Collaborative Research: The Nitrogen Isotope Systematics of the Icelandic Mantle

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Nitrogen is the most abundant gas in the atmosphere and is present at trace levels in the Earth's mantle. The nitrogen isotopic composition of the upper mantle, as sampled at mid-ocean ridges, is estimated to be ~ -4 to -5 per mil. In contrast, organic sediments on the ocean floor are enriched in 15N, with d15N values from + 5 to + 7 per mil. Therefore, the large difference in abundance and isotopic composition between the mantle and the surface gives nitrogen great potential as a tracer of volatile recycling between the Earth's surface and interior reservoirs. The isotopic composition of the Earth's primitive mantle is controversial. Recent measurements of Ocean Island Basalt (OIB) glasses show positive values up to +6 per mil leading to the idea that extensive nitrogen recycling from the surface to the deep mantle has occurred since the Archean. If indeed the N-isotopic composition of the deep mantle is not primordial but the result of N recycling, localities with the highest primordial noble gas signatures (highest 3He/4He ratios) are expected to show positive d15N signatures. The currently published database of d15N from high 3He/4He hot spots consists of only ~ 20 measurements of hydrothermal gases and even less of OIB glasses. This project aims to investigate the N-He-Ar systematics of the Icelandic mantle to produce a N-isotope data base essential to addressing such fundamental issues as (a) the volatile composition of the silicate Earth at the time of accretion, (b) the subsequent degassing history of the mantle, and (c) the nature of recycling between the atmosphere and the mantle. The investigators will sample mineral separates, submarine basalt glasses (from the Reykjanes Ridge), sub-glacial glasses and volcanic/hydrothermal gases that span 3He/4He ratios from MORB (8+/-1 RA) to 38 RA. The central goals of the project are to test the following hypotheses: 1) the ocean island with the highest 3He/4He ratios samples nitrogen that has an isotopic signature distinct from depleted MORB mantle (DMM) - possibly a lower mantle signature; 2) observed variations in helium and radiogenic isotopes suggest mixing between DMM and FOZO (focal zone) source in Iceland, this mixing is also seen in nitrogen isotopic signatures; 3) as in arcs, nitrogen isotopes of phenocrysts and geothermal fluids from the same volcano are essentially identical and sample predominantly mantle-derived volatiles; 4) the nitrogen isotopic composition of submarine glasses erupted under different confining pressures does not vary with the extent of degassing.Nitrogen, the most abundant gas in the Earth's atmosphere, bubbled up from the interior through active volcanism, leaving the Earth's mantle very much depleted in this element. This work, therefore, will contribute to a better understanding of how the Earth's atmosphere and deep mantle evolved over time and broaden our understanding of long-term chemical changes occurring on Earth, such as the water, carbon and nitrogen cycles. Through a public outreach component the investigators will educate pre-service teachers about such cycles and emphasize that even though the air we are breathing is 70% nitrogen some of that has come from deep within the Earth and may even be returning to there in a continuous cycle.