Abstract The equilibrium boron isotope fractionations (α3–4, Δ11B(Tur-fluid)) between tourmaline mineral groups and fluids at 0.5 GPa, 600–1000 K are investigated using density functional theory calculations. The first solvent shell controls boron isotope fractionation in solution, where the β values of both H3BO3 and B(OH)4− decrease with increasing numbers of hydrogen bonds. In supercritical fluids, the weakening of hydrogen bonds and the diversity in configurations of hydrated boron species both contribute to variations in the vibrational frequencies. The 1000lnαTur-fluid value increases with increasing Mg/(Fe+Mg) ratios in the dravite-schorl solid solution series. This effect, together with the crystallization sequence predicted from the Gibbs free energies of tourmaline formation, accounts well for the association of boron isotope and chemical zonation observed in zoned tourmaline grains. The dependence of boron isotope fractionation on the B O bond length in tourmaline group minerals reflects the changes in relevant molar volumes caused by differences in the mass and charge of the atoms occupying the X, Y, and Z sites of tourmalines. This study suggests that the chemical composition of the tourmaline plays an important role in controlling the boron isotope composition in tourmalines crystallized from hydrothermal systems.

中文翻译：

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电气石族矿物和流体中平衡硼同位素分馏驱动机制的探索：密度泛函理论研究

摘要 使用密度泛函理论计算研究了电气石矿物群与 0.5 GPa、600-1000 K 流体之间的平衡硼同位素分馏 (α3-4, Δ11B(Tur-fluid))。第一个溶剂壳控制溶液中的硼同位素分馏，其中 H3BO3 和 B(OH)4− 的 β 值随着氢键数量的增加而降低。在超临界流体中，氢键的减弱和水合硼物种构型的多样性都会导致振动频率的变化。1000lnαTur-fluid 值随着 dravite-schorl 固溶体系列中 Mg/(Fe+Mg) 比率的增加而增加。这种效应，连同根据电气石形成的吉布斯自由能预测的结晶顺序，很好地解释了在分区电气石颗粒中观察到的硼同位素和化学分区的关联。硼同位素分馏对电气石族矿物中 BO 键长的依赖性反映了由于占据电气石 X、Y 和 Z 位点的原子的质量和电荷差异引起的相关摩尔体积的变化。这项研究表明，电气石的化学成分在控制从热液系统结晶的电气石中的硼同位素组成方面起着重要作用。