PHYSICOCHEMICAL CONTROLS ON THE MG-CONTENT OF CALCITE PRECIPITATED FROM DILUTE SOLUTIONS

The magnesium content of calcite has been used as a proxy for temperature and aqueous Mg/Ca content of fluids in numerous studies with varying degrees of success. Confounding factors that potentially complicate the relationship between the solid and liquid phases include kinetic effects and environmental factors such as the PCO₂ of the gas phase in equilibrium with the precipitating fluid. To better understand the influence of these factors on the partitioning of Mg in calcite, thirty three laboratory experiments were conducted at 25°C in which Mg-calcite was precipitated on pure calcite substrate under tightly controlled physicochemical conditions using the chemostat technique. Experiments were run over a range of Mg_(aq) (6-24 mmolL^-1), Ca_(aq) (12-24 mmol L^-1), Mg_(aq)/Ca_(aq) (0.5-1.0), alkalinity (10-20 meq L^-1) and PCO₂ (0.075-0.3 atm) conditions. Nine additional experiments were performed using carbonate substrates containing 10-98 mol% MgCO₃ to explore the role of solid state geochemistry on the Mg-content of the overgrowths.

The results show a strong positive relationship between aqueous Mg/Ca and the mol% MgCO₃ of the precipitated solid, while no relationship was observed between precipitation rate, solution PCO₂ and the mol% MgCO₃ of the solid. These results are consistent with previous studies that suggest precipitation rate nor PCO₂ have a measurable effect on the partitioning of Mg in calcite. On the other hand, the observed range of distribution coefficients (0.0215-0.0370; where D = Mg/Ca_solid / Mg/Ca_soln) was higher and more variable than classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (~0.008 std dev). Moreover, the Mg-content of the overgrowth appears to be dependent on the Mg-content of the carbonate substrate. This suggests that Mg is preferentially adsorbed on the surface layer when Mg/Ca(aq) is relatively low (e.g., <7.5), and/or that substrate composition (i.e., mol% MgCO₃) may exert an influence on the Mg-content of calcite precipitated inorganically from dilute solutions.

Once the effect of substrate geochemistry is better understood, the Mg/Ca content of inorganic calcite may be used to infer paleofluid compositions regardless of precipitation rate or solution PCO₂ within the boundaries of the present experimental conditions.