"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.
