Fluid inclusions, ranging from pure N2 to pure CO2, occur in olivine porphyroclasts in spinel dunite xenoliths (chrome-diopside suite) from two localities within the Quaternary to Historic alkaline lavas of Lanzarote, Canary Islands. This is the first report of fluid inclusions containing major amounts of N2 in mantle xenoliths. The nitrogen-rich fluid inclusions predate at least one generation of nitrogen-free carbon dioxide inclusions; textural evidence indicates that the inclusions were trapped within the upper mantle. Some of the nitrogen-rich fluid inclusions are intimately associated with solid inclusions of spinel. The nitrogen-rich fluid was most likely produced in-situ, by oxidation-dehydration reactions destabilizing ammonium-bearing silicate minerals (phlogopite, amphibole), increasing oxygen fugacity or, possibly, increasing temperature of the mantle. This process could be related to an event of CO2 and silicate melt injection at 6-8 kbar (Neumann et al., in press), or to some earlier event in the evolution of the mantle beneath Lanzarote. The existence of a N2-rich fluid phase in at least some mantle lithology(ies) at certain conditions is demonstrated by these data. This discovery has consequences for the understanding of the evolution of the mantle below the Canary Islands, as well as for the global nitrogen budget.