The Nitrogen Isotope Systematics of the Oceanic Mantle: A Combined Basalt, Xenolith and Geothermal Fluid Approach.

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There are many questions about the Earth's atmosphere that remain unanswered such as: is the atmosphere is getting bigger or smaller with time; what is the origin of the volatile elements and compounds that make up the atmosphere; and how deeply into Earth's interior do volatiles penetrate that are subducted into the mantle at subduction zones. Results of this research will significantly advance our knowledge of these issues by examining the cycling of volatiles between the surface and deep Earth through the study of nitrogen isotope geochemistry in a variety of rocks and fluids that have a mantle origin and come from different places in the ocean basins and from different tectonic settings. The research focuses on nitrogen because it is a sensitive tracer of interactions between Earth's interior and exterior volatile reservoirs due to its high abundance in air (about 78% of the atmosphere) and parts-per-million concentration in the mantle. Due to the ubiquitous presence of nitrogen in all surface environments and the small amount of nitrogen in mantle rocks, measuring the original signature of the nitrogen in the mantle, through its isotopes, without the interfering effects of atmospheric nitrogen is an major analytical challenge. As a result, the data on nitrogen isotopes in rocks and the mantle is extremely limited. The laboratory conducting this research is one of the few in the world that has the analytical capability of making such low-level nitrogen isotopic measurements. Samples of mantle-derived materials for analysis have been selected on the basis of the quality and comprehensiveness of other geochemical data and consist of materials from ocean islands, mid-ocean ridge spreading axes, back-arc basins, submarine basaltic lavas, mantle xenoliths, mafic phenocrysts, and geothermal fluids. These data will produce an essential database of the nitrogen isotopic variation in the oceanic mantle from which hypotheses on the origin and growth of the atmosphere and the cycling of volatiles between surface and deep Earth systems can be posed. Broader impacts of the work include the training of students who will be involved in all aspects of the work and will be trained on state-of-the-art analytical techniques. Students will also be involved in science communication and teaching activities and public outreach opportunities offered through University of San Diego Extension Program that provides college-level instruction to high-school students in under-served regions of San Diego County. The outreach activities will provide student instructors with a much-needed complement to the research focus of the study and will help transmit research results and the excitement of science to the public. Additional impacts include the refining and development of new laboratory techniques to allow measurement of vanishingly small amounts of nitrogen and its isotopes in geological materials. Nitrogen isotopes provide key constraints on the interaction between Earth's atmosphere and interior reservoirs, as well as on the evolutionary history of those reservoirs. Oceanic basaltic glasses from the Lau, Manus, and Alarcon basins will be studied using high-resolution, small-volume mass spectrometry that will allow the characterization of the nitrogen isotope variability in these samples to be determined, which in turn will help to determine the nitrogen isotopic variability of the mantle underlying the oceans. Samples targeted include those that indicate the presence of plume-influenced mantle (i.e., those that have the highest 3He/4He isotope ratios). Volcanic glasses will be analyzed where 3He/4He ratios fall in the nominal range of mantle compositions indicating that they are associated with the mantle source for mid-ocean ridge basalts. Samples that already are well-characterized geochemically for the radiogenic isotopic ratios of strontium, neodymium, and lead and the noble gases helium, neon, and argon as well as CO2 and H2O will be analyzed to test the degree to which the nitrogen isotope ratios of samples and these other petrogenetic tracers can be used to distinguish between mantle signatures and other controls on the isotopic composition of nitrogen, such as magma degassing and/or fractionation. Samples sets that include mafic phenocrysts and xenoliths from the Canary Islands hotspot plus phenocrysts from a number of ocean island basalts and geothermal fluids from Hawaii will also be analyzed. The results of this work will allow scrutiny and the determination of coupling between the nitrogen and helium isotope ratios and their relationship in terms of the variability of nitrogen isotopes in the ocean mantle from the perspective of different sampling media and regional controls. The mafic phenocrysts work will help define limits for sample Nitrogen-contents required for analysis, potentially leading to a significant increase in the sample base amenable to nitrogen isotopic analysis.