Ammonium (NH₄⁺), ammonia (NH₃) and N₂ are key nitrogen species in geological nitrogen recycling. NH₃ has also been proposed to play an important role in mobilizing base metals in the form of metal-ammine complexes in hydrothermal fluids. The nitrogen isotope fractionation factors among these nitrogen species in aqueous and gaseous phases are essential parameters to trace source signatures and geochemical properties in geological processes. However, the nitrogen isotope fractionation factors for metal-ammine complexes are largely absent, and the few existing nitrogen isotope fractionation factors for the aqueous NH₄⁺ – aqueous NH₃ pair show large discrepancy between experimental results and theoretical calculations. In this study, we employed the density functional theory to systematically calculate the nitrogen isotope fractionation factors among the nitrogen species that may occur in a hydrothermal system, i.e., gaseous N₂, gaseous and aqueous NH₄⁺ and NH₃, and ammine complexes of Co, Zn, Cu, Cd, Ag, Au, and Pt. Based on these new results, the large nitrogen isotope fractionations for the aqueous NH₄⁺ – aqueous NH₃ pair observed in previous experimental studies can be well explained by a combined effect of an equilibrium isotope fractionation between aqueous NH₄⁺ and aqueous NH₃ and a kinetic isotope fractionation during NH₃ degassing from the solution. This suggests that the nitrogen isotopic behavior during NH₃ degassing in natural hydrothermal system can be more complicated than previous thought. A numeric model is thus established here to quantify the combined isotopic effect on partial NH₃ degassing. Using the new results of metal-ammine complexes, we also tested the hypothesis that nitrogen mobilization could be controlled by copper-ammine complex based on the copper concentration-δ¹⁵N relationship previously observed in meta-gabbros.

铵（NH ₄ ⁺），氨（NH ₃）和N ₂是地质氮循环中的关键氮物种。还提出了NH ₃在以水-热流体形式以金属-胺配合物的形式移动贱金属中起重要作用。这些氮物质在水相和气相中的氮同位素分馏因子是追踪地质过程中的源特征和地球化学性质的基本参数。但是，金属-胺配合物的氮同位素分馏因子基本不存在，而NH ₄ ⁺ – NH ₃水溶液中存在的氮同位素分馏因子很少。对显示实验结果与理论计算之间存在很大差异。在这项研究中，我们采用密度泛函理论系统地计算了在水热系统中可能发生的氮物种中的氮同位素分馏因子，即气态N ₂，气态和水溶液NH ₄ ⁺和NH ₃以及氨的配合物。 Co，Zn，Cu，Cd，Ag，Au和Pt。基于这些新的结果，先前的实验研究中观察到的NH ₄ ⁺ -NH ₃水溶液对的较大氮同位素分馏可以很好地解释为NH ₄ ⁺水溶液之间的平衡同位素分馏的综合作用。NH ₃水溶液和NH ₃从溶液脱气过程中的动力学同位素分馏。这表明天然热液系统中NH ₃脱气过程中的氮同位素行为可能比先前的想法更为复杂。因此，在此建立了数值模型，以量化对部分NH ₃脱气的联合同位素效应。使用的金属-氨络合物的新的结果，我们还检验了氮动员可通过铜氨络合物基于所述铜浓度-δ来控制¹⁵预先在元辉长岩观察到N的关系。