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

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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.