Stability of ammonia in C-O-H-N fluids at 3.0-7.8 GPa and 750–1100 °C: implications for the efficiency of nitrogen flux into the mantle

The knowledge of Nitrogen speciation in the C-O-H-N fluid phase has important implications for the efficiency of Nitrogen flux and recycling in the mantle. The composition of nitrogen-rich C-O-H-N fluids is studied experimentally at pressures from 3.0 to 7.8 GPa, temperatures from 750 to 1090°С, and oxygen fugacity (fO₂) at the iron-wȕstite (IW) to fayalite-magnetite-quartz (FMQ) buffers simulating the conditions of a slab subducted to mantle depths from 100 to 250 km. Gas chromatography-mass spectrometry of the experimental quenched fluids reveals ammonia (NH₃), N₂, H₂O, CO₂, CH₄, as well as minor amounts of light alkanes and oxygen-bearing organics, while the NH₃/(N₂ + NH₃) ratio ranges from 0.01 to 0.97. Most of the experimental C-O-H-N fluids contain low fractions of NH₃ (≤ 0.12), while higher values are possible only at very low fO₂ unrealistic for subduction zones at depths < 250 km. A semi-empirical model, with the equilibrium constant K*(K*=\(\:{X}_{{NH}_{3}}^{2}\)/\(\:{X}_{{N}_{2}}\)), is suggested to predict nitrogen speciation for a sediment-derived mobile phase with reduced H₂O contents for typical P-T-fO₂ ranges. As inferred previously, the fraction of NH₃ can exceed the N₂ fraction (\(\:{X}_{{NH}_{3}}^{}\)/\(\:{X}_{{N}_{2}}\) >1) only in diluted aqueous fluids generated at temperatures of warm subduction and at a pressure of ~3.0 GPa, but it remains negligible in the hot subduction conditions. The transformation of the aqueous fluid into a supercritical fluid-melt (with reduced H₂O fraction) in warm or hot slabs subducted to depths equivalent to P≥3.5-5.5 GPa, is demonstrated to decrease significantly the fraction of NH₃ in the mobile phase, the redox conditions being invariable. In general, the aqueous fluid-to-SCFM transformation is expected to reduce the concentrations of nitrogen species compatible with phengite. Correspondingly, the coefficient of nitrogen partitioning between the mobile phase and phengite becomes lower with increasing pressure, which reduces the flux of nitrogen carried by both warm and hot slabs to depths >150 km.