Elsevier

Geochimica et Cosmochimica Acta

Volume 295, 15 February 2021, Pages 80-97
Geochimica et Cosmochimica Acta

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

https://doi.org/10.1016/j.gca.2020.12.010Get rights and content

Abstract

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.

Introduction

Geological nitrogen cycle, which is commonly mediated by hydrothermal fluids (e.g., Busigny and Bebout, 2013, Li et al., 2007, Li et al., 2014, Halama et al., 2010, Halama et al., 2017), involves transformation of various nitrogen species within or between Earth’s reservoirs, e.g., the atmosphere, crust, and mantle (e.g., Halama et al., 2014; Mikhail and Sverjensky, 2014; Bebout et al., 2016). In particular, the nitrogen transfer between the atmosphere, in which nitrogen occurs mainly as N2, and the lithosphere, in which nitrogen mainly occurs as NH4+ substituting K+ in mineral lattices (Honma and Itihara, 1981), may pass through an intermediate nitrogen species of NH3 (e.g., Brandes et al., 1998, Li et al., 2007, Li et al., 2009, Li et al., 2014). NH3 may also play an important role in alkaline fluids occurring in a variety of geological settings, such as the deep aquifer in ophiolites (e.g., the Oman ophiolite, the Coast Range ophiolite; see Holm et al., 2006 and reference therein), the deep subsurface fracture waters in Precambrian cratons (e.g., South Africa; Onstott et al., 2006), ridge flank hydrothermal systems (e.g., Lost City, Rainbow; see the discussion in Li et al., 2012 and reference therein), subduction zones (e.g., Mariana forearc; Wheat et al., 2008), hot spots (e.g., Yellowstone; Holloway et al., 2011), and alkaline lakes (e.g., Lake Bosumtwi, Ghana; Talbot and Johannessen, 1992). This is because NH4+ in alkaline fluids can be dissociated into NH3, which can be further removed from the fluid by degassing. The relative proportions of NH4+, NH3, and N2 in hydrothermal fluid are strongly dependent on the redox and pH conditions (e.g., Duit et al., 1986; Li et al., 2012; Li and Keppler, 2014; Mikhail and Sverjensky, 2014).

Besides being a key species in geological nitrogen cycle, NH3 may also play an important role in hydrothermal enrichment and mobilization of base metals because it is an effective ligand to form metal-ammine complexes with transition metals, such as Cu (Hathaway and Tomlinson, 1970, Han et al., 1974, Chu et al., 1978), Ni (Gupta and Sarpal, 1967), Co (Meek and Ibers, 1970), Zn (Eßmann, 1995), and Ag (Geddes and Bottger, 1969, Widmer-Cooper et al., 2001, Fox et al., 2002). This property of NH3 has been applied in industry to recover transition metals from ore deposits (e.g., Meng and Han, 1996, Katsiapi et al., 2010). In natural hydrothermal system, a possible coupling between NH3 and Cu has been proposed based on geochemical signatures of hydrothermally altered gabbros (Busigny et al., 2011). If NH3 can promote the solubility and mobility of base metals in hydrothermal system, it may potentially act as an important agent for ore genesis (Irving and Williams, 1953, Martell and Hancock, 1996).

Nitrogen isotopes have been used as a robust tool to trace nitrogen remobilization (e.g., Bebout et al., 1999, Busigny et al., 2005, Li et al., 2007) and geological nitrogen recycling (e.g., Bebout and Fogel, 1992; Busigny et al., 2003, Svensen et al., 2008, Halama et al., 2010, Li et al., 2009, Li et al., 2014). In order to apply nitrogen isotope system to constrain nitrogen sources and fluxes in geological nitrogen recycling pathways, the nitrogen isotope fractionation factors between involved nitrogen species are crucial prerequisite parameters. However, the nitrogen isotope fractionation factors between aqueous NH3 and metal-ammine complexes have not been well constrained yet, despite some early efforts (e.g., Gupta and Sarpal, 1967, Ishimori, 1960a). Several previous studies (Urey, 1947, Scalan, 1958, Hanschmann, 1981, Petts et al., 2015) have investigated the equilibrium nitrogen isotope fractionations among NH4+, NH3 and N2 by theoretical calculations and given very different results. These calculations were based on vibrational frequencies of nitrogen species in gas phases. However, in natural systems, particularly in hydrothermal systems, NH4+ and NH3 mostly exist in aqueous phases (hereafter referred as NH4+·nH2O or NH4+(aqueous), and NH3·nH2O or NH3(aqueous), respectively). One previous laboratory experimental study (Li et al., 2012) showed that, under hydrothermal condition, partial dissociation of NH4+·nH2O coupled with complete degassing of the produced NH3 induced large 15N enrichments in the remaining NH4+·nH2O, which cannot be explained by the theoretically predicted equilibrium fractionation factors between the NH4(gaseous)+ - NH3(gaseous) pair (Urey, 1947, Scalan, 1958, Hanschmann, 1981). To solve the discrepancy between the experimental and theoretical results, Li et al. (2012) proposed that the NH4+·nH2O dissociation – NH3 degassing process involved an intermediate step that NH4+·nH2O was first equilibrated with NH3·nH2O, from which NH3 was further exsolved and degassed (Li et al., 2012). Such a process can be described as Eq. (1):NH4+·nH2O + OH ⇋ NH3·nH2O + H2O → NH3(gaseous) + (n + 1)H2O

