Collaborative Research: The Nitrogen Isotope Systematics of the Icelandic Mantle

氮是大气中含量最丰富的气体，在地球地幔中含量很低。 在大洋中脊取样的上地幔的氮同位素组成估计为每密耳-4至-5。相比之下，洋底的有机沉积物富含15 N，d15 N值为每密耳+5至+7。因此，地幔和地表在丰度和同位素组成上的巨大差异，使氮很有可能成为地幔和地表之间挥发性循环的示踪剂。地球表面和内部水库。 地球原始地幔的同位素组成是有争议的。最近对大洋岛玄武岩（OIB）玻璃的测量显示出高达+6/mil的正值，这导致了自太古代以来发生了广泛的氮从表面到地幔深处的循环。 如果深地幔的N同位素组成确实不是原始的，而是N再循环的结果，那么具有最高原始惰性气体特征（最高的3 He/4 He比值）的地区预计将显示出正的d15 N特征。 目前公布的来自高3 He/4 He热点的d15 N数据库仅包含约20个热液气体的测量值，OIB玻璃的测量值更少。 该项目的目的是调查冰岛地幔的氮氦氩系统学，以建立一个氮同位素数据库，这对解决以下基本问题至关重要：（a）硅酸盐地球在吸积时的挥发性成分，（B）地幔随后的脱气历史，和（c）大气和地幔之间循环的性质。调查人员将对矿物分离物、海底玄武岩玻璃（来自雷克雅内斯海岭）、冰下玻璃和火山/热液气体进行采样，这些气体的3 He/4 He比率从MORB（8+/-1 RA）到38 RA。该项目的主要目标是检验以下假设：1）具有最高~ 3 He/~ 4 He比值的洋岛样品中的氮具有不同于亏损MORB地幔（DMM）的同位素特征-可能是下地幔特征; 2）氦和放射性同位素的观测变化表明DMM和FOZO之间存在混合（焦点区）源在冰岛，这种混合也见于氮同位素特征; 3）在弧，斑晶和地热流体的氮同位素从同一火山基本上是相同的，样品主要是地幔衍生的挥发物;（4）在不同围压下喷发的海底玻璃的氮同位素组成不随脱气程度而变化。氮是地球大气中最丰富的气体，通过活火山作用从地球内部冒出，使地幔中的氮元素非常贫乏。 因此，这项工作将有助于更好地了解地球大气层和地幔深处如何随时间演变，并扩大我们对地球上发生的长期化学变化，如水、碳和氮循环的了解。 通过公共宣传部分，调查人员将教育职前教师了解这种循环，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，甚至可能在连续的循环中返回那里。

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.