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

Abstract

Fluid inclusions (FIs) ( n = 34) in spinel lherzolite xenoliths derived from the subcontinental lithospheric mantle from Jeju Island (S. Korea), the Rio Grande Rift (New-Mexico, USA) and Mt. Quincan (Australia) were analysed using confocal Raman spectroscopy. Despite the significant geographical distances, the studied rocks and their FIs show similar petrographic features. We show evidence for the presence of small amounts of nitrogen in extreme high density (≥1 g/cm 3 ) FIs from the deep lithospheric mantle using Raman spectroscopy with a spectral resolution between 0.6 and 0.7 cm −1 . The only nitrogen-bearing species found in the inclusions was N 2 . The N 2 Raman signal appeared as two adjacent, partially overlapping bands. The band at higher Raman shift is identical with the nitrogen-in-air (N 2 air ) band. The band at lower Raman shift is therefore nitrogen within the FIs (N 2 fluid ). The relative amounts of CO 2 and N 2 within the FIs were calculated using the integrated band areas. In the studied ultramafic xenoliths, N 2 concentrations varied between 0.2 and 1.8 mol%, with one exceptionally high concentration of 3.1 mol%. We believe that microthermometric measurements and/or Raman spectroscopy with an inferior spectral resolution may leave N 2 undetected in CO 2- rich upper mantle FIs. Based on our observations, we propose that N 2 may be a minor but common component in subcontinental lithospheric upper-mantle fluids.