Evidence for extreme SmNd fractionation during chemical weathering
Evidence for extreme SmNd fractionation during chemical weathering
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DOI：
10.1016/j.chemgeo.2021.120284
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发表时间：
2021-04
期刊：
影响因子：
3.9
通讯作者：
Jin-Liang Feng;Jifeng Zhang;Shao-peng Gao;Chaowen Li;J. Ju;Xiao-Ming Liu;Le-Le Pei-Le;Kun-Ying Wang-Kun-Yin
中科院分区：
文献类型：
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作者：
Jin-Liang Feng;Jifeng Zhang;Shao-peng Gao;Chaowen Li;J. Ju;Xiao-Ming Liu;Le-Le Pei-Le;Kun-Ying Wang-Kun-Yin
在陆上化学风化、海底风化等壳内作用过程中，Smsingle - bondNd的化学分选仍不确定，而对可能发生的Smsingle - bondNd分选的条件、规模和幅度、具体过程和实际机制都知之甚少。本文选择白云岩上发育的原位土风化剖面作为研究对象。基于连续萃取实验和矿物及地球化学组成（包括单键nd同位素组成）的测量，我们认为，陆地化学风化作用不仅导致了大量的单键nd的运移、再分配和二次富集，而且导致了单键nd的极端分选，明显的143nd同位素均质化，147sm单键143nd同位素系统几乎完全重置。初始143nd /144Nd比值异常，风化产物143nd模式年龄（TDM）无意义。我们发现镧系元素四元效应，而不是矿物学控制，是观察到的大量单键nd分馏和其他稀土元素分馏的原因。在镧系四元效应的支配下，Sm以溶解的有机配合物的形式迁移，其迁移率优先于Nd。研究结果还表明，风化产物中147smsingle - bond - 143nd同位素体系的重置是由外来溶解的Smsingle - bond - nd输入和附加的Smsingle - bond - nd分馏的共同作用控制的。最后，我们认为类似的情景可能发生在海底风化、成岩作用和其他地质过程中。
During subaerial chemical weathering, submarine weathering and other intracrustal processes, Smsingle bondNd chemical fractionation remains uncertain, while a little is known about the conditions, scale and magnitude, specific processes and actual mechanism for possible Smsingle bondNd fractionation. Here, an in situ terra rossa weathering profile developed on dolostone is chosen for the study. Based on sequential extraction experiments and measurements of the mineral and geochemical compositions including Srsingle bondNd isotopic compositions, we suggest that subaerial chemical weathering not only leads to considerable Smsingle bondNd mobilization, redistribution and secondary enrichment, but also gives rise to extreme Smsingle bondNd fractionation, apparent143Nd isotope homogenization, nearly complete resetting of the147Smsingle bond143Nd isotope system, anomalous initial143Nd/144Nd ratios, and meaningless143Nd model age (TDM) for weathering products. We found that the lanthanide tetrad effect, rather than mineralogical control, is responsible for the observed substantial Smsingle bondNd fractionation and for the other rare earth element fractionation. Under domination of the lanthanide tetrad effect, the mobility of Sm is preferential to that of Nd during their migration in the form of dissolved organic complexes. Our results also illustrate that resetting of the147Smsingle bond143Nd isotope system in weathering products is controlled by the combined effect of the extraneous dissolved Smsingle bondNd input and additional Smsingle bondNd fractionation. Finally, we argue that similar scenarios may occur in submarine weathering, diagenesis and other geological processes.