In hydrothermal fluids, disulfur (S) and trisulfur (S) radical anions have been observed to coexist with the major hydrogen sulfide and sulfate species. These radical ions have potentially important effects on the solubility, transport and fractionation of metals and sulfur, with consequences for ore deposit formation and, more generally, for geochemical cycles of metals and volatiles. It is therefore essential to know the intrinsic isotopic properties of these important sulfur species in order to use sulfur isotopes for tracing different geological processes. Here, the theoretical equilibrium isotopic properties of the disulfur and trisulfur radical ions are computed and compared to the hydrogen sulfide (HS) and sulfate ion (SO), using, for the first time, a first-principles molecular dynamics (MD) approach. The isotopic properties are calculated directly from molecular dynamics trajectories using the vibrational density of states and the atomic kinetic energy, and then compared to the more established method based on sampling of several snapshots. This comparison allowed us to validate the new modelling method and to assess its advantages and limitations. The predicted equilibrium isotope fractionation in terms of S/S between S and S is small, i.e. <1‰, with a slight enrichment in the heavier isotope for S, over the temperature range 200–500 °C. Both radical ions are slightly depleted in the heavier isotope, by 1 to 2‰, relative to aqueous HS. Our results help tuning sulfur isotope fractionation models used for tracing the origin and evolution of hydrothermal fluids. Our method opens large perspectives for using the rapidly growing body of MD simulation data in geosciences on structure and stability of aqueous complexes to assess in parallel element isotope fractionations from MD-generated trajectories.

中文翻译：

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从分子动力学轨迹直接预测同位素性质：在热液中硫化物、硫酸盐和硫自由基离子的应用


在热液中，已观察到二硫 (S) 和三硫 (S) 自由基阴离子与主要的硫化氢和硫酸盐共存。这些自由基离子对金属和硫的溶解度、传输和分馏具有潜在的重要影响，对矿床形成产生影响，更一般地说，对金属和挥发物的地球化学循环产生影响。因此，有必要了解这些重要硫物质的固有同位素性质，以便使用硫同位素追踪不同的地质过程。在这里，首次使用第一原理分子动力学 (MD) 方法计算了二硫和三硫自由基离子的理论平衡同位素性质，并将其与硫化氢 (HS) 和硫酸根离子 (SO) 进行比较。同位素性质是使用振动态密度和原子动能直接从分子动力学轨迹计算出来的，然后与基于多个快照采样的更成熟的方法进行比较。这种比较使我们能够验证新的建模方法并评估其优点和局限性。在 200–500 °C 的温度范围内，S 和 S 之间的 S/S 预测平衡同位素分馏很小，即 <1‰，S 的较重同位素略有富集。相对于 H2S 水溶液，两种自由基离子的重同位素略有减少，减少了 1 至 2‰。我们的结果有助于调整用于追踪热液起源和演化的硫同位素分馏模型。 我们的方法为使用地球科学中快速增长的关于水复合物结构和稳定性的 MD 模拟数据来评估 MD 生成轨迹的平行元素同位素分馏开辟了广阔的前景。