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

合作研究：通过未脱气的大洋中脊玄武岩中碳、氮、氢和稀有气体的同位素变化检查上地幔挥发历史

• 批准号: 1558802
• 负责人: Peter Michael
• 金额: $ 11.12万
• 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: 2016
• 资助国家: 美国
• 起止时间: 2016-03-15 至 2021-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558802&HistoricalAwards=false
• 关键词: Collaborative; Research; Examining; Upper; Mantle

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.

对大洋中脊海底玄武岩中的二氧化碳、水、氮和惰性气体等挥发物的测量，为深入了解地球大气层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相当多的信息。 然而，几乎所有海底玄武岩在喷发时都会因脱气而失去大部分挥发性物质，这导致识别地幔原始化学特征所需的可靠化学数据的重大损失。因此，在我们对挥发物的起源及其进出地球的通量的理解中存在很大的不确定性和争议。这项研究对非常罕见的未脱气洋中脊玄武岩进行分析，以获得关于碳、氮、氢和稀有气体（氦、氖和氩）的地幔同位素组成的信息。总的来说，这些气体可以协同使用，以更好地了解地球的大气和海洋是如何形成和演变的。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。该项目产生的化学和同位素数据在这方面至关重要，将为今后几年的洋脊脱气模型和地球挥发物的起源和演变提供基准。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，该州收到的联邦资金相对较少（即，一个EPSCoR国家），并与法国科学家开展新的国际合作。它还使用了美国国家科学基金会资助的俄勒冈州州立大学的分析设施，并开发了分析岩石样品中惰性气体的新方法，为科学建立了基础设施。研究结果也将纳入研究人员讲授的课程，并用于为研究生开发一个新的研讨会，重点是化学地球动力学中的储层、过程和通量的建模。这项研究的科学目标是测试大洋中脊和大陆裂谷之下的上地幔区域在全球范围内挥发物丰度和同位素组成的相似程度。对未脱气的洋中脊玄武岩中的碳、氮、氢和惰性气体的分析将首次允许在不进行脱气校正的情况下测试惰性气体与CO2、N2和H2O中的主要挥发性同位素的相互关系。这使得对上地幔区域不均匀性的直接检验成为可能。研究还具体探讨了以下问题：（1）上地幔中挥发性C、N和H_2O的比例是原始的还是再循环的;（2）超贫化洋中脊玄武岩中的碳稳定同位素是否具有形成超贫化源所需的低度熔融早期贫化记录;（3）N在亏损上地幔中的行为和浓度;（4）上地幔稀有气体原始同位素比值中保存了早期地球过程的哪些记录。