Given that the produced NH3·nH2O was completely removed by NH3 degassing in the experiments, and more importantly, the 15N enrichments in the remaining NH4+·nH2O apparently fitted well to a batch model assuming equilibrium isotope fractionation between NH4+·nH2O and NH3·nH2O, Li et al. (2012) interpreted the strong 15N enrichments observed in the remaining NH4+·nH2O as a result of large equilibrium isotope fractionations (e.g., +45.4‰ at 23 °C and + 33.5‰ at 70 °C) between NH4+·nH2O and NH3·nH2O without considering kinetic isotopic effect from NH3 degassing. However, a recent laboratory experimental study (Deng et al., 2018) found significant kinetic nitrogen isotopic effect (-8.2‰ at 21 °C, and −5.2‰ at 70 °C) during degassing of NH3(gas) from NH3·nH2O. In addition, a recent theoretical calculation (Walters et al., 2019) using relatively simple HF/6-31G(d) and B3LYP/6-31G(d) levels of theory yielded significantly different nitrogen isotope fractionation factors between NH4+·nH2O and NH3·nH2O. Thus, it is necessary to reassess the isotopic behavior during NH3 degassing process described by Eq. (1).

To fill these knowledge gaps, we employed theoretical calculations to determine the equilibrium isotope fractionations among gaseous N2 and several other nitrogen species related to NH3 in hydrothermal fluids, including NH4+ and NH3 in both gaseous and aqueous phases and metal-ammine complexes of several important base metals, i.e., Co, Ni, Cu, Zn, Cd, Ag, Au, and Pt. Theoretical calculation is a robust and efficient way to estimate the equilibrium isotope fractionation factors among these species, given that they are difficult to be characterized by laboratory experiments.

Section snippets

Equilibrium isotope fractionation theory

The equilibrium isotope fractionation factor between a species and its atomic form can be described by the β factor (Urey, 1947, Bigeleisen and Mayer, 1947). The details of the Urey-Bigeleisen-Mayer model for theoretical calculation of equilibrium isotope fractionation factor have been intensively reviewed in the literature (e.g., Richet et al., 1977, Schauble et al., 2004, Liu et al., 2010; Young et al., 2015; Dauphas and Schauble, 2016; Blanchard et al., 2017). In brief, for an isotope

Anharmonic effect on isotope fractionation between gaseous NH4+ and NH3

The results of harmonic vibrational frequencies (wi) and anharmonicity constants (xij) for NH3 and NH4+ are listed in Table 1. Comparison between the lnαNH4+(gaseous)-NH3(gaseous) results with and without anharmonic corrections indicates that anharmonic effect on lnαNH4+(gaseous)-NH3(gaseous) is small, i.e., −1.4‰ at 0 °C, −0.6‰ at 400 °C, and −0.3‰ at 1000 °C (Table 2). Therefore, we did not apply the anharmonic corrections for the other nitrogen species in this study. For consistency, we used

Factors controlling the N isotope fractionations in the NH4+ – NH3 – metal-ammine complex system

Fig. 3 shows that 15N is most enriched in NH4+ and most depleted in NH3(gaseous). This is consistent with a more stable tetrahedral structure of NH4+ relative to the pyramidal structure of NH3. The hydration of NH3 induces significant 15N enrichment in NH3(aqueous) relative to NH3(gaseous), because of the additional N-H bond formed in NH3(aqueous) (Fig. 1). However, the hydration of NH4(gaseous)+ does not cause much more 15N enrichment in NH4+(aqueous) (+0.4‰; Table 3), because the bonding

