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Nitrogen variations in the mantle might have survived since Earth’s formation

Earth’s nitrogen-rich atmosphere contributes to the pleasant surface environment in which we live and breathe — but makes it very difficult to determine the nitrogen isotope composition of anything else. Pervasive atmospheric contamination of samples derived from Earth’s mantle poses a formidable challenge to anyone investigating the origins and transport of volatile species, such as nitrogen and the noble gases, in the deep Earth. In a paper in Nature, Labidi et al.1 report that they have used a ‘clumped isotope’ method to identify uncontaminated mantle nitrogen in volcanic-gas effusions and gases trapped in volcanic-rock samples. The relative abundances of isotopes in uncontaminated nitrogen vary among samples from different locations. The authors argue that these differences originate from Earth’s formation and have survived approximately 4.5 billion years of mixing associated with mantle convection.

There are two stable nitrogen isotopes, 14N and 15N, and their relative abundances are expressed as δ15N values — the parts per thousand deviation of the 15N/14N ratio from a standard value. The nitrogen isotopic compositions of mantle-derived samples can provide insight into a wide range of topics, from the mix of planetary building blocks that brought volatile species to Earth during its formation2, to the transport of atmospheric nitrogen into the mantle through the sinking of tectonic plates over time3.

Apart from the proportions of 14N and 15N in a sample, the way that isotopes are distributed between molecules also provides information. An isotopologue is a molecule that has a specific combination of isotopes of its constituent elements. For example, diatomic nitrogen molecules (N2, which constitute about 78% of the atmosphere by volume) can incorporate either 14N or 15N, yielding three possible isotopologues: 14N14N, 14N15N and 15N15N. Because the vast majority of nitrogen is 14N, the most common isotopologue is 14N14N. Substitution of a single 15N for 14N is rare; a doubly substituted isotopologue (15N15N) is rarer still. A random distribution of 14N and 15N between N2 molecules produces a specific mixture of the three isotopologues. Any measured deviation from the expected proportion of 15N15N is described as a clumped-isotope anomaly.

Earth’s atmospheric N2 exhibits a well-resolved clumped-isotope anomaly4, and Labidi et al. used this signature to identify atmospheric contamination of volcanic gases. The authors established that mantle N2 has no clumped-isotope anomaly by analysing nitrogen released from unusually gas-rich samples of mid-ocean ridge basalt, confirming the expectation that magmatic gases have a random distribution of isotopes among N2 isotopologues. With this information in hand, the authors examined nitrogen isotope compositions in hydrothermal gases sampled from Yellowstone National Park in the United States, Iceland and other volcanic localities. They identified the nitrogen isotope compositions of the mantle sampled at locations at which trends showing varying degrees of atmospheric contamination were evident.

In previous studies3,5 of nitrogen in mantle-derived gases, systematic variations among measured nitrogen and noble-gas compositions were sought to identify atmospheric contamination, but contradictory signatures were sometimes observed — some metrics indicated that there was contamination, whereas others suggested there was none. Labidi and colleagues show that data that might have been interpreted as mantle compositions on the basis of relationships between nitrogen and noble gases are, in fact, affected by atmospheric N2 contamination.

Their study also indicates that δ15N variations are produced in atmospheric N2 as it circulates through hydrothermal systems. However, the processes that generate such changes in the bulk proportions of 14N and 15N do not redistribute isotopes among isotopologues, so that the atmospheric clumped-isotope anomaly is preserved — which means that any contamination remains identifiable. There is no place for atmospheric N2 to hide if one is looking through a clumped-isotope lens.

An important feature of the authors’ analytical approach is that it is not necessary to measure pure, uncontaminated magmatic gas to estimate the mantle composition. Even if multiple atmospheric contaminants are present, evidence of mixing trends in the data can be used to identify the mantle δ15N value of magmatic gas, which has no clumped-isotope anomaly. Labidi et al. report a mantle δ15N value for the potentially deep-seated6,7 Yellowstone mantle plume that is distinct from those determined for mid-ocean ridge basalts. With uncertainties regarding atmospheric contamination eliminated, nitrogen isotope variations in the mantle can be interpreted in the context of Earth’s formation, differentiation into distinct layers, and the long-term coevolution of the deep Earth and surface owing to plate tectonics (Fig. 1).

Figure 1

Figure 1 | Nitrogen in the deep Earth. Labidi et al.1 report a new method for identifying contamination of volcanic gases by nitrogen from the atmosphere. The authors find that the nitrogen isotope composition of gases extracted from mid-ocean ridge basalts, which sample the convective mantle, is different from that of volcanic gas from Yellowstone National Park in the United States, which is thought to sample an upwelling mantle plume that originates in the deep mantle. By modelling transport of surface nitrogen into the mantle through subduction (the process in which one tectonic plate dives beneath another and descends into the mantle) and nitrogen loss (outgassing) from the mantle, the authors argue that only a limited amount of nitrogen from Earth’s surface has been incorporated into the mantle. They conclude that the observed variations in mantle nitrogen isotope compositions reflect differences that originated early in Earth’s history.

To test whether nitrogen exchange between the surface and mantle over time explains their results, Labidi and co-workers developed a mathematical model of nitrogen evolution in the mantle. Intriguingly, the results suggest that there has been a net loss of nitrogen from the convective mantle over most of Earth’s history, and little incorporation of surface nitrogen into the mantle. This contrasts with previously reported evidence of substantial incorporation of atmospheric xenon into the mantle8,9. Given the limited role of surface nitrogen in the mantle, the authors argue that the observed nitrogen isotope variations are a remnant from Earth’s formation and early differentiation, when volatile species were delivered to the growing Earth as it separated into the core, mantle, crust and atmosphere.

Evidence that early-formed mantle heterogeneities survive in the modern mantle has come from studies of signatures formed by rapidly decaying radioactive isotopes that decayed within the first 100 million years of Earth’s history8,10. It will be challenging to confirm that the nitrogen isotope variations identified by Labidi et al. arose early in Earth’s evolution, given that neither of the element’s two isotopes is produced by radioactive decay and that surface signatures might have a confounding role, however limited. Determination of δ15N values at other plume localities, including regions thought to be influenced by the recycling of surface materials11, would provide an interesting test of the authors’ primordial hypothesis. The application of clumped-isotope analysis reported by Labidi et al. provides an exciting method for such future studies — we now have an improved tool with which to view the origins and evolution of volatile species in the mantle.

Nature 580, 324-325 (2020)

doi: https://doi.org/10.1038/d41586-020-01020-5

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