The relationship between mantle pH and the deep nitrogen cycle

Abstract

8Nitrogen is distributed throughout all terrestrial geological reservoirs (i.e., the crust, mantle, and 9 core), which are in a constant state of disequilibrium due to metabolic factors at Earth's surface, 10 chemical weathering, diffusion, and deep N fluxes imposed by plate tectonics. However, the 11 behavior of nitrogen during subduction is the subject of ongoing debate. There is a general 12 consensus that during the crystallization of minerals from melts, monatomic nitrogen behaves like 13 argon (highly incompatible) and ammonium behaves like potassium and rubidium (which are 14 relatively less incompatible). Therefore, the behavior of nitrogen is fundamentally underpinned by 15 its chemical speciation. In aqueous fluids, the controlling factor which determines if nitrogen is 16 molecular (N 2 ) or ammonic (inclusive of both NH 4 + and NH 3 0 ) is oxygen fugacity, whereas pH 17 designates if ammonic nitrogen is NH 4 + and NH 3 0 . Therefore, to address the speciation of nitrogen 18 at high pressures and temperatures, one must also consider pH at the respective pressure-19 temperature conditions. To accomplish this goal we have used the Deep Earth Water Model 20 (DEW) to calculate the activities of aqueous nitrogen from 1-5 GPa and 600-1000 °C in equilibrium 21 with a model eclogite-facies mineral assemblage of jadeite + kyanite + quartz/coesite 22 (metasediment), jadeite + pyrope + talc + quartz/coesite (metamorphosed mafic rocks), and 23 carbonaceous eclogite (metamorphosed mafic rocks + elemental carbon). We then compare these 24 data with previously published data for the speciation of aqueous nitrogen across these respective 25 P-T conditions in equilibrium with a model peridotite mineral assemblage (Mikhail, S. Sverjensky, 26 D.A. 2014. Nature. Geoscience 7, 816-819). In addition, we have carried out full aqueous 27 speciation and solubility calculations for the more complex fluids in equilibrium with jadeite + 28 pyrope + kyanite + diamond, and for fluids in equilibrium with forsterite + enstatite + pyrope + 29 diamond. 30Our results show that the pH of the fluid is controlled by mineralogy for a given pressure and 31 temperature, and that pH can vary by several units in the pressure-temperature range of 1-5 GPa 32 2 and 600-1000 °C. Our data show that increasing temperature stabilizes molecular nitrogen and 33 increasing pressure stabilizes ammonic nitrogen. Our model also predicts a stark difference for the 34 dominance of ammonic vs. molecular and ammonium vs. ammonia for aqueous nitrogen in 35 equilibrium with eclogite-facies and peridotite mineralogies, and as a function of the total 36 dissolved nitrogen in the aqueous fluid where lower N concentrations favor aqueous ammonic 37 nitrogen stabilization and higher N concentrations favor aqueous N 2 . 38Furthermore, we present thermodynamic evidence for nitrogen to be reconsidered as an 39 extremely dynamic (chameleon) element whose speciation and therefore behavior is determined 40 by a combination of temperature, pressure, oxygen fugaci...

HighPGibbs, a Practical Tool for Fluid‐Rock Thermodynamic Simulation in Deep Earth and its Application on Calculating Nitrogen Speciation in Subduction Zone Fluids

“…Consistent with previous modeling results (Mikhail & Sverjensky, 2014; Mikhail et al, 2017), this study shows that the speciation of aqueous nitrogen is controlled mainly by the redox and total nitrogen content of the slab fluids. In altered marine basalts (Figures 2b–2d), which are more oxidized than their unaltered precursors, N 2 (aq) predominates over reduced nitrogen species (NH 3 (aq) and NH 4 + ) in a broader P‐T regime than that in fresh basalts (Figures 3b–3d).…”

“…Apart from mantle wedge redox, another factor that may influence the Earth's global budget of nitrogen circulation is whether nitrogen in subducting slabs can be recycled into the Earth's interior or released by slab fluids. To address this issue, Mikhail et al (2017) calculated the speciation of nitrogen in eclogite‐buffered fluids at four individual P‐T conditions (10 and 50 kbar and 600 and 1000 °C), showing that the subducting crust provides suitable pH and redox to stabilize the mineralogically incompatible nitrogen species N 2 (aq) or NH 3 (aq), thus favoring nitrogen liberation during devolatilization. These results demonstrated the importance of the lithology of subducting rock, which buffers fluid pH and f O 2 (g), in controlling the speciation of aqueous nitrogen.…”

“…It is noted that nitrogen‐bearing mineral end members are not included in the solid solutions and thus, the nitrogen exchanges between minerals and aqueous species are not directly simulated. Natural eclogites host ppm levels of nitrogen as substitution for K + in minerals such as phengite and clinopyroxene (Mikhail et al, 2017). Given the high solubility of nitrogen in the form of N 2 (aq) or NH 3 (aq), the incompatible nitrogen can easily be leached by aqueous fluids (e.g., Duan et al, 2000).…”

“…The model was coded as an Excel spreadsheet along with a revised thermodynamic database of aqueous species (available from Sverjensky et al, 2014). The spreadsheet calculates the standard partial molal Gibbs free energies of aqueous species as well as changes in Gibbs free energy of reactions among aqueous species, pure minerals, and gases at a given P‐T and was used to provide important insights in the nature of mantle fluids, such as the speciation of aqueous carbon (Sverjensky et al, 2014; Tumiati et al, 2017) and nitrogen (Mikhail & Sverjensky, 2014; Mikhail et al, 2017).…”

Состав Флюида Восстановленной Мантии По Экспериментальным Данным И Результатам Изучения Флюидных Включений В Алмазах

“…В рамках этой статьи мы не рассматривали влияние pH (меры кислотности-щелочности) на стабильность компонентов водных флюидов перидотитов и эклогитов. Согласно данным работ(Mikhail and Sverjensky, 2014;Mikhail et al, 2017) этот параметр в значительной мере может определять во флюидах область стабильности (NH4) + и NH3.…”