Towards Characterizing the Nitrogen Isotope Systematics of the Oceanic Mantle

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Nitrogen is far more abundant in the Earth’s atmosphere than in the mantle. Therefore, nitrogen can reflect interactions between Earth’s interior and exterior. However, few studies have investigated the nature of nitrogen in basalts that form as the mantle melts. The difficulty in measuring nitrogen in basalts is because of analytical challenges and the difficulty in obtaining representative samples. This is an important gap in knowledge because basalts make up the ocean crust that underlies two-thirds of the Earth's surface. This project will measure nitrogen isotopes from key mantle-derived samples. The samples were obtained from the main environments of basaltic magmatism in the world's oceans. The project thus aims to produce the best measure, to date, of the global nitrogen cycle over Earth history. The study will establish Woods Hole Oceanographic Institution as a key USA-based laboratory. The unique capability of the laboratory is the measure of high-precision nitrogen isotopes on extremely low concentration samples. Findings from the study will address many broader questions pertaining to the origin of the most abundant gas in our atmosphere. The recycling of this gas is tied to the evolution of the Earth and the solar system. Measurements of oceanic basalts from the Lau Basin, Manus Basin, Alarcon Basin, Galapagos and Mid-Atlantic Ridge Popping Rocks will allow us, for the first time, to define the scale of delta-N-15 variability in the oceanic mantle by targeting samples with high 3He/4He values (plume-influenced mantle) as well as other glasses with 3He/4He values within the nominal range of depleted MORB mantle (DMM). Given the well characterized geochemistry of these samples – including radiogenic isotopes (Sr-Nd-Pb), noble gases and major volatiles (He-Ne-Ar-CO2-H2O) –the relationship between delta-N-15 and petrogenetic tracers will be assessed to distinguish between mantle source features and secondary controls (e.g., degassing fractionation and/or surficial contamination). All samples will allow scrutiny of δ15N-3He/4He relationships and further characterization of the variability of delta-N-15 in the oceanic mantle from the perspective of different sampling media and regional controls. As a proof of concept, mafic crystals and xenoliths from the Canary Islands hotspot will also be measured, however this task is significantly more challenging due to extremely low N abundances in mafic crystals. Studies on mafic crystals will define limits for sample N-contents required for analysis, potentially leading to a significant increase in the sample base amenable to delta-N-15 studies. All results will provide important constraints on modeling the evolution of the DMM vs. plume-influenced mantle, and the advent of plate tectonics over Earth history. Specifically, N isotope data generated in the proposed study will be incorporated into a newly developed forward model that couples observations from N and other volatiles species (i.e., heavy noble gases) to understand the relative recycling efficiency of different volatile species. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.