Application of Uranium Disequilibrium in the Study of the Weathering Rate of the Granite in Northern Guangdong
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摘要: 为探讨粤北花岗岩化学风化过程中所表征的地貌演化和气候环境变化,选取锦江流域为研究区,运用铀系不平衡方法,测试流域内河流水中铀的质量浓度、风化壳及岩石中铀的质量分数、234U与238U的放射性活度比,计算流域中花岗岩的化学风化速率.结果显示:在当前水热条件下,本流域中平均CIA值为86.84的花岗岩风化壳的化学风化速率为0.038 mm/a,即风化1 m厚的花岗岩需要约26 000年;在热带亚热带地区,控制岩石风化的主要因素是高温潮湿的气候条件;铀系同位素之间产生的不平衡适用于量化评估风化作用下水-土-岩三者间的相互作用,也为岩石风化及地表过程的相关研究提供了一种新的示踪手段.Abstract: In order to explore the geomorphological evolution and climatic environment changes in the chemical weathering process of the granites in northern Guangdong, Jinjiang River Basin is selected as the research area and the uranium-series isotopes disequilibrium method is used to measure the uranium concentration and 234U/238U activity ratios of river water, weathering crust and rock in the basin and calculate of chemical weathering rate of grani-te in the basin. As the results show, under the current climatic conditions, the chemical weathering rate of the grani-te weathering crust with the CIA of 86.84 is 0.038 mm/year, which means that it will take about 26 000 years for 1 m thick granite to weather; in the tropical and subtropical regions, and the main factor for rock weathering is the climate conditions with high temperature and humidity. Uranium isotopes disequilibrium can be used to quantitatively evaluate the interaction among water, soil and rock under weathering, and it also provides a new means for research on rock weathering and earth surface processes.
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Key words:
- rock weathering rate /
- uranium disequilibrium /
- degree of weathering
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表 1 锦江流域岩石中238U的质量分数和234U与238U的放射性活度比
Table 1. The Uranium concentration and 234U/238U activity ratio in rocks of the Jingjiang River Basin
样品号 主要矿物组成 岩性 w(238U)/(μg·g-1) A(234U)/A(238U) CJ-Y1 钾长石,钠长石,角闪石,黑云母,石英 花岗岩 48.5 1.03±0.03 BSWXX-Y 钠长石,角闪石,石英,黑云母 花岗岩 22.3 1.01±0.05 表 2 锦江流域风化壳中238U的质量分数和234U与238U的放射性活度比
Table 2. The Uranium concentration and 234U/238U activity ratio in weathering crust of the Jinjiang River Basin
样品号 层级 深度/m w(238U)/(μg·g-1) A(234U)/A(238U) CJ-02 B 1.0 10.80 0.83±0.04 CJ-03 B 1.5 7.20 0.96±0.03 CJ-05 C 2.5 10.00 1.09±0.05 BSWXX-03 B 2.0 12.60 0.96±0.05 BSWXX-04 B 2.5 6.95 0.99±0.05 MW-01 B 1.5 16.30 0.99±0.03 MW-03 C 13.5 13.30 0.94±0.03 LTZ-L B 4.5 4.16 1.01±0.05 LTZ-R B 5.0 3.91 1.00±0.04 表 3 锦江流域河流水中238U的质量浓度和234U与238U的放射性活度比
Table 3. The Uranium concentration and 234U/238U activity ratio in water of the Jingjiang River Basin
样品编号 采样时间 ρ(238U)/(μg·L-1) A(234U)/A(238U) BSW-1 2013-06-27 2.15 1.21±0.08 HXSQ-1 2013-06-27 1.85 1.42±0.03 TKW-1 2013-06-27 1.33 1.38±0.02 TKW-2 2013-06-27 1.68 1.07±0.04 TKW-3 2014-11-08 0.55 1.11±0.08 BSW-2 2014-11-08 0.26 1.09±0.06 HXSQ-2 2014-11-08 0.56 1.08±0.05 TDLC 2014-11-08 0.19 1.04±0.07 HGT 2014-11-08 0.16 1.08±0.07 表 4 锦江流域风化壳和基岩的主要化学元素的质量分数
Table 4. The main chemical elements in weathering crust and rock from Jinjiang River Basin
% 样品号 w(Na2O) w(MgO) w(K2O) w(CaO) w(P2O5) w(Fe2O3) w(Al2O3) w(SiO2) BSWXX-03 0.28 0.64 5.34 0.67 0.045 7 5.50 21.59 66.59 BSWXX-04 0.22 0.42 6.63 0.83 0.050 1 4.50 21.08 66.66 CJ-02 0.19 0.32 1.53 0.20 0.025 0 6.40 25.92 65.26 CJ-03 0.09 0.12 1.43 0.18 0.019 3 6.30 25.97 63.11 CJ-05 0.20 0.18 3.76 0.47 0.029 8 5.50 23.03 69.34 MW-01 0.13 0.17 2.60 0.34 0.036 6 7.30 26.02 62.30 MW-03 0.11 0.39 3.06 0.39 0.078 6 4.70 23.90 69.18 LZT-L < 0.10 0.48 1.66 < 0.01 0.070 0 6.55 16.85 66.48 LZT-R 0.11 0.45 1.62 < 0.01 0.070 0 8.38 18.97 61.02 FX-01 0.10 0.58 1.90 < 0.01 0.080 0 7.98 18.69 62.35 FX-02 < 0.10 0.60 1.58 < 0.01 0.010 0 8.82 20.71 58.24 FX-03 0.11 0.57 2.39 < 0.01 0.090 0 6.96 16.74 65.78 FX-04 < 0.10 0.58 1.67 < 0.01 0.060 0 6.32 16.82 64.91 BSWXX-Y 7.50 0.66 0.11 2.19 0.080 4 2.90 12.47 72.84 CJ-Y1 2.24 0.24 5.22 0.90 0.038 1 1.50 12.15 76.36 表 5 表征风化程度的指标
Table 5. The proxy of characterizing the degree of weathering
样品号 硅铝率 硅铁率 硅铁铝率 CIA BSWXX-03 5.235 32.218 4.503 76.23 BSWXX-04 5.367 39.340 4.723 72.79 CJ-02 4.272 27.085 3.690 91.90 CJ-03 4.130 26.710 3.580 93.96 CJ-05 5.108 33.549 4.434 82.94 MW-01 4.063 22.689 3.446 88.89 MW-03 4.912 39.163 4.365 86.64 LZT-L 6.693 26.988 5.364 89.45 LZT-R 5.457 19.345 4.265 90.66 FX-01 5.671 20.822 4.457 89.29 FX-02 4.783 17.623 3.762 91.62 FX-03 6.683 25.195 5.282 85.70 FX-04 6.558 27.392 5.291 89.40 BSWXX-Y 9.913 66.610 8.628 43.12 CJ-Y1 10.662 135.202 9.883 52.50 -
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