Abstract Barium isotopes could be a novel tracer in low-temperature geochemical processes such as the Ba cycle in rivers and oceans. Equilibrium Ba isotope fractionation between Ba-hosting minerals and aqueous solution is of great importance for the applications of Ba isotopes in geochemistry, but it remains poorly constrained. In this study, we performed first-principles calculations based on the density functional theory (DFT) to determine the equilibrium Ba isotope fractionation between minerals and aqueous solution (103lnαmineral-Ba_aq of 137Ba/134Ba). The structural properties of aqueous Ba2+ are well predicted by the first-principles molecular dynamics (FPMD) simulation and 121 snapshots are extracted from FPMD trajectories to estimate the reduced partition function ratio (β factor or 103lnβ of 137Ba/134Ba) of aqueous Ba2+. The 103lnβ decreases in the sequence of aragonite > calcite > aqueous Ba2+ ∼ witherite > barite. The β factor is dominantly determined by the force constant, which is affected by both the average Ba-O bond length and the coordination number. Our results show that 103lnαaragonite-Ba_aq and 103lnαwitherite-Ba_aq are 0.36‰ and -0.02‰ at 300 K, respectively, consistent with results of experimental studies at equilibrium. The depletion of heavy Ba isotopes observed in natural corals relative to seawater suggests that kinetic effects play an important role in Ba isotope fractionation during coral growth. The 103lnαbarite-Ba_aq is only -0.17‰ at 300 K, indicating limited Ba isotope fractionation caused by the Ba removal stemmed from inorganic barite precipitation. Overall, the equilibrium Ba isotope fractionation factors between minerals and aqueous Ba2+ calculated in this study provide a guideline for applications of Ba isotopes in low-temperature geochemistry.

中文翻译：

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根据第一性原理计算平衡矿物和水溶液之间的钡同位素分馏

摘要 钡同位素可能是河流和海洋中的钡循环等低温地球化学过程中的一种新型示踪剂。Ba赋存矿物与水溶液之间的平衡Ba同位素分馏对于Ba同位素在地球化学中的应用具有重要意义，但它仍然缺乏约束。在本研究中，我们基于密度泛函理论 (DFT) 进行了第一性原理计算，以确定矿物和水溶液之间的平衡 Ba 同位素分馏（103lnαmineral-Ba_aq of 137Ba/134Ba）。Ba2+ 水溶液的结构特性通过第一性原理分子动力学 (FPMD) 模拟得到了很好的预测，并且从 FPMD 轨迹中提取了 121 个快照，以估计 Ba2+ 水溶液的减少分配函数比（β 因子或 137Ba/134Ba 的 103lnβ）。103lnβ按文石>方解石>Ba2+水溶液～毒重石>重晶石的顺序减小。β 因子主要由力常数决定，它受平均 Ba-O 键长和配位数的影响。我们的结果表明，103lnαaragonite-Ba_aq 和 103lnαwitherite-Ba_aq 在 300 K 下分别为 0.36‰ 和 -0.02‰，与平衡时的实验研究结果一致。在天然珊瑚中观察到的重 Ba 同位素相对于海水的消耗表明动力学效应在珊瑚生长过程中 Ba 同位素分馏中起着重要作用。103lnαbarite-Ba_aq 在 300 K 时仅为 -0.17‰，表明由无机重晶石沉淀引起的 Ba 去除引起的 Ba 同位素分馏有限。全面的，