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

大洋地幔的氮同位素系统学：玄武岩、捕虏体和地热流体相结合的方法。

• 批准号: 1657083
• 负责人: David Hilton
• 金额: $ 28.16万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-02-15 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1657083&HistoricalAwards=false
• 关键词: Nitrogen; Isotope; Systematics; Oceanic; Mantle

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.

关于地球大气层有许多问题仍然没有答案，例如：大气层是随着时间的推移而变大还是变小;组成大气层的挥发性元素和化合物的来源是什么;以及在俯冲带俯冲到地幔中的挥发物渗透到地球内部的深度。 这项研究的结果将大大提高我们对这些问题的认识，通过研究各种岩石和流体中的氮同位素地球化学，研究地球表面和深部之间的挥发物循环，这些岩石和流体具有地幔起源，来自海洋盆地的不同地方和不同的构造环境。 这项研究的重点是氮，因为它是地球内部和外部挥发性储层之间相互作用的敏感示踪剂，因为它在空气中的丰度很高（约占大气的78%），在地幔中的浓度为百万分之几。由于氮在所有地表环境中普遍存在，而地幔岩石中的氮含量很小，因此通过其同位素测量地幔中氮的原始特征，而不受大气氮的干扰影响，这是一项重大的分析挑战。因此，岩石和地幔中氮同位素的数据极其有限。进行这项研究的实验室是世界上少数几个具有进行这种低水平氮同位素测量的分析能力的实验室之一。已根据其他地球化学数据的质量和全面性选择了用于分析的幔源物质样品，包括来自洋岛、洋中脊扩张轴、弧后盆地、海底玄武质熔岩、地幔捕虏体、镁铁质斑晶和地热流体的物质。这些数据将产生一个关于海洋地幔中氮同位素变化的基本数据库，据此可以提出关于大气层起源和增长以及地表和地球深部系统之间挥发物循环的假设。这项工作的更广泛影响包括培训学生，他们将参与工作的各个方面，并将接受最先进的分析技术培训。学生还将参与科学交流和教学活动，并通过圣地亚哥大学推广计划提供公共宣传机会，该计划为圣地亚哥县服务不足地区的高中生提供大学水平的指导。外展活动将为学生导师提供急需的补充研究的研究重点，并将有助于向公众传播研究成果和科学的兴奋。 其他影响包括改进和开发新的实验室技术，以测量地质材料中几乎不存在的少量氮及其同位素。氮同位素对地球大气和内部储层之间的相互作用以及这些储层的演化历史提供了关键的限制。将使用高分辨率、小体积质谱法研究来自Lau、Manus和Alarcon盆地的海洋玄武岩玻璃，从而确定这些样品中氮同位素变异性的特征，这反过来将有助于确定海洋下地幔的氮同位素变异性。目标样本包括那些表明存在地幔柱影响的地幔（即，具有最高的3 He/4 He同位素比率的那些）。火山玻璃将进行分析，其中3 He/4 He比值落在地幔成分的标称范围内，表明它们与洋中脊玄武岩的地幔源有关。将对已经在地球化学上充分表征了锶、钕和铅的放射性同位素比率以及稀有气体氦、氖和氩以及CO2和H2O的样品进行分析，以测试样品的氮同位素比率和这些其他岩石成因示踪剂可用于区分地幔特征和其他对氮同位素组成的控制的程度，例如岩浆脱气和/或分馏。还将分析包括来自加那利群岛热点的镁铁质斑晶和捕虏体以及来自夏威夷的一些海洋岛屿玄武岩和地热流体的斑晶的样品集。这项工作的结果将允许审查和确定氮和氦同位素比率之间的耦合关系，以及从不同取样介质和区域控制的角度来看，在海洋地幔中氮同位素的可变性方面的关系。镁铁质斑晶的工作将有助于确定分析所需的样品氮含量的限制，可能导致适合氮同位素分析的样品基础的显着增加。