Collaborative Research: The Nitrogen Isotope Systematics of the Icelandic Mantle

氮是大气中含量最丰富的气体，在地幔中以微量形式存在。在大洋中脊采集的上地幔的氮同位素组成估计为每毫升~-4至-5。相比之下，海底有机沉积物富含15N，d15N值为+5至+7/mil。因此，地幔和地表在丰度和同位素组成上的巨大差异使氮成为地球表面和内部水库之间挥发性循环的示踪剂。地球原始地幔的同位素组成存在争议。最近对大洋岛玄武岩(OIB)玻璃的测量显示，正值高达+6/mil，这表明自太古宙以来，从地表到地幔深处发生了广泛的氮循环。如果深部地幔的氮同位素组成确实不是原始的，而是N循环的结果，那么原始稀有气体特征最高的地方(最高的3He/4He比值)有望显示出正的d15N特征。目前公布的高3He/4He热点的d15N数据库只包括~20个热液气体的测量数据，甚至更少的OIB玻璃数据。该项目旨在调查冰岛地幔的N-He-Ar系统学，以建立一个对解决下列基本问题至关重要的N同位素数据库：(A)吸积时硅酸盐地球的挥发性成分；(B)其后地幔的脱气历史；以及(C)大气层与地幔之间循环的性质。研究人员将采样矿物分离、海底玄武岩玻璃(来自雷克扬斯山脊)、冰下玻璃和火山/热液气体，范围从MORB(8+/-1 RA)到38 RA。该项目的中心目标是检验以下假设：1)具有最高3He/4He比值的大洋岛的氮具有与贫化的MORB地幔(DMM)不同的同位素特征--可能是较低的地幔特征；2)观测到的氦和放射性同位素的变化表明DMM和冰岛FOZO(震源区)源之间的混合，这种混合也可以在氮同位素特征中看到；3)就像在弧形中一样，同一火山的斑晶和地热流体的氮同位素基本相同，样本主要来自地幔的挥发物；4)在不同围压下喷发的海底玻璃的氮同位素组成并不随脱气程度的变化而变化。氮是地球大气中最丰富的气体，通过活火山作用从内部冒出气泡，使地幔中的这种元素非常贫化。因此，这项工作将有助于更好地了解地球大气层和深部地幔是如何随着时间的推移而演变的，并拓宽我们对地球上发生的长期化学变化的理解，例如水、碳和氮循环。通过公共宣传部分，调查人员将教育任职前教师关于这种循环的知识，并强调即使我们呼吸的空气中有70%是氮气，其中一些来自地球深处，甚至可能会以连续的循环返回那里。

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.