碳酸盐岩风化过程的铀、钍地球化学行为

Geochemical Behaviors of Uranium and Thorium during Weathering of Carbonate Rocks

铀、钍作为亲石元素普遍存在于地壳中,基岩风化使它们广泛参与地表及近地表环境中的各种地球化学过程。碳酸盐岩风化成土和侵蚀过程是喀斯特环境生态系统中铀系核素的重要来源和生物地球化学循环关键环节。采集典型喀斯特小流域后寨河6个碳酸盐岩风化剖面基岩和土壤样品,分析了风化过程中铀钍含量、分布和迁移特征,研究结果显示,碳酸盐岩化学风化作用产生的质量亏损平均在70%以上,纯石灰岩或白云岩剖面化学风化亏损系数达到90%以上,处于极度亏损状态。铀和钍受风化溶蚀残余作用的影响在岩土界面上出现富集突变特征,钍的富集更为明显,剖面中两者发生二次分布。化学风化亏损系数表明U和Th化学活动性的差异明显,U风化过程中易溶解而活动性较强,Th多吸附于土粒残余富集而活动性较弱,碳酸盐岩风化成土过程中Th/U比值的阶段性变化指示着风化作用强度,为提取更多的地球化学信息提供可能。

Uranium and thorium, as lithophile, are ubiquitous in earth crust and bedrock. The weathering of bedrock makes uranium and thorium participate extensively in various surficial geochemical processes. Both weathering-pedogenesis and erosion of carbonate rocks are important sources of uranium-series radionuclides in karst-ecological systems. Also be key process of global biogeochemical cycling of uranium-series radionuclides. Bedrock and soil samples are collected from 6 typical carbonate rock profiles in Houzhai catchment. The content, distribution and characterization of mobility of uranium and thorium are analyzed during weathering process of carbonate rocks. Carbonate rocks profiles are characterized by extreme mass defect which dominated by chemical-weathering. Chemical-weathering depletion fraction(CDF) is more than 70% due to dissolubility of carbonate rocks,and the CDF of pure limestone and dolomite is more than 90%. A sudden enrichment of uranium and thorium appear in the rocksoil interface because of chemical weathering and residual dissolution. Thorium-rich is more obvious than uranium, and both have secondary distribution in profile. Uranium is easily dissolved and more mobile, yet thorium is mostly adsorbed by soil particles less mobility in the process of weathering. The CDF indicates obvious difference in chemical mobility of uranium and thorium.Multi-stage variation of Th/U implies weathering intensity and more potential geochemical information during weatheringpedogenic process of carbonate rocks.