Wednesday, November 04, 2020, 12:00PM - 1:00PM
The Geological Sciences Department is hosting this talk in the Joint Earth Seminar Series. Dr. Labidi will be staying around on the GeoColloquium ZOOM link for an hour after the JESS talk to hang out, chat science and answer any detailed questions you might have for him.
Hydrothermal 15N15N abundances: deception and clarity on the origins of planetary nitrogen
Nitrogen is the main constituent of the earth's atmosphere, but its provenance in the earth's mantle is uncertain. Here, we constrain the origin of nitrogen in multiple mantle reservoirs and show that subduction may not be as important as previously thought to account for mantle nitrogen. We use the rare 15N15N isotopologue of N2 as a novel tracer of air contamination in volcanic gas effusions. By correcting for air contributions in the gases using this tracer, we derive new estimates for mantle delta15N and N2-3He ratios from multiple volcanic regions. We focus on Yellowstone, a primitive hotspot, and volcanoes from the Central American subduction zone. One of the main results is that hydrothermal processes can cause deception. For instance, negative delta15N 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. O2/N2 and N2/Ar ratios are also fractionated by hydrothermal processes. In fact, fractionated air (rather than pristine air) is the major nitrogen contributor to hydrothermal gases. Using two-component mixing models to correct for the contribution of fractionated air, the 15N/15N data allow unambiguous extrapolations that characterize mantle end members. We show that the subduction zone region has elevated delta15N and N2/3He relative to the convective mantle, consistent with slab nitrogen being added to mantle sources. In contrast, we find that while the Yellowstone plume has delta15N substantially higher than the convective mantle, resembling surface components, it shows N2-3He ratios indistinguishable from the convective mantle. This observation is challenging to reconcile with significant nitrogent addition from the surface. Our 15N/15N-based analysis suggests that plume nitrogen may be a primitive component.