Detection of small amounts of N2 in CO2-rich high-density fluid inclusions from mantle xenoliths

Fluid inclusions (FIs) (n = 34) in spinel lherzolite xenoliths derived from the subcontinental lithospheric mantle from Jeju (S. Korea), the Rio Grande Rift (New-Mexico, USA) and Mt. Quincan (Australia) were analysed using confocal Raman . Despite the significant geographical distances, the studied rocks and their FIs show similar petrographic features. We evidence for the presence of small amounts of nitrogen in extreme high density (≥1 g/cm3 ) FIs from the deep lithospheric mantle Raman spectroscopy with a spectral resolution between 0.6 and 0.7 cm-1 . The only nitrogen-bearing species found in the was N2. The N2 Raman signal appeared as two adjacent, partially overlapping bands. The band at higher Raman shift is with the nitrogen-in-air (N2 ) band. The band at lower Raman shift is therefore nitrogen within the FIs (N2 ). The relative of CO2 and N2 within the FIs were calculated using the integrated band areas. In the studied ultramafic xenoliths, N2 varied between 0.2 and 1.8 mol%, with one exceptionally high concentration of 3.1 mol%. We believe that measurements and/or Raman spectroscopy with an inferior spectral resolution may leave N2 undetected in CO2- upper mantle FIs. Based on our observations, we propose that N2 may be a minor but common component in subcontinental upper-mantle fluids.