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Self-consistent ion-by-ion growth model for kinetic isotopic fractionation during calcite precipitation


Microscopic mechanisms operating at the mineral-aqueous interface control rates of growth and dissolution, isotope fractionation and trace element partitioning during crystal growth. Despite the importance of characterizing surface kinetic controls on isotopic partitioning, no self-consistent microscopic theory has yet been presented which can simultaneously model both mineral growth rate and isotopic composition. Using a kinetic theory for . AB or di-ionic crystal growth, we derive a model to predict precipitation rate and isotope fractionation as a function of growth solution oversaturation and solution stoichiometry and apply the theory to calcium isotope fractionation during calcite precipitation.Our model assimilates the current understanding of surface controlled isotope fractionation with kinetic theories of ion-by-ion mineral growth to predict isotopic partitioning during the growth of ionic crystals. This approach accounts for the effect of solution composition on microscopic mineral surface structure and composition, providing numerous testable hypotheses for growth of sparingly soluble . AB crystals such as calcite, namely:. (1)Both oversaturation and solution stoichiometry control growth rate and partitioning of isotopes during precipitation;(2)for growth driven primarily by step propagation, distinct expressions describe dislocation- and 2D nucleation-driven growth rates, while the expression for isotope fractionation is the same for both mechanisms;(3)mineral precipitation occurring via the formation of an amorphous precursor will generate isotope effects that are not compatible with ion-by-ion growth theory and may therefore be excluded from comparison; and,(4)the absolute kinetic limit of isotope fractionation may not be accessible at high oversaturation due to the formation of amorphous precursors.Using calcite as a model system, we derive expressions for growth rate and isotopic fractionation as a function of oversaturation and Ca2+:CO32- in solution. Increasing oversaturation increases mineral growth rate and drives isotope partitioning towards the kinetic limit, while increasing the concentration of Ca 2+ relative to CO32- at a given oversaturation tends to drive crystal growth towards isotopic equilibrium. These competing effects attenuate the magnitude of isotope fractionations observable in terrestrial environments. © 2012 Elsevier Ltd.

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