MAGNESIAN CALCITE AS A PALEOENVIRONMENTAL INDICATOR

Previous studies have shown that temperature and Mg/Ca_(aq) ratio are the two main factors that control the Mg-content of calcite in low-temperature settings, however, other factors potentially complicate the relationship (e.g., kinetic effects and solution pCO₂). To better understand the role some of these factors play in controlling the Mg-content of calcite, 42 experiments were conducted where Mg-calcite was precipitated under tightly controlled aqueous conditions using the chemostat technique.

Experiments were run over a range of Mg_(aq) (4.4-66.8 mmol L^-1), Ca_(aq) (3.8-14.5 mmol L^-1), alkalinity (7.4-95.2 meq L^-1) and pCO₂ (0.03-0.50 atm) values with Mg/Ca_(aq) ratio ranging from 0.4 to 12.0 and Mg-calcite overgrowths were precipitated on pure calcite seed. Of the 25 experiments that produced at least 5 mg of pure Mg-calcite overgrowth, the mol% MgCO₃ ranged from 0.8 to 23.0 and precipitation rate ranged from 10^2.49 to 10^3.95 μmolm^-2 h^-1.

The Mg-content of the solid showed no dependence on precipitation rate or pCO₂ but a significant linear relationship was observed between Mg/Ca_(aq) and the mol% MgCO₃ of the overgrowth (mol% Mg = 0.0167•Mg/Ca_(aq) + 0.0062; R² = 0.82) that is consistent with previous studies.

On the other hand, solution chemistry and precipitation rate were not constant over the course of identical experiments conducted at relatively high Mg/Ca_(aq) ratios of 7.0 and 11.0. For these experiments, solution pH, Ca_(aq) and alkalinity increased initially and then decreased until steady state chemistry was achieved. These trends suggest that at relatively high Mg/Ca_(aq) ratio an incubation period is required to facilitate the precipitation of a Mg-bearing phase probably through the structural modification of the calcite surface. These results suggest that surface structure should be considered in the characterization and use of kinetic parameters to better understand the occurrence and distribution of Mg-calcite in surficial environments.