Complicated isotopic effect during NH3 degassing

NH3 degassing may occur in alkaline fluids in a variety of geological settings (see discussion in Li et al., 2012, Deng et al., 2018). Li et al. (2012) carried out laboratory experiments to simulate NH3 degassing in the field at a temperature range from 2 °C to 70 °C. The experiments were started by adding inadequate hydroxyl to partially dissociate NH4+ in a solution to drive NH3 degassing, i.e., the processes described by Eq. (1). The nitrogen isotopic compositions of the remaining NH4+ after

Conclusion

Theoretical calculations of equilibrium nitrogen isotope fractionation factors between gaseous and aqueous ammonium, ammonia, N2 and metal-ammine complexes indicate that 15N is enriched following the order of NH4+ >N2 > NH3(aqueous) > NH3(gaseous), with all but one metal-ammine complexes lying between NH4+ and NH3(aqueous). Our calculation suggests anharmonic effect is not significant on the isotope fractionation between NH4+ and NH3. In the metal-ammine complexes, coordination number may play

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study is supported by the NSERC-Discovery Grant to LL, the China Postdoctoral Science Foundation [2019M660811] to YH, and Chinese NSF project [41530210] to YL. We thank Y. Zhang for performing the anharmonic tests and valuable discussions. The paper has been greatly improved by constructive comments from Associate Editor Dr. M. Blanchard, Dr. S. Mikhail, and two anonymous reviewers.

References (109)

  • B. Hathaway et al.

    Copper (II) ammonia complexes

    Coord. Chem. Rev.

    (1970)
  • P.S. Hill et al.

    Modeling the effects of bond environment on equilibrium iron isotope fractionation in ferric aquo-chloro complexes

    Geochim. Cosmochim. Acta

    (2008)
  • J.M. Holloway et al.

    Ammonium in thermal waters of Yellowstone National Park: processes affecting speciation and isotope fractionation

    Geochim. Cosmochim. Acta

    (2011)
  • H. Honma et al.

    Distribution of ammonium in minerals of metamorphic and granitic rocks

    Geochim. Cosmochim. Acta

    (1981)
  • A. Katsiapi et al.

    Cobalt recovery from mixed Co-Mn hydroxide precipitates by ammonia-ammonium carbonate leaching

    Miner. Eng.

    (2010)
  • K.S. Kim et al.

    Structures and energetics of Zn(NH3)2+ n (n= 4–6). Coordination number of Zn2+ by ammine

    Chem. Phys. Lett.

    (1993)
  • Y. Li et al.

    Nitrogen speciation in mantle and crustal fluids

    Geochim. Cosmochim. Acta

    (2014)
  • L. Li et al.

    Nitrogen concentration and δ15N of altered oceanic crust obtained on ODP Legs 129 and 185: insights into alteration-related nitrogen enrichment and the nitrogen subduction budget

    Geochim. Cosmochim. Acta

    (2007)
  • L. Li et al.

    Kinetic nitrogen isotope fractionation associated with thermal decomposition of NH3: Experimental results and potential applications to trace the origin of N2 in natural gas and hydrothermal systems

    Geochim. Cosmochim. Acta

    (2009)
  • L. Li et al.

    Ammonium stability and nitrogen isotope fractionations for NH3(aq)–NH3(gas) systems at 20–70 °C and pH of 2–13: applications to habitability and nitrogen cycling in low-temperature hydrothermal systems

    Geochim. Cosmochim. Acta

    (2012)
  • L. Li et al.

    The nitrogen record of crust-mantle interaction and mantle convection from Archean to present

    Earth Planet. Sci. Lett.

    (2014)
  • X. Li et al.

    Equilibrium Se isotope fractionation parameters: a first-principles study

    Earth Planet. Sci. Lett.

    (2011)
  • Q. Liu et al.

    On the proper use of the Bigeleisen–Mayer equation and corrections to it in the calculation of isotopic fractionation equilibrium constants

    Geochim. Cosmochim. Acta

    (2010)
  • Y. Liu et al.

    Ab initio molecular orbital calculations for boron isotope fractionations on boric acids and borates

    Geochim. Cosmochim. Acta

    (2005)
  • M. Méheut et al.

    Equilibrium isotopic fractionation in the kaolinite, quartz, water system: Prediction from first-principles density-functional theory

    Geochim. Cosmochim. Acta

    (2007)
  • K.B. Nilsson et al.

