Yttrium speciation in subduction-zone fluids from ab initio molecular dynamics simulations
Yttrium speciation in subduction-zone fluids from ab initio molecular dynamics simulations
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DOI：
10.5194/se-11-767-2020
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发表时间：
2020-05
期刊：
影响因子：
3.4
通讯作者：
J. Stefanski;S. Jahn
中科院分区：
文献类型：
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作者：
J. Stefanski;S. Jahn
抽象的。稀土元素是高级变质作用等地质过程的重要地球化学示踪。水流体被认为是各种地质环境中稀土元素的重要载体，包括与俯冲带有关的环境。流体动员稀土的能力在很大程度上取决于其化学成分以及是否存在合适的配体，如氟化物和氯化物。在这项研究中，我们用从头算分子动力学模拟方法，研究了温度为800∘C，压力范围为1.3Gpa至4.5Gpa的氯化钇和氟化钇水溶液的结构和热力学性质。在压力范围内，水和卤化物离子对Y的总配位从7个变化到8个。对于氯化钇物种，观察到最多有三个氯化物配体。所得热力学数据表明，在与板坯脱水相关的条件下，在富氯和富氟的环境中，水合稀土氟化物比氯化钇络合物更稳定。Y(Cl，F)混合络合物即使在分子动力学时间尺度上也是不稳定的。此外，与野外观测相反，热力学模拟表明，在高级交代系统中，Y应该在相当低的氟浓度下被活化。这些结果表明，大多数俯冲带流体中的氟化物活性相当低，因为Y是活动性最小的稀土元素之一。此外，模拟表明，正如在其他高场强元素中观察到的那样，Y驱动水合水分子的自电离。这可能是水溶液中高电荷阳离子在高温高压条件下的一般性质。
Abstract. The rare Earth elements (REEs) are important geochemical tracers for geological processes such as high-grade metamorphism. Aqueous fluids are considered important carriers for the REEs in a variety of geological environments including settings associated with subduction zones. The capacity of a fluid to mobilize REEs strongly depends on its chemical composition and on the presence of suitable ligands such as fluoride and chloride. In this study, we present structural and thermodynamic properties of aqueous yttrium–chloride and yttrium–fluoride species at a temperature of 800 ∘ C in a pressure range between 1.3 and 4.5 GPa derived from ab initio molecular dynamics simulations. The total yttrium coordination by H2O and halide ions changes from seven to eight within the pressure range. For the yttrium–chloride species, a maximum number of three chloride ligands was observed. The derived thermodynamic data show that aqueous yttrium–fluoride complexes are more stable than their yttrium–chloride counterparts in chloride- and fluoride-rich environments at conditions relevant to slab dehydration. Mixed Y(Cl,F) complexes are found to be unstable even on the molecular dynamics timescale. Furthermore, in contrast to field observations, thermodynamic modeling indicates that yttrium should be mobilized at rather low fluoride concentrations in high-grade metasomatic systems. These results suggest a rather low fluoride activity in the majority of subduction-zone fluids because yttrium is one of the least-mobile REEs. Additionally, the simulations indicate that yttrium drives the self-ionization of hydration water molecules as it was observed for other high-field-strength elements. This might be a general property for highly charged cations in aqueous solutions under high-temperature and high-pressure conditions.