Barium isotope fractionation in barite–fluid systems at chemical equilibrium
Barium isotope fractionation in barite–fluid systems at chemical equilibrium
复制标题
重晶石-流体体系中化学平衡时的钡同位素分馏
DOI：
10.1016/j.chemgeo.2023.121453
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发表时间：
2023-03
期刊：
影响因子：
3.9
通讯作者：
J. Middleton;Wei Hong;A. Paytan;M. Auro;E. Griffith;T. Horner
中科院分区：
文献类型：
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作者：
J. Middleton;Wei Hong;A. Paytan;M. Auro;E. Griffith;T. Horner
沉积重晶石（BaSO 4，硫酸钡）的钡同位素组成正在成为现代和古代海洋环境中钡的来源和循环的强有力的示踪剂。为了可靠地使用Ba同位素来询问海洋Ba循环，重要的是要识别和约束过程，压裂BaSO 4中Ba的同位素组成。特别感兴趣的是离子交换：在化学平衡下发生在矿物流体系统中的微尺度溶解和沉淀。这一过程在矿物（如BaSO 4）和流体长时间接触的系统中通常很重要;然而，离子交换对BaSO 4中Ba同位素组成的影响尚不清楚。为了限制BaSO 4流体系统中离子交换的速率和同位素效应，我们在海洋相关条件下进行了一系列实验，并使用多相时间依赖性数值反应器模型解释了结果。通过一系列同位素示踪实验，我们发现BaSO 4-流体离子交换的速率在5 ~ 53 pmol m−2s−1之间。在一组平行实验中，用于评估Ba的质量相关同位素分馏，发现化学平衡时BaSO 4溶解和沉淀的综合效应导致Ba同位素的持续演化，并在同位素平衡时产生Δ 138巴长石-dBa = −0.10 ± 0.05 ‰的模拟偏移。然后，我们约束的同位素分馏的幅度BaSO 4溶解过程中拟合我们的数据在数值反应器模型，并使用以前的估计Ba同位素分馏BaSO 4沉淀（沉淀= 0.99968 ± 0.00002）。在化学平衡下，我们发现我们的数据最好解释为BaSO 4溶解= 0.99978 ± 0.00006，这意味着BaSO 4溶解释放同位素“轻”Ba到溶液中。由于与BaSO 4沉淀和溶解有关的同位素效应的大小是不平衡的，离子交换将倾向于改变共位BaSO 4和流体的同位素组成，直到两相抵消约0.10 ‰。沉积BaSO 4的这种影响的重要性可能取决于几个因素，我们建议多个网站筛选标准，以最大限度地利用这一新兴的代理。
The barium isotope composition of sedimentary barite (BaSO4, barium sulfate) is emerging as a powerful tracer of the sources and cycling of Ba in modern and ancient marine environments. To reliably use Ba isotopes to interrogate the marine Ba cycle, it is important to identify and constrain processes that fractionate the isotope composition of Ba in BaSO4. Of particular interest is ion exchange: micro-scale dissolution and precipitation that occurs in mineral–fluid systems at chemical equilibrium. This process is often important in systems where minerals, such as BaSO4, and a fluid remain in contact for prolonged periods of time; however, the impact of ion exchange on Ba isotope compositions in BaSO4is unknown. To constrain the rate and isotopic effect associated with ion exchange in BaSO4–fluid systems, we conducted a series of experiments under marine-relevant conditions and interpreted the results using a multi-phase time-dependent numerical reactor model. From a series of isotope-tracer experiments, we find that BaSO4–fluid ion exchange progresses at a rate between 5 and 53 pmol m−2s−1. In a parallel set of experiments used to assess mass-dependent isotope fractionation of Ba, the combined effect of BaSO4dissolution and precipitation while at chemical equilibrium was found to result in the continued evolution of Ba isotopes and produced a modeled offset of Δ138Babarite–dBa= −0.10 ± 0.05 ‰ at isotopic equilibrium. We then constrained the magnitude of isotopic fractionation during BaSO4dissolution by fitting our data in the numerical reactor model and using previous estimates of Ba isotope fractionation during BaSO4precipitation (⍺precipitation= 0.99968 ± 0.00002). At chemical equilibrium, we find our data are best explained by an ⍺dissolution= 0.99978 ± 0.00006, implying that BaSO4dissolution releases isotopically ‘light’ Ba to solution. Since the magnitude of the isotope effects associated with BaSO4precipitation and dissolution are imbalanced, ion exchange will tend to alter the isotope composition of co-located BaSO4and fluids until the two phases are offset by ≈0.10 ‰. The importance of this effect on sedimentary BaSO4likely depends on several factors and we suggest multiple site-screening criteria to maximize the utility of this emerging proxy.