NITROGEN, HELIUM AND ARGON IN BASALT: A STATIC MASS SPECTROMETRY STUDY

Abstract

An analytical system for the simultaneous determination of nitrogen, helium and argon contents and , ratios in rocks by static mass spectrometry has been designed. Gases are extracted following either stepwise heating or vacuum crushing. The purification of nitrogen can be controlled by monitoring signals at masses 12, 14, 28 and 29. Helium and argon isotopic ratios can be analysed in the same fraction, allowing control on the distribution of mantle-derived and atmosphere-derived components. Concentrations of nitrogen down to few ppb N are routinely measured. The concentration and distribution of volatiles in a N-MORB sample from the East Pacific Rise having a low vesicularity of 0.18% have been investigated. The mean N2 content is (1.66 ± 0.49) · 10−8 mol g−1 (0.46 ± 0.14 ppm), associated with a mean, non-atmospheric ratio of 8435 ± 2279. The ratio of the vesicle fraction is 79, significantly lower than the bulk-rock value (), suggesting either contrasted N2Ar solubilities, kinetic disequilibrium between N2 and Ar, or, more probably, selective addition of surface-derived nitrogen after vesiculation. This work illustrates the potential of a combined rare gas-nitrogen approach when observing mantle-derived samples with very low amounts of nitrogen.