Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen

Abstract

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 unclear1,2,3,4,5,6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 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 δ15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7,8,9,10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12,13,14,15, its N2/36Ar and N2/3He 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 δ15N values may both be dominantly primordial features.

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Fig. 1: The nitrogen isotopic composition of volcanic gases and volatile-rich MORBs.
Fig. 2: The relationship between Δ30 and N2/3He ratios in volcanic gases.
Fig. 3: The relationship between Δ30 and argon isotopes in volcanic gases.
Fig. 4: The evolution of δ15N and nitrogen abundances in the convective mantle and at the Earth’s surface as a function of time.

Data availability

Nitrogen isotopologue and noble gas data are archived on EarthChem at https://doi.org/10.1594/IEDA/111481. Source data for Figs. 1–3 are provided with the paper.

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Acknowledgements

This study was supported by the Deep Carbon Observatory through Sloan Foundation grant numbers G-2018-11346 and G-2017-9815 to E.D.Y. The Deep Carbon Observatory also supported field trips via grant numbers G-2016-7206 and G-2017-9696 to P. H.B. We thank S. Mukhopadyay for providing a sample of popping rock; K. Farley for lending equipment; and J. Dottin, M. Bonifacie, V. Busigny, P. Cartigny and A. Shahar for helpful discussions.

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Authors

Contributions

E.D.Y. designed the study. J.L. made the nitrogen isotopologue measurements of all mantle-derived samples and most cratonic samples, interpreted the data and wrote the manuscript with feedback from E.D.Y. E.D.Y. and J.L. constructed the box models. I.E.K. made nitrogen isotopologue measurements of some of the cratonic samples. P.H.B., D.V.B., M.W.B., T.P.F. and A.C. measured noble gas abundances and isotope ratios in mantle-derived samples. O.W. and T.G. measured major element chemistry in cratonic gases. P.H.B., D.V.B., M.W.B., T.P.F. and B.M. conducted field trips and sample collection in Yellowstone, USA. D.V.B., M.W.B. and B.M. conducted a field trip and sample collection in Eifel, Germany. P.H.B., A.S. and S.A.H. conducted a field trip and sample collection in Iceland. B.M. and T.P.F. conducted a field trip and sample collection in East Africa. T.P.F. conducted a field trip and sample collection in Hawaii. B.S.L., O.W. and T.G. conducted multiple field trips and sample collections in the Kidd Creek and Sudbury mines, Canada. G.A. assisted in acquiring data for popping rocks. M.D.K. contributed popping rock samples. All authors contributed to the final manuscript preparation.

Corresponding authors

Correspondence to J. Labidi or E. D. Young.

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The authors declare no competing interests.

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Peer review information Nature thanks Rita Parai and Yuji Sano for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Probability density plots for N2/36Ar, N2/3He and δ15N based on literature data and our study.

The relative probabilities are scaled so that each probability is visible on the same plot. The probability densities for δ15N are taken from the mean and standard deviation of the reported measured values in the cases of Yellowstone and Eifel. The δ15N MORB3,49,86 and metasediment12,14,15,87 data were compiled from the literature. The probability densities for molecular ratios were calculated by taking the ratios of Monte Carlo draws for numerator and denominator and propagating nominal 20% errors assigned to each molecular concentration. Literature data for chondritic N2/36Ar were obtained using N and 36Ar concentrations in individual chondrites19,88,89. The dataset includes all major types of carbonaceous, enstatite and ordinary chondrites. No systematic difference could be observed between chondrite groups. Using this global dataset, we find that chondritic estimates used earlier3,90 for N2/36Ar cannot be replicated. N2/36Ar and N2/3He estimates for metasediments are from Sano et al.32, assuming a normal distribution for the uncertainties. The convective mantle N2/3He and N2/36Ar data are from ref. 11 and references cited in Extended Data Fig. 5.

Extended Data Fig. 2 3He/4He of Icelandic gases plotted against their 4He/20Ne ratios normalized to air.

