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Project Progress

Year five accomplishments center on the inorganic growth of magnesium calcite from solution. Thirty chemo-stat experiments (Fig. 1) were performed to better understand the effect of temperature, chemical composition, pCO₂, pH and precipitation rate on Mg isotope fractionation in aqueous systems that contain magnesium calcite.

One liter containers of 1.0 M stock solution of MgCl₂•₆H₂O, CaCl₂•₂H₂O, and NaHCO₃ were prepared and bubbled with pure CO_2(g) until chemical equilibrium with the gas phase was achieved (determined by constant solution pH). Individual solutions of targeted chemical composition were mixed from these stock solutions and deionized water (18 MΩ cm) to form master solutions, from which precipitation experiments were conducted. A Mg:Ca ratio of ~3.33:1 was used for 19 chemo-stat experiments and while a ratio of 10:1 was used for 11 experiments. The alkalinity of each master solution was fixed at 0.010 ± 0.002 N. Titrant solutions were prepared in an analogous fashion, with solution chemistry tailored to contribute excess ions for the precipitation reaction while maintaining a fixed chemical composition for the master solution. Prior to an experiment, the master and titrant solutions were re-equilibrated with a CO₂/N₂ gas mixture at a specified temperature to readjust the pH of the master solution to attain a metastable state of supersaturation with respect to Mg-calcite at a pre-selected pCO₂. The physiochemical composition of the solution was adjusted so the master solution was incapable of spontaneous nucleation over the duration of an experiment but sufficiently supersaturated to support the heterogeneous growth of a solid on a pre-existing carbonate template. To initiate an experiment, calcite seed (~80 mg; Fig. 2) was added to the master solution to initiate the precipitation of Mg-calcite overgrowths. The composition of master solution was maintained by the synchronized and constant injection of two titrants (one containing aqueous Mg and Ca, and the other dissolved inorganic carbon) into the master solution by a peristaltic pump. At the conclusion of an experiment, solid was separated from the solution by filtration (0.45 µm) and an aliquot of solution was immediately analyzed for alkalinity. A second aliquot was acidified for Ca and Mg analysis by ICP-MS to confirm the chemical composition of the fluid, and a third aliquot of acidified solution was reserved for Mg isotope analysis at the University of Wisconsin. The solid run products were analyzed by x-ray diffraction (Fig. 3), scanning electron microscopy (SEM: Fig. 4), electron microprobe using WDS detector (Fig. 5) and field emission scanning electron microscopy (FESEM: Fig. 6) to visualize and quantify the mole% MgCO₃ in the overgrowth. The final solid was also digested for Ca and Mg analysis by ICP-MS to estimate the mole% MgCO₃ in the solid. Experimental conditions for each experiment are reported in Table 1.

Results
No relationship was observed between pCO₂ and mole% MgCO₃ in overgrowth which is consistent with the results reported by Hartley and Mucci (1996), nor was a relationship observed between precipitation rate and mole% MgCO₃ in overgrowth. The results show an inverse relationship between solution Mg/Ca ratio and precipitation rate, which is consistent with results reported by Zhang and Dawe (2000). A positive relationship was observed between temperature and the mole% MgCO₃ in overgrowth (Fig. 7; slope = 0.1672) which is consistent with the results reported by Lopez et al. (2009; slope = 0.1846). However, this relationship is influenced by the Mg/Ca ratio of the solution as well. According to Mucci and Morse (1983), the distribution coefficient (D) for Mg in calcite is constant above an aqueous Mg:Ca ratio of ~7.5, while below this value D increases with a decrease solution Mg:Ca ratio. Two solution Mg:Ca ratios were used in these experiments, 10:1 and 3.33:1, for which DMg values of 0.0123 and 0.0195 may be used to predict the Mg/Ca ratio in the overgrowth, respectively. The difference between the predicted and observed value for the Mg/Ca ratio in the overgrowth is compared to temperature in Fig. 8, providing a more meaningful estimate for the temperature dependence of Mg incorporation in calcite. Below 25°C, the predicted Mg/Ca ratio in the solid is higher than the observed value whereas above 25°C the predicted Mg/Ca ratio is less than the observed value.
The results will help to better constrain the temperature dependence of Mg- isotope fractionation in calcite (e.g. Li et al., 2012) once solution and solid Mg- isotope analyses are completed.

References-
Hartley, G., and Mucci, A. (1996). The influence of P_CO2 on the partitioning of magnesium in calcite overgrowths precipitated from artificial seawater at 25° and 1 atm. total pressure, Geochim. Comochim. Acta., 60: 315-324.
Lopez, O., Zuddas, P., and Faivre, D. (2009). The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice, Geochim. Comochim. Acta., 73: 337-347.
Mucci, A., and Morse, J. W. (1983). The incorporation of Mg²⁺ and Sr²⁺ into calcite overgrowths: influences of growth rate and solution composition, Geochim. Comochim. Acta., 47: 217-233.
Zhang, Y., and Dawe, R. (2000). Influence of Mg²⁺ on the kinetics of calcite precipitation and calcite crystal morphology, Chemical Geology, 163: 129-138.