Nitrogen isotope fractionations among gaseous and aqueous NH4+ NH3, N2, and metal-ammine complexes: Theoretical calculations and applications

Ammonium (NH4+), ammonia (NH3) and N-2 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 N-2, 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-delta N-1(5) relationship previously observed in meta-gabbros. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Ammonium, ammonia and N-2 are key nitrogen species in geological nitrogen recycling, with ammonia playing an important role in mobilizing base metals in hydrothermal fluids. The nitrogen isotope fractionation factors are essential parameters in tracing source signatures and geochemical properties. However, the fractionation factors for metal-ammine complexes are largely unknown, highlighting the complex behavior of nitrogen isotopes in natural hydrothermal systems.