    The coordination chemistry of the copper(II), zinc(II) and cadmium(II) ions in liquid and aqueous ammonia solution, and the crystal structures of hexaamminecopper(II) perchlorate and chloride, and hexaamminecadmium(II)chloride

    J. Molecular Liquids

    (2007)
  • M. Pavelka et al.

    Theoretical description of copper Cu (I)/Cu (II) complexes in mixed ammine-aqua environment. DFT and ab initio quantum chemical study

    Chem. Phys.

    (2005)
  • D. Petts et al.

    A nitrogen isotope fractionation factor between diamond and its parental fluid derived from detailed SIMS analysis of a gem diamond and theoretical calculations

    Chem. Geol.

    (2015)
  • C.E. Rees

    A teady-state model for sulphur isotope fractionation in bacterial reduction processes

    Geochim. Cosmochim. Acta

    (1973)
  • J.R. Rustad et al.

    Isotopic fractionation of Mg 2+(aq), Ca 2+(aq), and Fe 2+(aq) with carbonate minerals

    Geochim. Cosmochim. Acta

    (2010)
  • M.M. Savard et al.

    δ15N values of atmospheric N species simultaneously collected using sector-based samplers distant from sources – Isotopic inheritance and fractionation

    Atm. Environ.

    (2017)
  • E. Schauble et al.

    Theoretical estimates of equilibrium chromium-isotope fractionations

    Chem. Geol.

    (2004)
  • E. Schauble et al.

    Preferential formation of 13C–18O bonds in carbonate minerals, estimated using first-principles lattice dynamics

    Geochim. Cosmochim. Acta

    (2006)
  • T. Shoeib et al.

    A study of complexes Mg(NH3)n and Ag (NH3) n+, where n = 1–8: competition between direct coordination and solvation through hydrogen bonding

    Inter. J. Mass Spectrom.

    (2000)
  • H. Svensen et al.

    Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa

    Geochim. Cosmochim. Acta

    (2008)
  • M.R. Talbot et al.

    A high resolution palaeoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter

    Earth Planet. Sci. Lett.

    (1992)
  • P.R. Varadwaj et al.

    Low-spin complexes of Ni2+ with six NH3 and H2O ligands: A DFT–RX3LYP study

    J. Mol. Struct. (THEOCHEM)

    (2009)
  • M. Barnet et al.

    The Co–N bond lengths in Co II and Co III hexammines

    Chem. Commun. (London)

    (1966)
  • G.E. Bebout et al.

    Nitrogen-isotope record of fluid-rock interactions in the Skiddaw Aureole and granite, English Lake District

    Amer. Mineral.

    (1999)
  • G.E. Bebout et al.

    Pathways for nitrogen cycling in Earth’s crust and upper mantle: A review and new results for microporous beryl and cordierite

    Amer. Mineral.

    (2016)
  • A.D. Becke

    Density-functional thermochemistry. III. The role of exact exchange

    J. Chem. Phys.

    (1993)
  • J. Bigeleisen et al.

    Calculation of equilibrium constants for isotopic exchange reactions

    J. Chem. Phys.

    (1947)
  • M. Blanchard et al.

    Equilibrium fractionation of non-traditional isotopes: A molecular modeling perspective

    Rev. Mineral. Geochem.

    (2017)
  • J.A. Brandes et al.

    Abiotic nitrogen reduction on the early Earth

    Nature

    (1998)
  • V. Busigny et al.

    Nitrogen in the silicate Earth: Speciation and isotopic behavior during mineral–fluid interactions

    Elements

    (2013)
  • V. Busigny et al.

    Nitrogen content and isotopic composition of oceanic crust at a superfast spreading ridge: a profile in altered basalts from ODP Site 1256, Leg 206

    Geochem. Geophys. Geosyst.

    (2005)
  • D. Casanova et al.

    Restricted active space spin-flip configuration interaction approach: theory, implementation and examples

    Phys. Chem. Chem. Phys.

    (2009)
  • F. Chen et al.

    Electronic, structural, and hyperfine interaction investigations on rydberg molecules: NH4, OH3, and FH2

    J. Phys. Chem. A

    (2001)
  • T.-Y.-J. Chu et al.

    Removal of complex copper-ammonia ions from aqueous wastes with fly ash

    J. (Water Pollution Control Federation)

    (1978)
  • N. Dauphas et al.

    Mass spectrometry and natural variations of iron isotopes

    Mass Spectrom. Rev.

    (2006)
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