Nitrogen isotope fractionations among gaseous and aqueous NH4+, NH3, N2, and metal-ammine complexes: Theoretical calculations and applications
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 results with and without anharmonic corrections indicates that anharmonic effect on 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.
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