New Insights on Mantle Volatiles from Noble Gas Radionuclides

这个奖项是根据2009年美国复苏和再投资法案(公法111-5)资助的。稀有气体对地球上挥发分的来源以及大气和地幔的演化提供了至关重要的限制。然而，大气污染地幔挥发物，通常使解释变得复杂。这项提议涉及与地幔特征有关的大气成分的来源，包括裂谷带和与俯冲有关的火山，使用稀有气体放射性核素并结合其他化学和同位素数据。这些放射性核素以极低的丰度存在于大气中(81Kr~10^-12，t1/2=229000年；85Kr~10^-11，t1/2=10.8年；39Ar~10^-15，t1/2=269年)。这项提议的主要目标是使用这些放射性核素来解决“原始”挥发性样品中的表面空气污染，以及可能通过俯冲循环到地幔的大气成分。这项工作将侧重于两个地点(哥斯达黎加的里奥格兰德裂谷和Poas火山)，这两个地区的特点很好，很容易进行采样。这些分析需要使用以前用于研究富含二氧化碳的地热气体的方法从大量散装气体(5000升STP)中提取和提纯Kr和Ar。Kr的提纯将在伊利诺伊大学芝加哥分校(UIC)进行，惰性气体放射性核素将通过Argonne国家实验室的原子陷阱痕量分析(ATTA；81Kr和85Kr)和伯尔尼大学的低水平计数(39Ar)进行分析。样本的更多数据将在UIC、伯克利同位素地球化学中心和新墨西哥大学(UNM)获得。拟议的测量可能提供决定性的限制，有助于解决一些长期存在的问题，这些问题涉及陆地挥发性物质的起源、大气和海洋的演化、地表-地幔挥发性物质交换、地幔不均质性以及俯冲过程中挥发性元素的行为。领头的Pi是一名职业生涯早期的女性调查员。UIC和UNM的几名本科生将参加采样仪器的开发和测试以及实地工作。该项目将扩大在这些大学学习的学科的多样性，并为不同群体的学生提供更广阔的视野和独特的机会。该项目还将加强哥斯达黎加和瑞士地球科学家之间的国际合作。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit. Noble gases provide crucial constraints on the origin of volatiles on Earth, and the evolution of the atmosphere and mantle. However, atmospheric contamination of mantle-derived volatiles, generally complicates interpretation. This proposal addresses the origin of the atmospheric component associated with the mantle He signature, both for a rift zone and a subduction-related volcano using noble gas radionuclides in conjunction with other chemical and isotopic data. These radionuclides are present in the atmosphere at extremely low abundance (81Kr~10^-12, t1/2=229000yr; 85Kr~10^-11, t1/2=10.8yr; 39Ar~10^-15, t1/2=269yr) and they act as chronometers when isolated from their source. The main objective of this proposal is to use these radionuclides to resolve superficial air contamination in 'primordial' volatile samples, as well as atmospheric components that may be recycled to the mantle via subduction. This work will focus on two localities (Rio Grande Rift and Poas Volcano, Costa Rica) that are well characterized and readily accessible for sampling. The analyses require the extraction and purification of Kr and Ar from large quantities of bulk gas (5,000 liter STP) using methods previously applied to study of CO2-rich geothermal gases. Purification of Kr will be performed at University of Illinois at Chicago (UIC), and noble gas radionuclides will be analyzed via Atom Trap Trace Analysis (ATTA; 81Kr and 85Kr) at Argonne National Laboratory and by low-level counting (39Ar) at University of Bern. Additional data for the samples will be obtained at UIC, Berkeley's Center for Isotope Geochemistry, and the University of New Mexico (UNM). The proposed measurements may provide decisive constraints that could help to resolve a number of long-standing questions regarding implications for the origin of terrestrial volatiles, evolution of the atmosphere and ocean, surface-mantle volatile exchange, mantle heterogeneity, and the behavior of volatile elements during subduction.Broader Impacts. The lead PI is an early career female investigator. Several undergraduate students from UIC and UNM will participate in development and testing of sampling apparatus as well as field work. This project will broaden the diversity of subjects studied at these universities, and provide broader views and unique opportunities to diverse populations of students. This project will also enhance international collaboration between geoscientists from Costa Rica and Switzerland.