Equilibrium-Kinetic Calculation of Olivine Serpentinization. A Comparison with the Model Experiment

Abstract

The GEOCHEQ_M and GEOCHEQ_Isotope databases and codes were used to simulate the evolution of chemical interactions with time in the closed systems aqueous solution 0.495 М NaCl + 0.0194 M NaHCO3 + olivine (Fo91) at temperatures of 200 and 320°C and a pressure of 350 bar. The solution to rock mass ratios, selected temperature and pressure, the chemical composition and size of the olivine grains, the chemical composition of the aqueous solution, and the duration of interactions correspond to those in experiments described in (McCollom et al., 2016; McCollom, 2016). The carbon isotope composition of NaHCO3 was the only one to change. Instead of NaH13CO3 used in the experiment, our simulations were carried out assuming the carbon isotope composition of belemnite from the Peedee Formation, South Carolina, United States. The results of these simulations were compared with the experimental results. As in the experiments, the newly formed mineral assemblage consists of Mg-rich serpentine, brucite, and magnetite, with a minor magnesite amount at 200°С. The calculated rate of olivine serpentinization is close to the experimental one. The calculations yield higher dissolved methane contents than in the experiments. This may be explained by ignoring kinetic constraints of the reaction between dissolved CO2,aq and hydrogen generated during serpentinization. Fractionation of carbon isotopes was calculated between dissolved inorganic carbon (DIC), newly formed hydrocarbons, and magnesite. The hydrocarbons are significantly enriched in the light isotope with respect to DIC and with respect to magnesite at 200°C.