PHYSICOCHEMICAL CONTROLS ON THE MG-CONTENT OF CALCITE PRECIPITATED FROM DILUTE SOLUTIONS
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/Casolid / Mg/Casoln) 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.