Fast chemical and isotopic exchange of nitrogen during reaction with hot molybdenum

Reika Yokochi (1) , Bernard Marty (1, 2)

1 CRPG - Centre de Recherches Pétrographiques et Géochimiques
2 ENSG - Ecole Nationale Supérieure de Géologie

Molybdenum crucibles are commonly used to extract nitrogen from geological samples by induction heating. Because nitrogen is known to be reactive with certain metals (e.g., Ti and Fe), we have tested the reactivity of gaseous nitrogen with a Mo crucible held at 1800°C. The consumption of nitrogen, determined by monitoring the N₂/⁴⁰Ar ratio of the gas phase, varied between 25 and 100%, depending on the reaction duration. Nitrogen of the reacted gas was found to be systematically enriched in ¹⁵N relative to ¹⁴N by 10‰ compared to the initial isotopic composition, without any correlation with nitrogen consumption. We propose that a rapid isotopic exchange occurs between nitrogen originally trapped in the crucible and nitrogen from the gas phase, which modifies the isotopic composition of the reacted gas. This process can significantly bias the isotopic determination of nitrogen in rocks and minerals when a Mo furnace is used for gas extraction. Meanwhile, the rate of N-Mo chemical bonding may be controlled by the formation of nitride (rather than solid solution), a process slower than the isotopic exchange. The use of a Mo furnace for the extraction of trace nitrogen from rocks and minerals should therefore be avoided.

Domaines: Planète et Univers [physics]; Sciences de la Terre

Reika Yokochi, Bernard Marty. Fast chemical and isotopic exchange of nitrogen during reaction with hot molybdenum. Geochemistry, Geophysics, Geosystems, 2006, 7, ⟨10.1029/2006GC001253⟩. ⟨insu-03619228⟩