Geochemical and C-O Isotopic Study of Ophiolite-Derived Carbonates of the Barzaman Formation, Oman: Evidence of Natural CO₂ Sequestration Via Carbonation of Ultramafic Clasts

Carbonate precipitation through atmospheric CO₂ uptake by alkaline-hyperalkaline waters offers a potential approach to mitigating anthropogenic CO₂ emissions. The Oman Ophiolite produces high-pH water characterized by continuous sequestration of CO₂ at the air-water interface. The geochemical and isotopic data of carbonates from the Barzaman Formation is used to assess the amount of atmospheric CO₂ stored in the dolomite-calcite assemblage. Post Archean Australian Shale -normalized rare earth elements patterns, with the exception of La and Ce anomalies, are similar to those of the bulk oceanic and lower crusts, with increasing LREE and flat HREE trends, and a positive Eu anomaly. The δ¹³C_VPDB and δ¹⁸O_VSMOW isotope values of the analyzed samples show two distinct end-members, in which dolomite (−7.77‰ and +27.3‰) is isotopically heavier than calcite (−9.93‰ and +21.5‰). The estimated carbonate growth temperatures (18°C–56°C) are indistinguishable from the previously reported range (18°C–66°C). The C-O isotope model for calcite, groundwater, and atmospheric CO₂ shows that an ophiolite-derived calcite sample absorbed an unequivocal amount of atmospheric CO₂ (78% ± 11%) during precipitation. At the same time, dissolved inorganic carbon (DIC) in water accounts for the remaining carbon contribution (22% ± 9%). DIC is closely associated with different carbonate lithofacies and ophiolite-derived soil, exhibiting large variations in C-O isotopic compositions caused by isotopic disequilibrium. Taken together, geochemical and isotopic properties confirm that the carbonates were formed under oxic conditions triggered by the water-rock interaction. For a reliable estimate of CO₂ sequestered by carbonates of the Barzaman Formation, a systematic groundwater analysis is recommended to determine the contribution of CO₂ in DIC.