Rate-controlled calcium isotope fractionation in synthetic calcite

Abstract

The isotopic composition of Ca (Δ^(44)Ca/^(40)Ca) 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 NH_3 and CO_2, provided by the decomposition of (NH_4)_2CO_3, following the procedure developed by previous workers. Alkalinity, pH and concentrations of CO_3^(2−), HCO_3^−, and CO_2 in solution were determined. The procedures permitted us to determine Δ(^(44)Ca/^(40)Ca) 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 [Ca^(2+)]>>[CO_3^(2−)] was met. A wide range in Δ(^(44)Ca/^(40)Ca) 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(^(44)Ca/^(40)Ca) 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(^(44)Ca/^(40)Ca) = −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(^(44)Ca/^(40)Ca) and gave values of −0.42 ± 0.14‰, with the largest effect at low Ω. It is concluded that the diffusive flow of CO_3^(2−) into the immediate neighborhood of the crystal-solution interface is the rate-controlling mechanism and that diffusive transport of Ca^(2+) 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(^(44)Ca/^(40)Ca) ∼ −1.5 ± 0.25‰; and b) diffusive inflow of CO_3^(2−) causes supersaturation, thus precipitating Ca from the regions exterior to the narrow zone of equilibrium. The result is that Δ(^(44)Ca/^(40)Ca) 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 [Ca^(2+)]>>[CO_3^(2−)] + [HCO_3^−]. 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.

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