREE such as La and Nd, which leads to the strong intense negative anomalies. Ce migrating down to the rock as the formations of complexes with carbonate or phosphate ligands is not the main process that causes the intense negative anomalies. 6. The character of carbonate and the good rainy condition contribute to forming the special, environment such as alkali barrier at the weathering frontier. It is the most important factor that causes the enrichment and fractionation of REE', in the weathering profile and that the behavior of REE differs from other weathering profiles. In the weathering process REE can restructure and redistribution apparently, so the distribution and the fractionation of REE, which can't be used to indicate the provenance of the red residue, only reflect the behavior of REE in the weathering process. |
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