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Séminaire de Jabrane Labidi
19 November 2019 à 14 h 00 min - 17 h 00 min
Title: Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen
Nitrogen (N) is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle is uncertain. The relative contributions of primordial nitrogen inherited from Earth’s accretion versus that subducted from the Earth’s surface remain unclear. Here, we show evidence that mantle nitrogen is inherited during Earth’s accretion. We use the rare 15N15N isotopologue of N2 as a novel tracer of air contamination in volcanic gas effusions. By correcting for air contamination in the gases using this tracer, we derive new estimates for mantle d15N, N2/36Ar and N2/3He ratios from three volcanic regions: Iceland, Eifel (Germany), and Yellowstone (USA). Our results show that negative d15N values observed in gases, previously regarded as indicating a mantle origin for the nitrogen, 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 unambiguous extrapolations that characterize mantle endmember d15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly elevated d15N and N2/36Ar relative to the convective mantle, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that while the Yellowstone plume has d15N significantly greater than the convective mantle, resembling surface components, it possesses N2/36Ar and N2/3He ratios indistinguishable from the convective mantle. This observation is challenging to reconcile with significant nitrogen addition from the surface. We conclude that while d15N variation may occur within the deep Earth as a result of subduction as illustrated by the Eifel data, our data for Yellowstone are most consistent with a lack of surface-derived subducted components. Our 15N15N-based analysis suggests that convective mantle and plume d15N values are both primordial features.