Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen

• Published in: Nature (London) 2020-04, Vol.580 (7803), p.367-371
• Main Authors: Labidi, J., Barry, P. H., Bekaert, D. V., Broadley, M. W., Marty, B., Giunta, T., Warr, O., Sherwood Lollar, B., Fischer, T. P., Avice, G., Caracausi, A., Ballentine, C. J., Halldórsson, S. A., Stefánsson, A., Kurz, M. D., Kohl, I. E., Young, E. D.
• Format: Article
• Language: English
• Subjects: 704/445/209; 704/445/598; Air pollution; Atmospheric models; Basalt; Contamination; Deposition; Earth atmosphere; Earth mantle; Earth Sciences; Earth surface; Equilibrium; Fractionation; Gases; Geochemistry; Geological time; Humanities and Social Sciences; Hydrothermal systems; Hypotheses; multidisciplinary; Nitrogen; Nitrogen cycle; Nitrogen isotopes; Oceans; Provenance; Science; Science (multidisciplinary); Sciences of the Universe; Subduction (geology); Volcanic gases
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-020-2173-4

Nitrogen is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth’s accretion versus that subducted from the Earth’s surface is unclear 1 – 6 . Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15 N 15 N isotopologue of N 2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ 15 N (the fractional difference in 15 N/ 14 N from air), N 2 / 36 Ar and N 2 / 3 He. Our results show that negative δ 15 N values observed in gases, previously regarded as indicating a mantle origin for nitrogen 7 – 10 , in fact represent dominantly air-derived N 2 that experienced 15 N/ 14 N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15 N 15 N data allow extrapolations that characterize mantle endmember δ 15 N, N 2 / 36 Ar and N 2 / 3 He values. We show that the Eifel region has slightly increased δ 15 N and N 2 / 36 Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts 11 , consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ 15 N values substantially greater than that of the convective mantle, resembling surface components 12 – 15 , its N 2 / 36 Ar and N 2 / 3 He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ 15 N values may both be dominantly primordial features. A rare nitrogen isotopologue is used to detect contamination by air in volcanic gas effusions, and thereby derive the isotopic compositions of mantle endmembers.