Collaborative Research: Carbon-Helium-Argon Isotope Relations at High-3He/4He Hotspots and Implications for Mantle Dynamics

合作研究：高 3He/4He 热点的碳-氦-氩同位素关系及其对地幔动力学的影响

• 批准号: 1763259
• 负责人: Peter Michael
• 金额: $ 7万
• 依托单位: University of Tulsa
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-05-15 至 2022-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1763259&HistoricalAwards=false
• 关键词: Collaborative; Research; Carbon; Helium; Argon

Carbon isotope ratios are a primary source of information for understanding the global carbon cycle, which has a role in the generation of greenhouse gases; the movement of carbon from Earth's surface back into its interior; and the escape to Earth's surface of primordial carbon that has been trapped deep in the planet since it formed 4.5 billion years ago. These isotope ratios, however, are impacted by a variety of geologic processes, such as the degassing of magma during volcanic eruptions. This causes fractionation that changes the original ratio of the isotopes, making them difficult to interpret fully. Without some means of recognizing these changes, it is not possible to fully understand the cycle of carbon on Earth and the story it tells. One way to overcome the carbon isotope fractionation problem and get a handle on the amount and composition of carbon in the deep Earth is to look at the isotopes of noble gases that are erupted along with carbon dioxide (CO2) that comes out of volcanoes. Although we know a lot about carbon in shallow Earth reservoirs, its concentration, isotopic composition, and behavior in the deep mantle is not well understood. This research uses the content and isotopic composition of two noble gases: Helium (He) and Argon (Ar) from volcanic rocks at two locations in the Pacific (Hawaii and the Lau Spreading Center in the western Pacific), to unravel the story of carbon in the deep mantle and improve our knowledge of the carbon cycle, carbon fluxes, and carbon sources emanating from the Earth. Broader impacts of the research include laboratory and research training of a graduate student whose gender is underrepresented in the sciences; the training of undergraduates at an institution in Oklahoma, an institution in an EPSCoR (Experimental Program to Stimulate Competitive Research) state; support of an NSF-funded state-of-the-art noble gas analytical facility in the state of Oregon; and incorporation of research results into courses. This research utilizes coupled measurements of He, Ar and CO2 abundances in both the vesicles and glasses of submarine basalts erupted at two different locations in the Pacific Ocean. One is Loihi seamount off the island of Hawaii and the other is the Lau Spreading Center, both of which have been influenced by the presence of mantle plumes that tap deep mantle regions. Measurements will include the concentration and isotopic positions of CO2, He, Ar, and H2O trapped in volcanic glasses and vessicles. Release of the gases takes place in a state-of-the-art noble gas laboratory at Oregon State University. Gases in vesicles in the lavas will be released via crushing the lava in a vacuum. He and Ar concentrations and isotopic compositions in the glass will be released via step heating/fusion. Dissolved CO2 and H2O in the glasses will be determined by FTIR (Fourier Transform Infrared Spectroscopy). These data will be used to deconvolute changes in the carbon isotopic composition resulting from magma degassing so the primordial and recycled components of the carbon in the samples can be determined. Key research questions being addressed include: (1) What are the values of CO2/3He and the carbon isotopic ratio in the source region of mantle plumes; (2) is primordial carbon preserved in the deep source of mantle plumes; and (3) what is the CO2 flux at mantle plume localities. Results of the work will be integrated into geodynamic models addressing the cycling of carbon and noble gases between Earth's surface and deep reservoirs.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.

碳同位素比率是了解全球碳循环的主要信息来源，全球碳循环在产生温室气体、碳从地球表面返回地球内部以及自45亿年前地球形成以来一直被困在地球深处的原始碳逃逸到地球表面方面发挥了作用。然而，这些同位素比率受到各种地质过程的影响，例如火山喷发期间岩浆的脱气作用。这会引起分馏，改变同位素的原始比例，使它们很难完全解释。如果没有一些方法来认识这些变化，就不可能完全理解地球上的碳循环及其所讲述的故事。克服碳同位素分馏问题并掌握地球深处碳的数量和组成的一种方法是观察火山喷发出的稀有气体和二氧化碳(CO2)的同位素。尽管我们对浅层地球储集层中的碳了解很多，但它的浓度、同位素组成和在深部地幔中的行为还不是很清楚。这项研究利用太平洋两个地点(夏威夷和西太平洋的刘扩散中心)火山岩中的氦(He)和Ar(Ar)两种稀有气体的含量和同位素组成，来揭示地幔深部的碳故事，并提高我们对地球碳循环、碳通量和碳源的认识。这项研究的更广泛影响包括对一名在科学界性别比例较低的研究生进行实验室和研究培训；在俄克拉何马州的一所机构对本科生进行培训，这是一所位于EPSCoR(刺激竞争研究的实验计划)的机构；支持由NSF资助的俄勒冈州最先进的惰性气体分析设施；以及将研究成果纳入课程。这项研究利用了对太平洋两个不同地点喷发的海底玄武岩的气泡和玻璃中He、Ar和CO2丰度的耦合测量。一个是夏威夷岛外的Loihi海山，另一个是刘扩散中心，这两个中心都受到了来自深部地幔区域的地幔热柱的影响。测量将包括被困在火山玻璃和容器中的二氧化碳、He、Ar和H2O的浓度和同位素位置。气体的释放在俄勒冈州立大学最先进的惰性气体实验室进行。熔岩中囊泡中的气体将通过在真空中粉碎熔岩来释放。玻璃中的He和Ar浓度和同位素组成将通过阶梯加热/熔融释放出来。玻璃中溶解的CO2和H2O将通过傅立叶变换红外光谱(FTIR)进行测定。这些数据将被用来解开岩浆脱气导致的碳同位素组成的变化，从而可以确定样品中碳的原始成分和再循环成分。正在进行的主要研究问题包括：(1)地幔热柱源区的CO2/3He和碳同位素比值是什么；(2)地幔热柱深源中保存的原始碳是什么；(3)地幔热柱地区的二氧化碳通量是多少。这项工作的结果将被整合到地球动力学模型中，研究地球表面和深层之间的碳和惰性气体的循环。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。