The effect of curvature on weathering rind formation: Evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe
The effect of curvature on weathering rind formation: Evidence from Uranium-series isotopes in basaltic andesite weathering clasts in Guadeloupe
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DOI：
10.1016/j.gca.2011.11.038
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发表时间：
2012-03
影响因子：
5
通讯作者：
Lin Ma;F. Chabaux;E. Pelt;M. Granet;P. Sak;J. Gaillardet;M. Lebedeva;S. Brantley
中科院分区：
文献类型：
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作者：
Lin Ma;F. Chabaux;E. Pelt;M. Granet;P. Sak;J. Gaillardet;M. Lebedeva;S. Brantley
为了量化热带和潮湿气候下风化碎屑的外皮形成率，并确定控制风化反应的因素，我们分析了从瓜德罗普的巴斯特尔岛采集的玄武安山岩风化碎屑中的铀系列同位素组成和微量元素浓度。U，Th和Ti元素的配置文件显示，Th和Ti的行为保守的外皮形成过程中，但U是从外部源添加到外皮。在外壳中，风化反应包括原生矿物如辉石、斜长石和玻璃基质的溶解，以及铁氢氧化物、三水铝石和少量高岭石的形成。稀土元素（REE）剖面显示了显着的Eu负异常形成于碎屑风化，与斜长石溶解。主要是由于斜长石溶解在果皮中形成显着的孔隙。新的孔隙度被推断为允许土壤水携带外部衍生的，溶解的U流入。由于这种流入，U沉淀沿着与新形成的粘土矿物和羟基氧化物的外皮。Th的保守行为和U的连续加入充分解释了在壳中观察到的（238 U/232 Th）和（230 Th/232 Th）活度比的系统趋势。根据开放系统铀添加模型测量的铀系活度比确定的外壳形成速率，从核壳边界的低曲率到高曲率部分（0.018 - 0.12 mm −1）增加了1.3倍（0.18 - 0.24 mm/kyr），表明曲率影响外壳形成速率，正如扩散限制外壳形成所预期的那样。因此，U系列地质年代学提供了第一个直接证据，表明界面的曲率控制着碎屑尺度上风化层形成的速率。如果表面粗糙度等于约1300-2200的值，则在碎屑尺度上确定的风化率可以与在分水岭或土壤剖面尺度上确定的风化率相一致。
To quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions, we analyzed Uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from Basse-Terre Island in Guadeloupe. U, Th, and Ti elemental profiles reveal that Th and Ti behave conservatively during rind formation, but that U is added from an external source to the rind. In the rind, weathering reactions include dissolution of primary minerals such as pyroxene, plagioclase, and glass matrix, as well as formation of Fe oxyhydroxides, gibbsite and minor kaolinite. Rare earth element (REE) profiles reveal a significant Eu negative anomaly formed during clast weathering, consistent with plagioclase dissolution. Significant porosity forms in the rind mostly due to plagioclase dissolution. The new porosity is inferred to allow influx of soil water carrying externally derived, dissolved U. Due to this influx, U precipitates along with newly formed clay minerals and oxyhydroxides in the rind. The conservative behavior of Th and the continuous addition of U into the rind adequately explain the observed systematic trends of (238U/232Th) and (230Th/232Th) activity ratios in the rind. Rind formation rates, determined from the measured U-series activity ratios with an open system U addition model, increase by a factor of ∼1.3 (0.18–0.24mm/kyr) from a low curvature to a high curvature section (0.018–0.12mm−1) of the core–rind boundary, revealing that curvature affects rates of rind formation as expected for diffusion-limited rind formation. U-series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale. The weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300–2200.