| 其他摘要 | In order to obtain more information about the weathering-pedogenic processes of carbonate rocks in Guizhou, Province and lay the foundation for the future research in determining the rate of weathering-pedogenesis, the weathering-pedogenic processes of two profiles of carbonate rocks from Guizhou Province has been studied in this research, by comparing the geochemical characteristics of uranium and thorium to that of major elements, trace elements and rare earth elements and the study of the U-Th disequilibria. The general conclusions are summarized as follows:
The 238U－234U－230Th disequilibria of the weathering profiles of carbonate rocks from Guizhou Province indicated that the U-Th disequilibria correlated to the weathering processes and to the evolution characteristics of minerals and ferruginous caps in the weathering profiles tightly. The weathering profiles were affected not only by the simple accumulating or leaching processes but also by the complex reorganization within each horizon. The U-Th disequilibria also suggested that the perturbation of the systems could correspond to the late middle Pleistocene climatic variations.The U-Th disequilibria in the weathering profiles of the carbonate rocks were caused by the weathering of parent carbonate rocks, the dissolving of uranium by the weathering liquids, the adsorption capacity of the organic matters, ferruginous oxides and clay minerals such as illite, kaolinite to uranium and thorium in the surface soil zone, the ferrous bands and the moderately weathered zone, respectively. They were also caused by α-recoil and the reduction of microorganism.
And the detailed conclusions are as follows:
(1) Comparing to the bedrock, uranium and thorium in the two profiles were strongly accumulated in the moderately weathered zone. Uranium and thorium in the Anshun dolomite profile accumulated in the lower part of the completely weathered zone. While the uranium accumulated in the middle of the completely weathered zone and thorium accumulated in the upper of the completely weathered zone in the Zunyi limestone profile. Then they decreased gradually to the top of the profile.
(2) Comparing to the bedrock, uranium and thorium in the two profiles were strongly accumulated in the moderately weathered zone. The reason is that during the weathering processes, the primary minerals in the bedrock dissolving, eroded and formed new secondary clay minerals, i.e. illite. The affinity of illite to uranium and thorium is strong. The accumulation of uranium and thorium in the weathering profiles resulted from the eluviation of surface water and the further weathering of ferruginous caps. The dissolving of ferruginous caps released the adsorbed uranium and thorium, the downward remobilization of uranium and thorium also made them accumulate in the middle and bottom of the completely weathered zone.
(3) The distribution characteristics of uranium and thorium in the weathering profiles suggested that the content of uranium and thorium correlated to the weathering processes and to the evolution characteristics of minerals and ferruginous caps in the weathering profiles tightly. The leaching degree of uranium and thorium in the Zunyi limestone profile is lower than that of Anshun dolomite profile indicted that the weathering degree of Zunyi limestone profile is lower than that of Anshun dolomite profile.
(4) Most of the 234U/238U activity ratio in the Anshun dolomite profile changed between <1 and >1. Except in the middle of the profile, 230Th/238U≈1, almost all of 230Th/238U are higher than 1.
(5) The 238U －234U－230Th disequilibria in the Anshun dolomite profile suggested that the 238U －234U－230Th disequilibria in the weathering profile were caused by the weathering of parent carbonate rocks, the dissolving of uranium by the weathering liquids, the adsorption capacity of the organic matters, ferruginous oxides and clay minerals such as illite, kaolinite to uranium and thorium in the surface soil zone, the ferrous bands and the moderately weathered zone, respectively. They were also caused by α-recoil and the reduction of microorganism.
(6) Except few sampling points, 234U/238U in most sampling points of Zunyi limestone profile are lower than 1, and 230Th/238U in most sampling points are higher than 1.
(7) The 238U －234U－230Th disequilibria in the Zunyi limestone profile suggested that the 234U－238U disequilibrium were mainly caused by the dissolving of uranium by surface water and the infiltration water and α recoil. While the 230Th－238U disequilibrium were mainly caused by the adsorption of organic matters in the surface soil zone, kaolinite, ferruginous oxides and illite to thorium and α recoil.
(8) In this study, application of the modeling of U mobility to the weathering profiles of the carbonate rocks suggested that these two profiles were all affected by the recent accumulating or leaching processes of uranium. The weathering systems were not in steady states but transient states. The uranium remobilization could bring the systems back to steady states.
(9) Calculated with the isochrone dating method, the age ranges of Anshun dolomite profile are from 87.0±7.8 ka to 479.2±47.9ka and that of Zunyi limestone are from 62.3±8.7 ka to 353.3±31.8ka.
(10) From the isochrone dating method, we know that the steady state of the two weathering systems will be reached in 1.1Ma.
(11) The mobility processes of uranium in the weathering profiles of carbonate rocks which were inferred from the modeling of U mobility is consistent with the mobility processes of major elements and trace elements suggested that the selection of the model is correct.
(12) The 238U－234U－230Th disequilibria of the whole weathering profiles suggested that the perturbation of the systems could correspond to the late middle Pleistocene climatic variations. The weathering profiles of carbonate rocks are affected by remobilization processes of uranium. In summary, each unit, and even each sample, has a complex history with superimposition of old formation steps and further remobilization processes. |
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