RARE Eases have proved to be particularly useful in modelling the early evolution of the Earth’s atmosphere(1-3). But it is not straightforward to extend this approach to the main volatile species (such as hydrogen, carbon and nitrogen) that comprise the atmosphere, hydrosphere and sediments, as these elements are chemically reactive and may have experienced different geodynamic histories. A way around this problem is to calibrate major volatile species relative to rare gases(4-8). Here I use a recently developed static mass spectrometry method that allows simultaneous analysis of nitrogen, carbon, helium and argon(9) to analyse gases trapped in vesicles of mid-ocean-ridge basalt glasses. The results show that the abundances of N-2 and Ar-40 (a radiogenic isotope that has been produced through geological time by the decay of K-40 in the solid Earth) correlate well over several orders of magnitude, suggesting that the N-2/Ar-40 ratio in the mantle source is near-constant and comparable to the present-day atmospheric value. In contrast, the inferred mantle N-2/Ar-36 ratio (where Ar-36 is a primordial isotope of argon) is two orders of magnitude higher than the atmospheric ratio. This observation, when combined with argon isotope systematics, allows a better estimate to be made of the nitrogen content of the mantle.