Nitrogen content of the mantle inferred from N₂–Ar correlation in oceanic basalts

Abstract

RARE gases 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 argon⁹ to analyse gases trapped in vesicles of mid-ocean-ridge basalt glasses. The results show that the abundances of N₂ and ⁴⁰Ar (a radiogenic isotope that has been produced through geological time by the decay of ⁴⁰K in the solid Earth) correlate well over several orders of magnitude, suggesting that the N₂/⁴⁰Ar ratio in the mantle source is near-constant and comparable to the present-day atmospheric value. In contrast, the inferred mantle N₂/³⁶Ar ratio (where ³⁶Ar 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.