RUI: Collaborative Research: Constraining peridotite alteration timescales with environmental tracers (3H, 39Ar, 14C and 81Kr)

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).Peridotite is the main constituent of the Earth's upper mantle. Peridotite alteration through water-rock interaction plays an important role in the global carbon budget and produces hydrogen gas (H2) that can support lifeforms fueled by chemical energy. This project will use the Samail ophiolite in Oman, the largest chunk of peridotite rock at the Earth’s surface, as a field laboratory to investigate the process of low temperature (25-60oC) peridotite alteration. Natural environmental tracers in groundwater within the ophiolite will be used to provide a maximum timescale for low temperature alteration and the source and supply rate of H2. These results will eventually help to calculate the rate of low temperature peridotite alteration and fluid flow through the ophiolite system and thus inform the use of ophiolites for geological carbon dioxide storage to combat climate change. They will also provide insight into the amount of chemical energy available to support life in this extreme hyperalkaline environment and in other locations of low temperature peridotite alteration on Earth and potentially elsewhere, like Mars. This project will be transformative in the education and training of geoscientists at two primarily undergraduate institutions: California State University, Sacramento, a minority-serving institution, and Barnard College in New York City, a women's college. The project will support student research experiences through both faculty-mentored student projects and course-based undergraduate research experiences (CUREs). CURE classroom modules will be publicly shared for use by other hydrogeology/geochemistry instructors throughout the nation. The project will also support the development of groundwater demonstration tanks to illustrate the use of environmental tracers and the impact that precipitation of secondary minerals can have on groundwater flow. The aim of this project is to use a suite of environmental tracers in the hyperalkaline (pH 11-12) groundwater of the Samail ophiolite to determine the maximum timescale of low temperature peridotite alteration and the source and supply rate of H2 gas. Groundwater samples will be collected using boreholes and sampling equipment recently installed in the mantle peridotite of the Samail ophiolite as part of the International Continental Scientific Drilling Program's Oman Drilling Project. Groundwater ages are currently unknown and estimates span several orders of magnitude. For this project, age distributions will be determined with environmental tracers 3H, 14C, stable noble gases, 39Ar and/or 81Kr. These ages will place upper bounds on the timescale of low temperature peridotite alteration and help determine if the water-rock interaction causing this alteration is taking place in the hydraulically conductive near-surface or if there are deeper, and/or older contributions. Other tracers (dissolved He and H2, 3He/4He ratios, and δ2H in H2) will be used to distinguish between several previously proposed sources of H2 in hyperalkaline groundwater: local production by low temperature water-rock interaction, transport from deeper sources, or release through weathering of fluid inclusions formed by peridotite alteration at higher temperature. Finally, the combination of groundwater ages and measured H2 concentrations will be used to quantify a minimum H2 accumulation rate in the aquifer. This H2 accumulation rate can then be used in bioenergetic calculations of the capacity to support chemosynthetic microbial life in this unique ecosystem.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.