A primary magmatic source of nitrogen to the Earth’s crust

Abstract

The igneous portion of Earth’s continental crust represents a long-term sink of terrestrial nitrogen, but the origin of the nitrogen in this reservoir remains ambiguous. Possible sources include magmatic differentiation of mantle-derived melts (that is, magmatic nitrogen) and/or the burial of biomass (that is, fixed atmospheric nitrogen). Identifying the sources of crustal nitrogen is required to accurately reconstruct the evolution of Earth’s atmospheric pressure, and therefore habitability, over geologic timescales. Here we present analyses of the nitrogen geochemistry of extrusive igneous rocks from Hekla volcano, Iceland, which has been previously used as a natural laboratory to study the effects of magmatic differentiation on stable isotope systems. We find that bulk rock nitrogen abundance increases as rocks become more evolved, with up to 23 μg g−1 of nitrogen in felsic igneous samples and non-systematic and negligible nitrogen isotopic fractionation across the suite. Our findings indicate that this nitrogen is magmatic in origin and provides evidence that nitrogen behaves as an incompatible trace element during magmatic differentiation. Assuming Hekla is representative of differentiating systems more broadly, the observed nitrogen enrichment would satisfy 31–52% of Earth’s felsic crust-hosted nitrogen. We suggest that continental crust formation can act as nitrogen trap between the mantle and the atmosphere. Therefore, nitrogen degassing from Earth’s interior to the atmosphere over geological time may have been previously overestimated.