MN INCORPORATION INTO CALCITE: INFLUENCE OF INTERFACIAL DYNAMICS ON TRACE ELEMENT PARTITIONING, CALCIUM ISOTOPE FRACTIONATION, AND GROWTH KINETICS

Crystal growth kinetics in complex solutions and the associated incorporation of trace elements are controlled by reactions occurring at the fluid-mineral interface, but their description in predictive models remains challenging. For the case of sparingly soluble minerals such as calcite, mechanistic ion by ion models have been developed that describe the impact of foreign ions on crystal growth via their influence on the kinetics of growth unit attachment and detachment from surface sites – including surface attachment dynamics (e.g. kink blocking) and the solid solution thermodynamic effects of foreign ion incorporation. However, such models have only been developed for and tested on incompatible elements where relatively little trace element is incorporated into the lattice. Here we investigate the partitioning of Mn into calcite as a representative compatible trace element that is readily concentrated in the mineral lattice, examining the interplay between aqueous Mn activity, Mn partitioning, growth rates, and Ca isotope fractionation to probe the mass-dependent reactions occurring at the fluid-mineral interface and their representation in ion by ion models.

Seeded Mn-calcite growth experiments were run under different Mn loadings (Mn/Ca 0.001 – 0.15) using a chemostat reactor. All experiments were performed at the same supersaturation with respect to calcite (SI 0.8), pH (8.0), and ionic strength (0.1M). Solid solutions of Ca_xMn_1-xCO₃ were precipitated ranging from 3-60 mol % Mn. For Mn/Ca = 0.001–0.05, increased Mn/Ca yielded slower growth rates (4.4E-6 to 2.4E-7 mol/m²/s) and greater partition coefficients (6-20). However, for Mn/Ca = 0.1 and 0.15, growth rates were observed to increase with increasing Mn/Ca (3.4 and 4.4E-7 mol/m²/s), accompanied by declining partition coefficients, consistent with the previously reported strong negative correlation between growth rates and Mn partitioning. Preliminary Ca isotope results suggest Δ⁴⁴Ca decreases with increasing Mn/Ca and fractionation is somewhat greater than in pure calcite grown at comparable rates, possibly due to lattice contraction decreasing the detachment flux of Ca. These findings indicate that both kink blocking and lattice strain induced by Mn incorporation contribute significantly to Mn-calcite growth inhibition.