Continental sedimentary processes decouple Nd and Hf isotopes
Continental sedimentary processes decouple Nd and Hf isotopes
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DOI：
10.1016/j.gca.2013.07.027
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发表时间：
2013-11
影响因子：
5
通讯作者：
M. Garçon;C. Chauvel;C. France‐Lanord;P. Huyghe;J. Lavé
中科院分区：
文献类型：
--
作者：
M. Garçon;C. Chauvel;C. France‐Lanord;P. Huyghe;J. Lavé
大多数地壳和地幔岩石的钕和铪同位素组成相互关联，形成了“地阵”。然而，现在已经确定，尽管粗碎屑沉积物遵循这一趋势，但细颗粒海洋沉积物相对于它们的Nd同位素比率具有较高的Hf比率。两种同位素系统的这种“解耦”是否只发生在海洋环境中，还是由大陆环境中的沉积过程引起，目前仍不确定。在本研究中，明确地利用大型河流沉积物的铪和钕同位素组成来约束这两种同位素系统在侵蚀和沉积物从大陆向海洋运输过程中的行为。我们报告了恒河及其支流的床载、悬浮载和河岸的主要元素和微量元素浓度以及Nd和Hf同位素组成，这些河流流经喜马拉雅山脉，即卡纳利河、纳拉亚尼河、科西河和马尔尚迪河。样本集包括尼泊尔喜马拉雅山脉、喜马拉雅山前、下游洪泛区和孟加拉国恒河出水口的沉积物样本。结果表明，矿物的水动力分选解释了河流沉积物中Hf同位素的整个范围，即大于10εHfunits，但不影响Nd同位素组成。在同一地点取样的河床和河岸沉积物的ε h值有系统地低于悬浮荷载。在ε hfvs .ε nd图中，粗粒沉积物位于陆基阵下方或之上。与此相反，细粒沉积物（包括大部分悬浮物）的ε hf相对于ε nd则偏离陆地阵，海洋陆源粘土的情况也是如此。观察到的Nd-Hf解耦可以用矿物学分选过程来解释，该分选过程使底部沉积物中富集了粗而致密的矿物，包括非放射性成因的锆石，而表面沉积物中富集了具有放射性成因Hf特征的细物质。Nd-Hf同位素解耦合随着河漫滩输沙量的增加而增加，在河口处解耦合最大。这意味着在世界范围内海洋粘土和河流沉积物中观测到的Nd-Hf同位素解耦可能具有相同的起源。最后，我们估计了在海洋沉积物从未俯冲的情况下，现今地幔的Nd-Hf同位素组成，并得出结论：随着时间的推移，海洋沉积物的异常Nd-Hf同位素组成缓慢地将地幔的组成转向更具有放射性的Hf值。
The neodymium and hafnium isotopic compositions of most crustal and mantle rocks correlate to form the “Terrestrial Array”. However, it is now well established that whereas coarse detrital sediments follow this trend, fine-grained oceanic sediments have high Hf ratios relative to their Nd isotopic ratios. It remains uncertain whether this “decoupling” of the two isotopic systems only occurs in the oceanic environment or if it is induced by sedimentary processes in continental settings. In this study, the hafnium and neodymium isotopic compositions of sediments in large rivers is expressly used to constrain the behavior of the two isotopic systems during erosion and sediment transport from continent to ocean.We report major and trace element concentrations together with Nd and Hf isotopic compositions of bedloads, suspended loads and river banks from the Ganges River and its tributaries draining the Himalayan Range i.e. the Karnali, the Narayani, the Kosi and the Marsyandi Rivers. The sample set includes sediments sampled within the Himalayan Range in Nepal, at the Himalayan mountain front, and also downstream on the floodplain and at the outflow of the Ganges in Bangladesh. Results show that hydrodynamic sorting of minerals explains the entire Hf isotopic range, i.e. more than 10εHfunits, observed in the river sediments but does not affect the Nd isotopic composition. Bedloads and bank sediments have systematically lowerεHfvalues than suspended loads sampled at the same location. Coarse-grained sediments lie below or on the Terrestrial Array in anεHfvs.εNddiagram. In contrast, fine-grained sediments, including most of the suspended loads, deviate from the Terrestrial Array toward higherεHfrelative to theirεNd, as is the case for oceanic terrigenous clays. The observed Nd–Hf decoupling is explained by mineralogical sorting processes that enrich bottom sediments in coarse and dense minerals, including unradiogenic zircons, while surface sediments are enriched in fine material with radiogenic Hf signatures. The data also show that Nd–Hf isotopic decoupling increases with sediment transport in the floodplain to reach its maximum at the river mouth. This implies that the Nd–Hf isotopic decoupling observed in worldwide oceanic clays and river sediments is likely to have the same origin. Finally, we estimated the Nd–Hf isotopic composition of the present-day mantle if oceanic sediments had never been subducted and conclude that the addition of oceanic sediments with their anomalous Nd–Hf isotopic compositions has slowly shifted the composition of the Earth’s mantle towards more radiogenic Hf values through time.