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.

关于地球大气的问题有很多问题，例如：随着时间的流逝，气氛越来越大还是较小；构成气氛的挥发性元素和化合物的起源是什么？以及在俯冲带俯冲到俯冲区域中俯冲到地幔的挥发物的深入到地球内部的深处。 这项研究的结果将通过研究具有地幔起源的各种岩石和液体中的氮同位素地球化学研究，从而大大提高了我们对这些问题的了解，并来自海洋盆地的不同位置以及来自不同的构造环境。 该研究的重点是氮，因为它是由于其在空气中的高丰度（约78％的大气中的78％）和地幔中的零件浓度，因此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。由于在所有表面环境中无处不在的氮以及地幔岩石中少量的氮，通过其同位素在地幔中测量氮的原始特征，而没有大气氮的干扰作用，这是一个重大的分析挑战。结果，关于岩石中氮同位素的数据，地幔非常有限。进行这项研究的实验室是世界上少数具有进行如此低水平氮同位素测量的分析能力的实验室之一。已经根据其他地球化学数据的质量和全面性选择了地幔衍生材料的样品，并由海洋岛，中山山脊传播轴，后弧盆地，海底底层熔岩，地幔质子质体，毛晶，Mafic Ancocrysts和Geothermalmalmalmalsmals组成。这些数据将产生一个必不可少的数据库，以构成海洋地幔中氮同位素变异的基本数据库，从中可以提出关于大气的起源和生长的假设以及表面和深地球系统之间挥发性的循环。这项工作的更广泛的影响包括培训将参与工作各个方面的学生，并将接受有关最先进的分析技术的培训。学生还将通过圣地亚哥大学扩展计划提供科学传播和教学活动以及公共推广机会，该计划为圣地亚哥县服务不足地区的高中生提供大学级别的教学。外展活动将为学生讲师提供急需的研究重点，并有助于将研究结果和科学兴奋传递给公众。 其他影响包括新实验室技术的精炼和开发，以允许测量消失的少量氮及其在地质材料中的同位素。氮同位素对地球大气与内部储层之间的相互作用以及这些储层的进化历史提供了关键限制。将使用高分辨率的小体积质谱法对LAU，MANUS和ALARCON盆地的海洋基底玻璃进行研究，这将允许确定这些样品中氮同位素变异性的表征，从而确定这些样品的氮同位素可变性，进而有助于确定海洋下层层的氮同位素同位素变异性。靶向的样品包括指示羽状地幔存在的样品（即3HE/4HE同位素比率最高的地幔）。将分析火山玻璃，其中3HE/4HE的比例位于标称的地幔组成范围内，这表明它们与中海脊玄武岩的地幔源相关。在地球化学上已经针对锶，近hododium和Lead和Lead的放射同位素比率以及贵族气体，NEON和ARGON以及CO2和H2O的样品进行了分析，以测试以哪些符号和其他对这些Petrogenotic的含量控制，将分析氦气，Neon和Argon以及CO2和H2O的样品。氮的同位素组成，例如岩浆脱气和/或分馏。还将分析来自加那利群岛热点的镁铁质表晶和异种石的样品集以及来自夏威夷的许多海洋岛玄武岩和地热液的苯晶。这项工作的结果将允许审查并确定氮和氦同位素比之间的耦合及其关系，从不同采样培养基和区域控制的角度来看，海底氮同位素的变异性。镁铁质的表晶工作将有助于定义分析所需的样品氮含量的限制，这可能导致样品基碱可显着增加，可将氮同位素分析延伸。