Collaborative Research: Examining Upper Mantle Volatile History Through Isotopic Variations of Carbon, Nitrogen, Hydrogen and Noble Gases in Undegassed Mid-Ocean Ridge Basalts

对大洋中脊海底玄武岩中二氧化碳、水、氮和惰性气体等挥发物的测量，为了解地球大气和海洋的起源，以及地球表面和内部循环的地质历史提供了重要的见解。然而，几乎所有的海底玄武岩在喷发时都因脱气而失去了大部分挥发性物质，这导致了可靠的化学数据的大量损失，而这些数据是识别地幔原始化学特征所必需的。因此，我们对挥发物的起源及其进出地球的通量的理解存在很大的不确定性和争议。本研究对极少出现的未脱气的洋中脊玄武岩进行分析，以获取地幔碳、氮、氢和稀有气体（氦、氖和氩）的同位素组成信息。总的来说，这些气体可以协同使用，以更好地了解地球大气和海洋是如何形成和演化的。洋中脊是地球表面的基本地质特征，地球科学中最有趣的挑战之一是了解它们的形成和演变过程。从这个项目得到的化学和同位素数据在这方面是基本的，并将在未来几年为海洋脊的脱气模型和地球挥发物的起源和演化提供基准。这项工作的更广泛影响包括资助俄克拉荷马州的一个机构，俄克拉荷马州接受的联邦资金相对较少（即EPSCoR州），并与法国科学家开展新的国际合作。它还使用美国国家科学基金会资助的俄勒冈州立大学的分析设备，开发分析岩石样本中惰性气体的新方法，为科学建设基础设施。研究结果也将被纳入研究人员讲授的课程中，并用于为研究生开设一个新的研讨会，以化学地球动力学中的储层、过程和通量建模为中心。这项研究的科学目标是测试海洋中脊和大陆裂谷下的上地幔区域在全球范围内挥发性丰度和同位素组成的相似程度。对未脱气的海中脊玄武岩中的碳、氮、氢和稀有气体的分析，将首次允许在不使用脱气校正的情况下，测试二氧化碳、N2和H2O中稀有气体和主要挥发性同位素的相互关系。这允许对上地幔的区域非均质性进行直接测试。该研究还具体解决了以下问题：(1)上地幔中挥发物C、N和H2O的原始比例与循环比例；(2)超贫洋中脊玄武岩的碳稳定同位素是否记录了形成超贫源所必需的较早的低熔体损耗；(3)衰竭上地幔中N的变化规律和浓度；(4)上地幔稀有气体的原始同位素比值保存了早期地球过程的哪些记录。

Measurements of volatiles such as carbon dioxide, water, nitrogen and noble gases in submarine basalts from mid-ocean ridges have provided considerable insight into the origin of Earth's atmosphere and oceans, and the geologic history of cycling between the surface of the Earth and its interior. However, almost all submarine basalts have lost much of their volatile inventory by degassing when they are erupted, which has led to a major loss of the reliable chemical data needed to identify pristine chemical signatures from Earth's mantle. As a result, there is significant uncertainty and controversy in our understanding of the origins of volatiles and their fluxes into and out of the Earth. This research carries out analyses of the very rare occurrences of undegassed mid-ocean ridge basalts to obtain information on the mantle isotopic composition of carbon, nitrogen, hydrogen and noble gases (helium, neon and argon). Collectively, these gases can be used synergistically to better understand how Earth's atmosphere and oceans formed and evolved. Mid-ocean ridges are fundamental geologic features of Earth's surface, and one of the most intriguing challenges in geosciences is understanding the processes by which they are created and evolve. The chemical and isotopic data resulting from this project are fundamental in this regard, and will provide benchmarks for years to come for models of degassing at ocean ridges and the origin and evolution of Earth's volatiles. Broader impacts of the work include funding an institution in Oklahoma, a state that receives comparatively little federal money (i.e., an EPSCoR state) and the development of new international collaborations with French scientists. It also uses NSF-funded analytical facilities at Oregon State University and develops new methods for analyzing noble gases in rock samples, building infrastructure for science. Findings will also be incorporated into classes taught by the investigators and used to develop a new seminar for graduate students centered on modeling of reservoirs, processes and fluxes in chemical geodynamics. The scientific goals of this research are to test the extent to which regions of the upper mantle beneath mid-ocean ridges and continental rifts are similar in their volatile abundances and isotopic compositions on a global scale. Analyses of carbon, nitrogen, hydrogen, and noble gases in undegassed mid-ocean ridge basalts will allow, for the first time, testing of the inter-relationships of noble gas and major volatile isotopes in CO2, N2, and H2O without employing degassing corrections. This allows a direct test for regional heterogeneity in the upper mantle. The research also specifically addresses the questions: (1) What proportions of the volatiles C, N and H2O in the upper mantle are primordial versus recycled; (2) do carbon stable isotopes in the ultra-depleted mid-ocean ridge basalts carry a record of earlier depletion in low degree melts that appears to be required in formation of the ultra-depleted source; (3) what is the behavior and concentration of N in the depleted upper mantle; and (4) what record of early Earth processes is preserved in the primordial isotope ratios of upper mantle noble gases.