NITROGEN AND ARGON ISOTOPES IN OCEANIC BASALTS

Abstract

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 δ15N values between −1.4‰ and −6.5‰ relative to the atmospheric composition, 8 have δ15N close to 0‰, and 5 present enrichments in 15N (from +2.1‰ to +5.4‰). A comparison between nitrogen and argon isotopic ratios suggests mixing between a surface-derived component enriched in 15N and a mantle end-member characterised by extremely high 40Ar/36Ar ratio and depletion in 15N of about −3 to ca. −5‰ relative to atmospheric N. Such isotopic compositions and the very high N/36Ar ratios compared to the atmospheric value of these mantle-derived samples are difficult to reconcile with derivation of atmospheric N2 from the convective mantle through time, and are more consistent with early volatile fractionation in the Earth–Atmosphere system.