Oxygen isotope exchange kinetics during high-temperature synthesis of dolomite and magnesite

The kinetics of oxygen isotope fractionation during the hydrothermal dolomitization of CaCO3 was experimentally studied in the temperature range 150 – 220 °C over a one-year reaction period. Synthesis of dolomite was achieved by reacting either synthetic low-Mg calcite or speleothem aragonite with a Mg-rich (200 mM) hydrothermal fluid that was highly depleted in oxygen-18 (δ18OVSMOW = -46.4 ‰) and either 50 or 100 mM of NaHCO3, within sealed autoclaves.
All experimental sets produced dolomite along with varied amounts of co-precipitated magnesite via a stepwise dissolution-precipitation reaction sequence adhering to Ostwald’s step rule. Exchange of oxygen isotopes between the reactive solution and precipitating carbonate phases was fast enough, that oxygen isotope equilibrium was reached in all experiments, which is a pre-requisite for kinetic considerations. Analysis of the oxygen isotope fractionation as a pseudo first order reaction allowed for the reaction rate constants (k), duration to reach 99.9 % of isotope equilibrium (t0.999) and activation energy for oxygen isotope exchange during dolomitization to be determined for the first time. Despite the use of different reactant CaCO3 phases and alkalinity conditions we found that the kinetics of oxygen isotope fractionation is the same at equal temperatures, isolating temperature as the sole control over kinetics within our experimental approach. Oxygen isotope exchange was significantly faster at higher temperature with t0.99 decreasing from ~ 210 days at 150°C to ~ 40 days at 220°C. Furthermore, despite differences in the relative abundance of dolomite and magnesite precipitated, oxygen isotope fractionation kinetics and values are the same within the analytical precision. We obtained from our data an activation energy for oxygen isotope exchange of 48.4 ± 7.2 (1σ error) kJ·mol-1 and an overall oxygen isotope fractionation factor (1σ error): 103ln(αdolomite±magnesite-water) = 2.702 (± 0.236) · (106 / T2) + 0.435 (± 1.290), which is generally consistent with the literature, sitting between fractionation lines for dolomite-water and magnesite-water.
Further work to separate the dolomite from co-precipitated magnesite using di-Na-EDTA is ongoing, so that the oxygen isotope fractionation factor and exchange kinetics can be determined and confidently reported for the individual phases (dolomite and magnesite) as opposed to the mixture.