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 δ13CVPDB and δ18OVSMOW 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. Plain Language Summary The sequestration of CO2 through mineralization has been a widely debated potential method for coping with rising levels of anthropogenic CO2 emissions. The C‐O stable isotope compositions of carbonates from the Barzaman Formation in Oman have been used to estimate the amount of atmospheric CO2 stored in them. These samples have geochemical signatures that suggest they are closely related to the oceanic crust, which is a precursor to the Oman Ophiolite. Based on the carbon and oxygen isotope model, we estimate that atmospheric CO2 contributed approximately 78% ± 11% (sd) of the overall carbon budget in a calcite sample at the t