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 N-2 (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 (2NH(3) -> N-2 + 3H(2)). Pure NH3 (with initial delta N-15(NH3) of similar to -2 parts per thousand, relative to air standard) was sealed into quartz tubes and thermally decomposed at 600, 700 or 800 degrees C from 2 hours to 500 days. With the progress of the reaction, the delta N-15 of the remaining NH3 and the accumulated N-2 increased from -2 to + 35 parts per thousand and from -20 to -2 parts per thousand, 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 (alpha(N2-NH3)) of 0.983 +/- 0.002 for 600, 700 and 800 degrees C. Applied to geological settings, this significant isotope discrimination (similar to 17 parts per thousand) associated with partial decomposition of NH3/NH4+ from crustal sources (delta N-15(average) similar to + 6.3 parts per thousand) can produce mantle-like (i.e. similar to -5 parts per thousand) or even lower delta N-15 values of N-2. This may explain the large variation of delta N-15 (-20 to + 30 parts per thousand) of N-2 in natural gas reservoirs. It can also possibly explain the extreme N-15-depletion of N-2 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 N-2. (C) 2009 Elsevier Ltd. All rights reserved.
Li, Long Cartigny, Pierre Ader, Magali