Experimental constraints on Mg isotope fractionation during clay formation: Implications for the global biogeochemical cycle of Mg
Experimental constraints on Mg isotope fractionation during clay formation: Implications for the global biogeochemical cycle of Mg
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DOI：
10.1016/j.epsl.2019.115980
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发表时间：
2020-02-01
影响因子：
5.3
通讯作者：
Tipper, Edward T.
中科院分区：
文献类型：
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作者：
Hindshaw, Ruth S.;Tosca, Rebecca;Tipper, Edward T.
粘土矿物形成过程中镁同位素分馏的方向和幅度是利用镁同位素研究临界带镁地球化学循环和海洋镁收支的基础。本研究提供了实验数据的镁分馏因子的两个蒙脱石组矿物（硅镁石和皂石）在地球表面过程相关的温度。所得固体通过X射线衍射（XRD）和傅里叶变换红外光谱（FT-IR）表征以确认产物的矿物学和结晶度。进行了一系列实验以评估温度和pH对同位素分馏的影响。用氯化铵处理块状固体样品以除去交换性Mg，以区分这些位点和八面体位点之间的Mg同位素分馏。与初始溶液相比，所有块状和残余固体都富集Mg-24，并且交换性池的δ Mg-26值低于初始溶液或在初始溶液的误差范围内。与初始溶液相比，最终溶液在Mg-26的误差内或富集Mg-26，这取决于从溶液中除去的Mg的分数（f（Mg））。对于具有小的或可忽略的f（M）（g）的实验，增加pH导致更高的反应速率和减少的从初始溶液的分馏。这可能指向动力学效应，但残余固体的组成（Mg/（Li+Mg）比）也取决于pH。组成的变化反映在FT-IR数据中Mg-3-OH伸缩的波数中，这是键强度的代表，并表明平衡控制。随着温度的升高，与初始溶液相比，减少分馏的观察结果进一步支持了平衡控制。Rayleigh分馏模型和批分馏模型分别得到0.9991和0.9990的分馏因子。我们将我们的结果与现有的现场和实验数据进行了比较，并提出了明显的矛盾周围的Mg同位素分馏成层状硅酸盐矿物的方向可能是由于粘土八面体网站和溶解Mg之间的Mg键长的相似性。因此，矿物结构或初始溶液条件的微小变化可能会导致键长的变化足以改变分馏的方向，这意味着镁同位素分馏成粘土矿物的幅度和方向可能取决于当地的现场条件。或者，如果在该领域的次生粘土矿物的沉淀优先采用轻镁，在本实验研究中观察到的，这意味着碳酸盐风化溶解镁通量的贡献被低估，与全球镁的地球化学循环的重大影响。(C)2019爱思唯尔B. V.保留所有权利。
The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg Isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectite-group minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites.All bulk and residual solids were enriched in Mg-24 compared to the initial solution and (delta Mg-26 values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, Mg-26 compared to the initial solution, depending on the fraction of Mg removed from solution (f(Mg)). For experiments with small or negligible f(M)(g), increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but the composition of the residual solid (Mg/(Li+Mg) ratio) was also dependent on pH. The change in the composition was reflected in the wavenumber of the Mg-3-OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively.We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length sufficient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg. (C) 2019 Elsevier B.V. All rights reserved.