Literature data50 illustrate a three-component mixing with air, the convective mantle and the plume mantle. The convective mantle endmember is characterized by 3He/4He of 8 RA and a relatively high 4He/20Ne relative to air44. The plume component is characterized by primordial 3He/4He values of up to 30 RA and a 4He/20Ne value lower than the convective mantle50,51. On this plot, our 11 samples have compositions with clear contributions from the three endmembers.

Extended Data Fig. 3 N2/He and N2/3He ratios versus δ15N in gases from Iceland, Yellowstone, Ayrolaf, Eifel and Hawaii.

Top, values for N2/He versus δ15N for air, the convective mantle and cratonic gases compared with the samples from this study are shown. This is a similar plot to that in Fischer et al.8 except that cratonic gases are shown for the first time, to our knowledge. The extremely low N2/He ratio for cratonic gases derived here results from substantial accumulation of 4He over geological times. See the Supplementary Discussion for definitions of the cratonic gases based on samples from the Canadian Shield (data in Supplementary Table 2). In this space, data are usually interpreted as representing ternary mixtures. However, this plot fails to account for the processes occurring in hydrothermal systems, whereby the extremely low δ15N values are from isotopically fractionated N2 degassing from geothermal waters, not mixing with mantle components (see main text). Bottom, we show the values for N2/3He versus δ15N for air, the convective mantle and cratonic gases compared with samples from this study. This is a similar plot to that in Sano et al.32 except that cratonic gases are defined (see the Supplementary Information). The samples with low N2/3He values relative to the cratonic endmember can be assumed to receive negligible cratonic nitrogen (see the main text and Supplementary Discussion on Yellowstone). The mantle gases are taken from ref. 3.

Extended Data Fig. 4 84Kr/36Ar and 132Xe/36Ar ratios versus δ15N in gases from hydrothermal systems with near-atmospheric values.

84Kr/36Ar, 132Xe/36Ar and δ15N values are shown for Iceland and Yellowstone samples for which we have heavy noble gas data and Δ30 values of 16‰ and higher. In the two plots, the data define a negative trend, implying that nitrogen loss causing δ15N variations occurs together with preferential Kr (top) and Xe (bottom) losses relative to argon. This is in contrast with predictions based on solubilities obtained in ideal conditions, where both Kr and Xe are expected to be more soluble than Ar29. We suggest that this represents degassing of air-saturated water under extreme temperature and pressure conditions, where gas solubilities deviate considerably from behaviour governed by Henry’s Law29. Here, the data would require Kr (and Xe) to become more insoluble than Ar.

Extended Data Fig. 5 N2/36Ar ratios versus 40Ar/36Ar ratios in basalts and rocks from the Kola plume.

Data are from the literature2,3,4,86 and illustrate mixing between mantle gases and air, most probably as the result of the introduction of air into rock cracks during eruption or sample handling. The highest 40Ar/36Ar ratio recorded in basalt crushing experiments with simultaneous N2/36Ar measurements is 42,366 ± 9,713. At this value, the corresponding N2/36Ar ratio is \({4.2}_{-1.5\,}^{+2.0}\times {10}^{6}\), which was assigned to the convective mantle86. The convective mantle is more likely to have a 40Ar/36Ar ratio of 25,000 ± 2,000 (ref. 44). At this 40Ar/36Ar ratio, the corresponding N2/36Ar value becomes \({2.0}_{-1.2\,}^{+1.0}\times {10}^{6}\). At a 40Ar/36Ar value of 5,000 (refs. 2,4), the plume N2/36Ar value would be lower, at \({0.4}_{-0.2\,}^{+0.2}\times {10}^{6}\). However, at the 40Ar/36Ar value of 10,000 suggested by recent studies51, we obtain N2/36Ar = \({0.7}_{-0.3\,}^{+0.5}\times {10}^{6}\) for the plume according to the correlation between N2/36Ar and 40Ar/36Ar.

