Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T
Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T
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DOI：
10.1016/j.gca.2012.10.007
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发表时间：
2012-10
影响因子：
5
通讯作者：
P. Kowalski;B. Wunder;Sandro Jahn Gfz German Research Centre for Geosciences-Sandro-Jahn-Gfz-German-Research-Centre-for-103271123;Telegrafenberg;Potsdam;H Germany-
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文献类型：
--
作者：
P. Kowalski;B. Wunder;Sandro Jahn Gfz German Research Centre for Geosciences-Sandro-Jahn-Gfz-German-Research-Centre-for-103271123;Telegrafenberg;Potsdam;H Germany-
在过去的十年中，实验研究表明，在三角和四面体配位位点中携带硼的材料之间存在大的B同位素分馏，但对产生所观察到的同位素特征的机制知之甚少。为了了解硼同位素分馏过程并更好地解释实验数据和在天然样品中观察到的同位素特征，我们采用基于密度泛函理论的第一性原理计算，结合从头算分子动力学和一种新的赝频率分析方法，研究了含硼矿物之间的B同位素分馏（例如电气石和云母）和含有H3 BO 3和H4 BO 4-物种的水性流体。我们证实了实验发现，同位素分馏主要是由分馏硼原子的协调驱动，并发现此外，所产生的同位素签名的强度与BO键长强烈相关。我们还证明了我们的计算方案的能力，以预测在极端压力下的流体的同位素签名显示的一致性，计算的压力依赖的β因子与测量的压力变化的BO振动频率的H3 BO 3和H4 BO 4-在水溶液中的流体。硼云母和中性流体之间的分馏因子的预测值与实测值的比较证实了实验发现的高P和T下中性流体中四面体硼物种的混合物的存在，这也解释了文献中报道的电气石-云母系统的各种测量之间的不一致。我们的研究表明，计算的平衡同位素分馏因子具有与实验相当的准确性，并提供独特的和有价值的洞察到原子尺度上的同位素分馏机制的过程。
Over the last decade experimental studies have shown a large B isotope fractionation between materials carrying boron incorporated in trigonally and tetrahedrally coordinated sites, but the mechanisms responsible for producing the observed isotopic signatures are poorly known. In order to understand the boron isotope fractionation processes and to obtain a better interpretation of the experimental data and isotopic signatures observed in natural samples, we use first principles calculations based on density functional theory in conjunction with ab initio molecular dynamics and a new pseudofrequency analysis method to investigate the B isotope fractionation between B-bearing minerals (such as tourmaline and micas) and aqueous fluids containing H3BO3and H4BO4-species. We confirm the experimental finding that the isotope fractionation is mainly driven by the coordination of the fractionating boron atoms and have found in addition that the strength of the produced isotopic signature is strongly correlated with the BO bond length. We also demonstrate the ability of our computational scheme to predict the isotopic signatures of fluids at extreme pressures by showing the consistency of computed pressure-dependent β factors with the measured pressure shifts of the BO vibrational frequencies of H3BO3and H4BO4-in aqueous fluid. The comparison of the predicted with measured fractionation factors between boromuscovite and neutral fluid confirms the existence of the admixture of tetrahedral boron species in neutral fluid at high P and T found experimentally, which also explains the inconsistency between the various measurements on the tourmaline–mica system reported in the literature. Our investigation shows that the calculated equilibrium isotope fractionation factors have an accuracy comparable to the experiments and give unique and valuable insight into the processes governing the isotope fractionation mechanisms on the atomic scale.