Carbonate mineral precipitation via capture of atmospheric CO₂ by alkaline-hyperalkaline waters offers a potential strategy-referred to as carbon capture and storage-to mitigate anthropogenic CO₂ emissions. Oman ophiolite produces high-pH waters characterized by continuous sequestration of atmospheric CO₂. The geochemical and isotopic data of carbonates are utilized to assess the CO₂ stored in dolomite-calcite assemblage of the Barzaman Formation. PAAS-normalized REE+Y patterns display increasing LREE, flat HREE, and a positive Eu-anomaly, identical to those of the bulk ocean crust and lower crust. Further, La(+), Ce(-), and Y(+) anomalies in studied samples are missing in the ocean crust, however, present in local groundwater and modern seawater. Carbon and oxygen isotopes show two distinct end-members, heavier dolomite than calcite. The estimated carbonate growth temperatures (18℃ to 65℃) are indistinguishable from literature values. The C-O isotope model of dolomite-calcite assemblage, water, and atmospheric CO₂ demonstrates that the C-budget of ophiolite-derived carbonates represent an unequivocal contribution from the latter. Meanwhile, the remaining contributors may include dissolved inorganic carbon (DIC) in waters closely associated with travertines and ophiolite-derived soil exhibiting large variations in their C-O isotope compositions caused by kinetic fractionation leading to isotopic disequilibrium. Taken together, REE+Y patterns and isotopic compositions verify that the carbonates were formed under oxic conditions through alterations triggered by the reaction between ophiolite and meteoric water. A detailed groundwater study is recommended to assess the contribution of atmospheric CO₂ in DIC for a thorough estimation of the amount of CO₂ sequestered by carbonates of the Barzaman Formation.