On the evolution of dolomite stoichiometry and cation order during high-temperature synthesis experiments: An alternative model for the geochemical evolution of natural dolomites

Data collected from a series of high-temperature dolomitization experiments in which dolomite replaces calcite in Mg–Ca–Cl solutions indicate that dolomite composition (mol% MgCO3) and cation order evolve independently as a function of reaction progress. Despite a wide range of initial solution Mg/Ca (0.43–1.50), the first product to form in all experiments is disordered dolomite. Both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial Mg/Ca in solution. Experimental solutions with lower Mg/Ca yield less stoichiometric (more Ca-rich) dolomite and have reactions that progress at a slower rate. Conversely, solutions with higher Mg/Ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates. In all experiments, dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower Mg/Ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution. Despite a dolomite composition that remains relatively constant, the degree of cation order increases with the percentage of dolomite. Shortly before complete consumption of calcite reactants, when \u3e 95% of the calcite reactants have been replaced by dolomite products, dolomite composition rapidly becomes more stoichiometric. After all calcite reactants have been consumed, dolomite is completely stoichiometric regardless of the Mg/Ca in solution at the onset of experimental conditions. However, the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions. The observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies. This suggests that the mechanism by which dolomite replaces CaCO3 is not clear and is more complex than previously recognized. The data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth. Thus, new material being added to the crystal surface is forced to follow the established composition. The data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite. We propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases (i.e., indicating further reaction progress). Rather, carbonate rocks with a higher dolomite-to-limestone ratio, where the dolomite is more stoichiometric, can be better explained as having been dolomitized in solutions with higher Mg/Ca, which cause both faster rates of dolomitization and more stoichiometric compositions. This model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path. The new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models, which numerically track chemical reactions and constituents