Kinetic nitrogen isotope fractionation between air and dissolved N₂ in water: Implications for hydrothermal systems

Abstract

Nitrogen isotopes are widely used to trace volatile sources in volcanic/hydrothermal systems. However, the nitrogen isotope composition of dissolved N₂ may not be the same as N₂ gas, due to minor isotope fractionation. In order to evaluate N isotope fractionation during dissolution of N₂ gas in water as a function of temperature, we measured δ¹⁵N values of dissolved N₂ over a temperature range of 5 to 60°C. Our experiments show that δ¹⁵N values are 0.91, 0.73, –0.04, and –0.42‰ at 5, 20, 40, and 60°C, respectively. The temperature dependent fractionation is stronger than published results and indicates an isotopic “crossover” at 40°C. A possible explanation for the steep temperature dependence is that a kinetic-based incorporation of the light isotope (¹⁴N) into water increases as temperature rises. We show that small negative δ¹⁵N values in natural springs could be due to kinetic fractionation between dissolved N₂ and N₂ in air.