GROUNDWATER FLOW DYNAMICS AND CO2 MINERALIZATION IN THE PERIDOTITE SECTION OF THE SAMAIL OPHIOLITE, OMAN

The Samail Ophiolite, Oman is a site of natural CO₂ mineralization in ultramafic rocks and is being investigated as a site for engineered geological CO₂ storage. The Samail Ophiolite is the largest and best exposed ophiolite in the world. Groundwater ages in the mantle peridotite section provide insight into groundwater flow dynamics within this fractured rock aquifer and provide constraints on water-rock reaction rates by serving as a maximum timeframe for water-rock interaction.

Groundwater in the peridotite section of the Samail Ophiolite has been evaluated using multiple age tracers: ³H, ³⁹Ar, and ¹⁴C. Samples collected in the winters of 2018 to 2020 show that Type I shallow Mg-HCO₃ groundwater has ³⁹Ar apparent ages ranging from modern to 250 years. δ¹³C in these samples is -15 to -9 ‰ and ³H is typically 1 to 2 TU, though one sample has 0.4 TU. This water may be considered the starting point for continued alteration in a system closed off from the atmosphere. The resulting Type II hyperalkaline (pH >10) Ca-OH waters have ³⁹Ar apparent ages ranging from modern to 530 years and are depleted in δ¹³C with values of -22 to -17 ‰. ³H concentrations are less than 0.7 TU. The distribution of these tracers suggests that two or more component mixing is likely an important process in this fracture flow dominated system. The relatively young (< 1000 years) ³⁹Ar ages of Type II groundwater indicate that the geochemical processes responsible for producing hyperalkaline Ca-OH water are either faster than previously thought or mixing and/or other chemical reactions are at play. Results from additional samples collected in 2024 may help to elucidate the groundwater flow and chemical dynamics of this system.

Additionally, a physical groundwater model was built to demonstrate the entire process from CO₂ capture to storage via mineralization in mafic or ultramafic rocks. The model has successfully been used to educate students and the public about CO₂ storage via mineralization in geological reservoirs.