Nitrogen distribution and nitrogen isotope fractionation in synthetic 2:1 phyllosilicates under hydrothermal conditions at 200 °C and saturated vapor pressure

Abstract

This study investigates nitrogen distribution and isotope fractionation within synthetic 2:1 phyllosilicates, simulating submarine hydrothermal environments at 200 °C and saturated vapor pressure. XRD and EDS results revealed the potential coexistence of multiple cations in the interlayer of synthetic 2:1 phyllosilicate, concurrently suggesting cation substitution in the tetrahedral and/or octahedral sheets. Meanwhile, the iron-enriched 25-5 sample exhibited restricted interlayer expansibility. NH₄⁺ absorptions were identified in the NH₄-stretching (3200–2800 cm⁻¹) and NH₄-bending (1450–1400 cm⁻¹) regions, with wavenumber shifts indicating the influence of interlayer water removal. At pH 10.56, over 95% of nitrogen was released into the gas phase, while at pH 8.88, nitrogen proportions in the liquid and gas phases were comparable (average 48–49%). Experiments with iron at pH ∼8.80 showed that the nitrogen proportion in the gas phase (average 28%) was more than twofold lower than that in the liquid phase (average 68%). Equilibrium isotope fractionation factors indicated discernible preference for heavier nitrogen isotopes in the solid phase (α_solid-liquid = 1.009–1.021 and α_solid-gas = 1.011–1.027). The α_liquid-gas range for sample 25–2 was 1.001–1.008, while that for the iron-enriched composite 25–5 was 0.997–1.010. Our experimental studies have confirmed that, in the absence of exchange interactions with external substances possessing different nitrogen isotope ratios, nitrogen isotope fractionation between ammonium and ammonia, controlled by variations in temperature and pH during mineralization, plays a crucial role in the variation of nitrogen isotope ratios. Additionally, we confirmed that metal-amines influence nitrogen isotope fractionation by modulating ammonia gas emission. These findings enhance our understanding of nitrogen cycling across the gas, liquid, and solid phases in submarine hydrothermal systems.