Extended Data Fig. 6 Mass balance applied to account for Eifel and Yellowstone, in terms of δ15N, N2/36Ar and N2/3He.

δ15N values of Eifel and Yellowstone are shown, as derived from Fig. 1. N2/36Ar and N2/3He values of Eifel and Yellowstone are also shown, as derived from Figs. 2, 3. Recycled components have high elemental ratios, according to ref. 32. These ratios might be even higher if N was less devolatilized than noble gases during slab devolatilization. Note that this would not change our conclusion, since the mixing curve would remain identical. a, In the N2/3He space, the position of Eifel and society basalts can be explained with a simple two-component mixing between the convective mantle and some recycled component. The values for the three Society basalts are taken from ref. 4. The dataset was filtered to show only basalts with the lowest levels of air contamination. We only used the three basalts with 40Ar/36Ar > 5,000. For this to work for Yellowstone, anomalously high δ15N would be required (see b). An alternative mixing requires a 3He-rich reservoir to be postulated with a low N2/3He ratio. We illustrate this speculation with a δ15N of –5‰, like that of the convective mantle. However, other δ15N values (typically between −10 and +10‰) would also fit the Yellowstone data. This is because N in the Yellowstone source would mostly be accounted for by the recycled component, not the 3He-rich endmember forced with a low N2/3He ratio b, In the N2/36Ar space, Eifel can be explained by conventional mixing. If such mixing involves the known convective mantle, Yellowstone requires a recycled endmember with a δ15N > 50‰, which is implausible. Again, for a mixing to account for the data, one would have to assign the 3He-rich reservoir with a low N2/36Ar ratio.

Extended Data Fig. 7 The evolution of δ15N and nitrogen abundances in the convective mantle and at the Earth’s surface as a function of time.

Various cases with time-dependent solutions are explored. Curves are calculated as for Fig. 4 (main text) using a two-box model described in the Methods and main text. Here, three cases are modelled on the basis of the combination of various temporal variations in volcanic outgassing and subduction fluxes shown in the bottom panel. Modern fluxes are taken from ref. 13. The blue curves (right) are for the surface (air + continental crust), and the red curves (left) are for the convective mantle. As in Fig. 4, the starting composition for the mantle was chosen to have an enstatite chondrite-like δ15N (ref. 6), and various initial N abundances. A critical result of the model is that varying fluxes can easily match the N abundances for the mantle and the surface, as well as the isotope composition of N in the surface. However, similar to the case where constant fluxes are used (Fig. 4), relatively high subduction fluxes pushes the N isotope cycle towards a steady state in which the mantle would have a higher average δ15N value than that of the surface, contrary to the relationship observed today. The model shown as Case 3 provides an acceptable match to the modern observations, where the mantle has a δ15N value of −1‰, after starting at −40‰. However, this predicts that the mantle evolves considerably through time in terms of δ15N. Thus, if correct, this prediction requires all peridotitic diamonds to be formed in roughly the past 500 Myr. However, peridotitic diamonds found in cratonic lithospheres as old as 3.3 Gyr old are dominated by a δ15N mode at −5‰ (ref. 18), seemingly ruling out the Case 3 model.

Supplementary information

Supplementary Table 1

Nitrogen and noble gas data for the springs and fumaroles studied. All the nitrogen data were acquired at UCLA. Noble gas data were acquired in various labs (see legend).

Supplementary Table 2

Nitrogen and noble gas data for cratonic samples studied here (see supplementary discussion). All the nitrogen isotope data were acquired at UCLA. Noble gas data were acquired at the university of Toronto (see legend).

Supplementary Table 3

Nitrogen isotope data for air standards of varying sizes. Data were acquired at UCLA over the course of this study.

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Labidi, J., Barry, P.H., Bekaert, D.V. et al. Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen. Nature 580, 367–371 (2020). https://doi.org/10.1038/s41586-020-2173-4

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