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

碳同位素比率是了解全球碳循环的主要信息来源，碳循环在温室气体的产生中起着重要作用；碳从地球表面回到地球内部的运动；以及自45亿年前形成以来一直被困在地球深处的原始碳逃逸到地球表面。然而，这些同位素比率受到各种地质过程的影响，例如火山爆发期间岩浆的脱气。这导致了分馏，改变了同位素的原始比例，使它们难以完全解释。如果没有某种认识这些变化的手段，就不可能完全了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与火山喷出的二氧化碳（CO2）一起喷发的稀有气体的同位素。虽然我们对地球浅层储层中的碳有很多了解，但对其浓度、同位素组成和在深部地幔中的行为却知之甚少。本研究利用两种稀有气体的含量和同位素组成：氦（He）和氩（Ar）来自太平洋两个地点（夏威夷和西太平洋的劳扩张中心）的火山岩，揭示了地幔深处碳的故事，提高了我们对地球碳循环、碳通量和碳源的认识。这项研究的更广泛的影响包括对一名在科学领域性别代表性不足的研究生进行实验室和研究培训；俄克拉何马州EPSCoR （Experimental Program to stimulation Competitive Research）机构的本科生培训；支持美国国家科学基金会在俄勒冈州资助的最先进的稀有气体分析设施；并将研究成果纳入课程。本研究利用在太平洋两个不同地点喷发的海底玄武岩的囊泡和玻璃中He， Ar和CO2丰度的耦合测量。一个是夏威夷岛附近的Loihi海山，另一个是劳扩张中心，两者都受到了地幔柱存在的影响，这些地幔柱进入了地幔深部区域。测量将包括困在火山玻璃和容器中的CO2、He、Ar和H2O的浓度和同位素位置。这些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。熔岩中的气泡中的气体将通过在真空中粉碎熔岩而释放出来。玻璃中的He和Ar浓度和同位素组成将通过逐步加热/聚变释放。溶解在玻璃中的CO2和H2O将用FTIR（傅里叶变换红外光谱）测定。这些数据将用于解卷积岩浆脱气引起的碳同位素组成的变化，从而确定样品中碳的原始成分和回收成分。研究的重点问题包括：(1)地幔柱源区的CO2/3He和碳同位素比值是多少；(2)原始碳保存在地幔柱的深层源；(3)地幔柱位置的CO2通量。这项工作的结果将被整合到地球动力学模型中，以解决地球表面和深层储层之间碳和稀有气体的循环问题。该奖项反映了美国国家科学基金会的法定使命，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。

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.