Towards Characterizing the Nitrogen Isotope Systematics of the Oceanic Mantle

地球大气中的氮比地幔中的氮丰富得多。因此，氮可以反映地球内部和外部之间的相互作用。然而，很少有研究调查的性质，氮的玄武岩形成的地幔熔融。测量玄武岩中氮的困难是因为分析挑战和难以获得代表性样品。这是一个重要的知识空白，因为玄武岩构成了占地球表面三分之二的海洋地壳。该项目将测量关键的幔源样品中的氮同位素。这些样品是从世界各大洋中玄武岩岩浆活动的主要环境中取得的。因此，该项目的目的是产生迄今为止对地球历史上全球氮循环的最佳测量。这项研究将使伍兹霍尔海洋研究所成为美国的一个重要实验室。该实验室的独特能力是对极低浓度样品进行高精度氮同位素测量。这项研究的结果将解决与我们大气中最丰富的气体的起源有关的许多更广泛的问题。这种气体的再循环与地球和太阳系的演化有关。对劳盆地、马努斯盆地、阿拉尔孔盆地、加拉帕戈斯和大西洋中脊爆裂岩的海洋玄武岩的测量将首次使我们能够，以具有高3 He/4 He值的样品为目标，确定大洋地幔中δ-N-15变化的规模（地幔柱影响的地幔）以及其它3 He/4 He值在亏损MORB地幔（DMM）标称范围内的玻璃。鉴于这些样品的地球化学特征良好-包括放射性同位素（Sr-Nd-Pb），稀有气体和主要挥发物（He-Ne-Ar-CO2-H2O）-将评估δ-N-15和岩石成因示踪剂之间的关系，以区分地幔源特征和次级控制（例如，脱气分馏和/或表面活性剂污染）。所有样品都将允许从不同取样介质和区域控制的角度仔细检查δ 15 N-3 He/4 He关系，并进一步表征海洋地幔中δ-N-15的变化。作为概念验证，还将测量加那利群岛热点的镁铁质晶体和捕虏体，但由于镁铁质晶体中的氮丰度极低，这项任务更具挑战性。对镁铁质晶体的研究将确定分析所需的样品N含量的限度，可能导致适合δ-N-15研究的样品基础显著增加。所有的结果将提供重要的制约因素，模拟DMM与地幔柱影响的地幔的演化，以及在地球历史上板块构造的出现。具体而言，在拟议的研究中产生的N同位素数据将被纳入一个新开发的前向模型，该模型将N和其他挥发性物质的观测结果（即，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.