HR: 0800h
 AN: V51E-0832    [Abstracts]
 TI: The Determination of Kinetic Nitrogen-Isotope Fractionation During Partial Decomposition of NH3 at 700 ° C And Its Potential Applications
 AU: * Li, L
 EM: longli@ipgp.jussieu,fr
 AF: Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Université Paris 7, 2 Place Jussieu, T54-64 E1, Paris, 75251, France
AU: Cartigny, P
 EM: cartigny@ipgp.jussieu.fr
 AF: Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Université Paris 7, 2 Place Jussieu, T54-64 E1, Paris, 75251, France
AB: Nitrogen (N) isotopes are potential proxies to trace the early Earth's evolution and detect the biosphere-deep Earth interactions. Yet their application is limited due to our poor knowledge of N-isotope fractionation (equilibrium and kinetic). To better understand the large variations of N isotopic compositions (δ15N from <-25‰ to >35‰) in Earth's materials, we experimentally studied the N-isotope behavior during partial decomposition of NH3 (2NH3 --> N2 + 3H2) at 700 ° C and low pressure (ranging between 3 and 9 bar) using sealed quartz tubes. The δ15N values of remaining NH3 (1.4 to 34.7‰) and produced N2 (-17.8 to -5.7‰) at different decomposition ratios (26 to 99%) reflect Rayleigh fractionation. We verified that there was no N loss during the experiments, thus the N- isotope fractionation factor was determined from isotope measurements of either NH3 or N2. The best fitted N-isotope fractionation factor between NH3 and N2 at 700 ° C (αNH3-N2) is 0.9829 ± 0.0006 (2σ). Following this kinetic effect, N2 with negative δ15N values can be easily produced from the sediments after limited/partial decomposition. This process may help understand the large N isotopic variations, especially the extremely low δ15N values (as low as -22‰) of N2 in the oil field, and also draw attention to the possibility of overestimating the contribution of mantle N2 in the volcanic gas.
DE: 0469 Nitrogen cycling
 DE: 1030 Geochemical cycles (0330)
 DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
 DE: 1041 Stable isotope geochemistry (0454, 4870)
 DE: 8430 Volcanic gases
 SC: Volcanology, Geochemistry, Petrology [V]
 MN: 2007 Fall Meeting