Rate-controlled calcium isotope fractionation in synthetic calcite

The isotopic composition of Ca (Δ⁴⁴Ca/⁴⁰Ca) in calcite crystals has been determined relative to that in the parent solutions by TIMS using a double spike. Solutions were exposed to an atmosphere of NH₃ and CO₂, provided by the decomposition of (NH₄)₂CO₃, following the procedure developed by previous workers. Alkalinity, pH and concentrations of CO₃²⁻, HCO₃⁻, and CO₂ in solution were determined. The procedures permitted us to determine Δ(⁴⁴Ca/⁴⁰Ca) over a range of pH conditions, with the associated ranges of alkalinity. Two solutions with greatly different Ca concentrations were used, but, in all cases, the condition Ca²⁺]>>CO₃²⁻] was met. A wide range in Δ(⁴⁴Ca/⁴⁰Ca) was found for the calcite crystals, extending from 0.04 ± 0.13‰ to −1.34 ± 0.15‰, generally anti-correlating with the amount of Ca removed from the solution. The results show that Δ(⁴⁴Ca/⁴⁰Ca) is a linear function of the saturation state of the solution with respect to calcite (Ω). The two parameters are very well correlated over a wide range in Ω for each solution with a given Ca]. The linear correlation extended from Δ(⁴⁴Ca/⁴⁰Ca) = −1.34 ± 0.15‰ to 0.04 ± 0.13‰, with the slopes directly dependent on Ca]. Solutions, which were vigorously stirred, showed a much smaller range in Δ(⁴⁴Ca/⁴⁰Ca) and gave values of −0.42 ± 0.14‰, with the largest effect at low Ω. It is concluded that the diffusive flow of CO₃²⁻ into the immediate neighborhood of the crystal-solution interface is the rate-controlling mechanism and that diffusive transport of Ca²⁺ is not a significant factor. The data are simply explained by the assumptions that: a) the immediate interface of the crystal and the solution is at equilibrium with Δ(⁴⁴Ca/⁴⁰Ca) ∼ −1.5 ± 0.25‰; and b) diffusive inflow of CO₃²⁻ causes supersaturation, thus precipitating Ca from the regions exterior to the narrow zone of equilibrium. The result is that Δ(⁴⁴Ca/⁴⁰Ca) is a monotonically increasing (from negative values to zero) function of Ω. We consider this model to be a plausible explanation of most of the available data reported in the literature. The well-resolved but small and regular isotope fractionation shifts in Ca are thus not related to the diffusion of very large hydrated Ca complexes, but rather due to the ready availability of Ca in the general neighborhood of the crystal-solution interface. The largest isotopic shift which occurs as a small equilibrium effect is then subdued by supersaturation precipitation for solutions where Ca²⁺]>>CO₃²⁻] + HCO₃⁻]. It is shown that there is a clear temperature dependence of the net isotopic shifts that is simply due to changes in Ω due to the equilibrium “constants” dependence on temperature, which changes the degree of saturation and hence the amount of isotopically unequilibrated Ca precipitated. The effects that are found in natural samples, therefore, will be dependent on the degree of diffusive inflow of carbonate species at or around the crystal-liquid interface in the particular precipitating system, thus limiting the equilibrium effect.