The cerium isotope fingerprints of redox fluctuation in bauxites
The cerium isotope fingerprints of redox fluctuation in bauxites
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DOI：
10.1016/j.epsl.2022.117962
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发表时间：
2023-01
影响因子：
5.3
通讯作者：
Wenshuai Li;Xiao-ming Liu;R. Nakada;Y. Takahashi;Yongfeng Hu;Mohsen Shakouri;Zhao-Yu Zhang;T. Okumura;Shinya Yamada
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文献类型：
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作者：
Wenshuai Li;Xiao-ming Liu;R. Nakada;Y. Takahashi;Yongfeng Hu;Mohsen Shakouri;Zhao-Yu Zhang;T. Okumura;Shinya Yamada
我们测量了哥伦比亚河玄武岩（CRBs）上铝土矿的Ce浓度、氧化态和同位素组成，以评估氧化还原和非氧化还原过程对地球表面Ce同位素变化的影响。从岩芯中回收的铝土矿显示出Ce浓度向上增加（5.4-88.7 μg/g）和Ce的总体贫化（传质系数τ Ce，Nb：-0.96至-0.52，即相对于其母体CRB净贫化52-96%）。Cowlitz铝土矿和哥伦比亚铝土矿的Ce/Ce比值分别为0.4 ~ 6.3和0.6 ~ 1.4。Ce的正异常主要出现在2-5 m的浅风化层中。铈主要以三价形式存在于沉积的外源物质（来自大陆古老风化地区的风吹尘埃）和原生CRBs中。有一个向下增加的部分Ce（IV）的风化层。Cowlitz剖面和哥伦比亚剖面的δ 142 Ce变化范围分别为-0.161 ±0.034 ~-0.018 ±0.043‰和-0.277 ±0.034 ~-0.046 ±0.034‰。Ce同位素记录反映了Ce和Mn的氧化还原循环，并被顶部的尘埃堆积所掩盖。Ce（IV）的富集发生在过渡带（浅风化层底部），Ce同位素从CRBs（δ 142 Ce从-0. 080 ± 0. 034到-0. 04 ± 0. 034 ‰）发生了轻微的偏移，这可能是由于与O2驱动的CeO 2沉淀有关的轻微同位素分馏作用造成的。在深风化层（5-9 m）中，Ce同位素相对于玄武岩母岩发生负位移，氧化态Ce和Mn富集，其中大的同位素分馏作用可能是由于Ce氧化吸附在MnO 2上所致。Ce/Ce比值与δ 142 Ce的垂向差异可能是由于Mn和Ce的气候氧化还原循环，使CeO 2颗粒分散在浅部风化层中，而与氧化锰矿伴生的轻同位素Ce则向下转移到深部风化层中。我们的结论是铈物种同位素和异常的组合可能记录陆地氧化还原状态和波动。
We measured the Ce concentration, oxidation state, and isotope composition of bauxites developed on the Columbia River Basalts (CRBs) to evaluate the impacts of redox and non-redox processes on the Ce isotopic variability on Earth's surface. Bauxites recovered from drill cores show upward increasing Ce concentration (5.4–88.7 μg/g) and overall depletion of Ce (mass transfer coefficient, τ Ce, Nb:-0.96 to-0.52, ie, net depletion of 52-96% relative to their parent CRBs). There is a wide range of the Ce anomaly (Ce/Ce⁎) from 0.4 to 6.3 and 0.6 to 1.4 in the Cowlitz and Columbia bauxites, respectively. The most positive Ce anomalies appear at the shallow regolith (2-5 m depth). Cerium primarily presents in its trivalent form in deposited exogenous materials (wind-blown dust from an old, weathered region of the continent) and primary CRBs. There is a downward increase in the fraction of Ce (IV) in the regolith. Bauxite δ 142 Ce ranges from-0.161±0.034 to-0.018±0.043‰ and-0.277±0.034 to-0.046±0.034‰ in the Cowlitz and Columbia profiles, respectively. The Ce isotope record reflects the redox cycle of Ce and Mn and is masked by dust accretion on the top. The enrichment of Ce (IV) occurs in the transition zone (the bottom of the shallow regolith) with minor Ce isotope shifts from the CRBs (δ 142 Ce from-0.080±0.034 to-0.04±0.034‰) likely caused by minor isotopic fractionation linked to O 2-driven CeO 2 precipitation. There are negative Ce isotope shifts from the basaltic parents and the enrichment of oxidized Ce and Mn in the deep regolith (5-9 m depth), where large isotopic fractionation probably induced by Ce oxidative adsorption on MnO 2 plays a vital role. The vertical difference between Ce/Ce⁎ and δ 142 Ce may be attributed to the climatic oxidation-reduction cycle of Mn and Ce, leaving discrete CeO 2 grains in the shallow regolith while transferring isotopically light Ce associated to oxidized Mn deposits down to the deep regolith. We conclude that a combination of Ce species isotopes and anomalies potentially records terrestrial redox status and fluctuation.