Résumé
Nitrogen is the most abundant constituent of Earth's atmosphere. The exchange of reduced and oxidized species forms the surficial nitrogen cycle well characterized in biogeochemistry. Besides this surficial cycle, nitrogen is also a constituent of high-pressure fluids. The analysis of mantle xenoliths for nitrogen generally yield low concentrations of 0.1-10 mu g g (super -1) N. These low concentrations are due to degassing of nitrogen as N (sub 2) and the incompatibility of nitrogen within anhydrous mantle minerals. In contrast, chondrites contain up to 100-1000 mu g g (super -1) nitrogen and thus indicate that Earth's interior may possess significant nitrogen-rich reservoirs. Hydrous mantle minerals, such as phlogopite, incorporate nitrogen in form of ammonium (NH (sub 4) (super +) ) which substitutes for K (super +) . When brought to the surface as mantle xenoliths, nitrogen is "protected" from degassing within the minerals' lattice. Thus, the analysis of ammonium in phlogopite enables constraints on nitrogen concentration and redistribution within the Earth's mantle. We measured phlogopite in various natural micaceous ultramafic rocks using an Elementar VarioCube CHNS analyzer. N concentrations in phlogopite are <60 mu g g (super -1) in intraplate mantle xenoliths and to up to 500 mu g g (super -1) in samples from subduction zone settings. Limits of detection of 30 mu g g (super -1) N are governed by the adhesion of atmospheric N on the sample surface. We are working on lowering this detection limit by drying samples in a vacuum oven coupled with gas exchange of air by Ar.