Carbonation processes in Oman peridotite: temperature-dependent reactions and Mg isotope composition of reacting fluids

The study of natural analogues of CO₂ mineral sequestration combined with the experimental quantification of carbonation reactions constitutes a fundamental approach to understand the spatial and structural distribution of carbonated bodies and the time scales by which large amounts of CO₂ can be stored in solid form into geologic formations. To better quantify the carbonation rates of ultramafic rocks and study the evolution of dissolved Mg isotope composition during their interaction with CO₂-rich fluids, a series of batch carbonation experiments using a partially serpentinized harzburgite from the Semail ophiolite (Oman) was conducted at 90–180 °C and at constant CO₂ partial pressures (∼ 15–20 bar). The yield of the carbonation reaction increased from ∼ 0 at 90 °C to a maximum of 31 mol % at 150 °C, decreasing to 12 mol % at 180 °C over a period of one month. Magnesites containing 3–9 wt% of Fe and silica polymorphs (SiO_2(am) and chalcedony) were the main reaction products, with a fraction of secondary Mg-silicates that increased with increasing temperature, significantly reducing the carbonation extent at 180 °C. The aqueous fluid became progressively enriched in heavy isotopes with the progress of the carbonation reaction. The apparent Mg isotope fractionations between the rock and bulk fluid (Δ²⁶Mg = δ²⁶Mg_solid − δ²⁶Mg_fluid) varied from −1.6 ‰ at 120 °C to −0.9 ‰ at 180 °C, consistent with the preferential uptake of ²⁴Mg by carbonate minerals and the decrease of isotope fractionation with increasing temperature. The average magnesite isotope compositions (−1.6 ‰ ≤ δ²⁶Mg ≤ -0.3 ‰) derived from mass-balance calculations were found to be within the range of δ²⁶Mg values reported for Oman listvenites, suggesting that the carbonation processes in this geological unit took place within the temperature range considered in this study (∼120–180 °C).

Comparison of experimental results with observations of a well-studied natural analogue provides new indications on the optimum conditions for the development of CO₂ sequestration methods in peridotites, or ex situ carbonation techniques using ultramafic minerals. The observed evolution of the Mg isotopic composition of the fluid also suggests that the use of Mg-isotopes could be an effective tool to monitor the spatial and temporal progress of carbonation reactions during field-scale CO₂ storage operations in ultramafic rocks.