In order to document the isotopic composition of nitrogen in the convective mantle, we have analysed nitrogen isotopes together with argon isotopes in oceanic basalt glasses. Gases were extracted from fluid inclusions by crushing and, after removal of interfering compounds, were analysed by static mass spectrometry. The results show that, of the 25 samples analysed, 12 have delta(15)N values between -1.4 parts per thousand and -6.5 parts per thousand relative to the atmospheric composition, 8 have delta(15)N close to 0 parts per thousand and 5 present enrichments in N-15 (from + 2.1 parts per thousand to + 5.4 parts per thousand). A comparison between nitrogen and argon isotopic ratios suggests mixing between a surface-derived component enriched in N-15 and a mantle end-member characterised by extremely high Ar-40/Ar-36 ratio and depletion in N-15 of about -3 to ca. -5 parts per thousand relative to atmospheric N. Such isotopic compositions and the very high N/Ar-36 ratios compared to the atmospheric Value of these mantle-derived samples are difficult to reconcile with derivation of atmospheric N-2 from the convective mantle through time, and are more consistent with early volatile fractionation in the Earth-Atmosphere system. (C) 1997 Elsevier Science B.V.