Abstract
Partitioning of strontium during spontaneous calcite formation was experimentally studied using an advanced CO 2-diffusion technique. Results at different precipitation rates and T = 5, 25, and 40 °C show that at constant temperature Sr incorporation into calcite is controlled by the precipitation rate ( R in μmol/m 2/h) according to the individual expressions log D Sr = ( 0.319 ± 0.042 ) · log R - 1.80 ± 0.11 log D Sr = ( 0.214 ± 0.026 ) · log R - 1.67 ± 0.09 log D Sr = ( 0.095 ± 0.031 ) · log R - 1.42 ± 0.09 at 5, 25, and 40 °C, respectively. High rates induce less Sr partitioning (i.e., high Sr distribution coefficients of Sr) in calcite ( D Sr). D Sr value and rate dependency decreases with increasing temperature. This indicates that effect of R is more significant at lower T. Both R and D Sr are also apparently influenced by solution chemistry (i.e., pH). The strontium incorporation behavior during precipitation of calcite can be reasonably explained by the Surface Entrapment Model (SEMO) adapted from Watson and Liang (Watson E. B. and Liang Y. (1995) A simple model for sector zoning in slowly grown crystals: implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks. Am. Miner. 80, 1179–1187). According to SEMO Sr is significantly enriched in the surface layer of the growing calcite. Sr incorporation into the final calcite crystal lattice depends on the estimated value of the Sr entrapment factor for the surface layer, crystal growth kinetics, and ion diffusion behavior within the proposed surface layer. SEMO is a valid theoretical concept to elucidate mechanisms and kinetics of Sr incorporation into inorganic calcite within the range of the experimental conditions used in this study. Accordingly, apparent D Sr values in natural and applied systems can be used as an environmental proxy for inorganic calcite growth behavior at the present aqueous molar Sr/Ca ratios between 0.01 and 0.00006.
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