Isotopic fractionation during congruent dissolution, precipitation and at equilibrium: Evidence from Mg isotopes

This study provides direct experimental evidence of magnesium (Mg) isotope fractionation between an aqueous fluid and magnesite during its congruent dissolution, precipitation and at equilibrium. Closed-system batch reactor experiments were performed at temperatures from 120 to 200 °C and at 15 or 30 bar CO2 pressure. During congruent magnesite dissolution the fluid became enriched in isotopically heavy Mg, with a steady-state δ26Mgfluid composition that was 0.4‰ higher than the dissolving magnesite at 15 bar of CO2 pressure and 0.15‰ higher at 30 bar of CO2 pressure. Magnesite precipitation was provoked by increasing the reactor temperature after equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the reactor temperature was increased and rapid magnesite precipitation occurred. However, isotopic exchange continued as the system equilibrated, eradicating this Rayleigh signal. The equilibrium 26Mg/24Mg fractionation factors (αeqm) for the magnesite-fluid system were found to be 0.99881 at 150 °C and 0.99912 at 200 °C. Taken together, these observations (1) support the theoretical hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and (2) indicate that isotopes will continue to exchange and equilibrate even if the mineral surfaces and co-existing fluids are in chemical equilibrium. The fact that isotopes continue to exchange at chemical equilibrium will tend to eradicate both kinetic and paleo-environmental isotopic signatures, and the degree to which such signatures are completely eradicated depends on how deep into the surface the isotopic exchange process penetrates.