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 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. Differentiated bodies from our Solar System have rocky mantles with 15 N/ 14 N ratios within ±15‰ of modern terrestrial air 16,17. This is true for Earth's convective mantle, which has a δ 15 N value of approximately −5 ± 3‰, based on measurements from diamonds 5,18 and basalts that have been filtered for air contamination 3,11. Conversely, volatilerich chondritic meteorites exhibit highly variable δ 15 N values between −20 ± 11‰ for enstatite chondrites and 48 ± 9‰ for CI carbonaceous chondrites 16,19. The distinct 15 N/ 14 N of rocky mantles relative to the chondrites may reflect inheritance of N from a heterogeneous mixture of chondritic precursors 3. Alternatively, the relatively high 15 N/ 14 N values could be the result of evaporative losses 20 , or equilibrium partitioning of N isotopes between metal cores and rocky mantles 21,22. For Earth, plate tectonics allows for another interpretation 1. Geochemists have suggested that mantle δ 15 N v...

The NC-CC Isotope Dichotomy: Implications for the Chemical and Isotopic Evolution of the Early Solar System

“…However, a dichotomy seems to exist for nitrogen isotope ( 15 N/ 14 N) ratios, with CC irons being enriched in 15 N compared to NC irons (Scott et al 2018). These observations are consistent with the general trend of increasing 15 N-enrichments with radial distance from the Sun (Füri and Marty 2015), i.e., from the Sun (δ 15 N Sun = -383 ± 8‰; Marty et al 2011) to the Earth-Moon system (δ 15 N Earth-Moon ≈ -40 to +26‰; Palot et al 2012;Cartigny and Marty 2013;Füri et al 2015;Labidi et al 2020) and Mars (δ 15 N Mars,mantle -30‰; Mathew and Marti 2001) and to comets (δ 15 N Comet ≈ +800 to 1000‰; Bockelée-Morvan et al 2015, and references therein). In contrast, nitrogen isotope ratios recorded by chondrites do not display the NC-CC dichotomy.…”

Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

“…nonthermal site and why the noble gases are less contaminated by atmosphere. The greater mantle volatile contribution in Brimstone Basin has also been further substantiated by a recent innovative study that used the rare 15 N 15 N isotopologue of N 2 to identify and correct for air contamination in volcanic gas emissions (39). This study revealed that not only did the Brimstone Basin samples contain the greatest contribution of mantlederived N 2 of all of the Yellowstone samples measured but also that the δ 15 N of the Yellowstone mantle source is distinct from MORB and in line with previous determinations of plumederived samples (39).…”

The Systematics of Chlorine, Lithium, and Boron andδ37Cl,δ7Li, and δ11B in the Hydrothermal System of the Yellowstone Plateau Volcanic Field

“…The emission rate and isotopic composition suggest that a significant amount of helium has been liberated from the Archean rocks over the past 2 million years since the crust was affected by magmatism (Lowenstern, Evans, et al., 2014). Other studies have shown some of the other noble gases and nitrogen emitted from the YPVF hydrothermal system are derived from the mantle (Broadley et al., 2020; Labidi et al., 2020).…”