Kinetic nitrogen isotope fractionation associated with thermal decomposition of NH3: Experimental results and potential applications to trace the origin of N2 in natural gas and hydrothermal systems

Ammonia (NH3) is the major intermediate phase in the pathway of nitrogen (N) transfer from the fixed N phases (e.g., in crustal material) to free N2 (e.g., in natural gas reservoirs and volcanic gases). Yet the N isotopic behavior during these N-cycling processes remains poorly known. In an attempt to contribute to the understanding of N cycling using N isotopes, we carried out laboratory experiments to investigate the N isotopic effect associated with thermal decomposition of ammonia (2NH3 → N2 + 3H2). Pure NH3 (with initial δ15NNH3NNH3 of ∼ −2‰, relative to air standard) was sealed into quartz tubes and thermally decomposed at 600, 700 or 800 °C from 2 hours to 500 days. With the progress of the reaction, the δ15N of the remaining NH3 and the accumulated N2 increased from −2 to +35‰ and from −20 to −2‰, respectively. The differences of the N-isotope fractionations at the three temperatures are not significant. Modeling using the Rayleigh distillation model yielded similar kinetic N-isotope fractionation factors (αN2–NH3αN2–NH3) of 0.983 ± 0.002 for 600, 700 and 800 °C. Applied to geological settings, this significant isotope discrimination (∼17‰) associated with partial decomposition of NH3/NH4+ from crustal sources (δ15Naverage ∼ +6.3‰) can produce mantle-like (i.e. ∼ −5‰) or even lower δ15N values of N2. This may explain the large variation of δ15N (−20 to +30‰) of N2 in natural gas reservoirs. It can also possibly explain the extreme 15N-depletion of N2 in some volcanic gases. This possibility has to be carefully considered when using N isotopes to trace geological N cycling across subduction zones by analysis of volcanic N2.