Dolomitization kinetics during induction: fluid geochemistry and mineralogical evidence for early nucleation

ABSTRACT Dolomitization is described as a stepwise replacement of CaCO3 via a dissolution–precipitation process that first involves formation of a metastable very-high-magnesium calcite (VHMC), which is then replaced by ordered, stoichiometric dolomite. Laboratory experiments consistently report a long induction period during which no mineral products are detected with X-ray diffraction. In contrast to previous studies, the current study investigates the induction period using a combination of fluid chemistry, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and data on in situ SEM-EDS elemental composition from dozens of high-temperature dolomitization experiments conducted over the range of 150–200°C. Consistent with previous experiments, XRD data show a prolonged induction period before Ca-carbonate reactants are replaced by VHMC, which are then replaced by dolomite. The fluid-geochemistry data show, however, that fluid Mg/Ca ratios decrease and Sr/Ca ratios increase immediately after the onset of experimental conditions and continue along linear trajectories until most of the CaCO3 reactants are consumed. These observations are consistent with calcite dissolution and VHMC precipitation during the first stages of the mineral-replacement reaction. SEM and SEM-EDS observations align with the fluid-geochemistry data in that they show VHMC crystals on calcite reactants, hours before VHMC is detected by XRD. Collectively, these data indicate VHMC nucleation and growth is not significantly inhibited, but rather that VHMC forms at a low rate. A revised three-stage model of dolomitization is presented whereby there is no induction period for VHMC. In this model, stage one is characterized by CaCO3 replacement by VHMC, which proceeds slowly at first and then rapidly. Stage two is characterized by VHMC replacement by poorly ordered dolomite, and stage three is characterized by recrystallization to ordered dolomite.