Precipitation of calcium carbonate minerals from aqueous solutions causes surface-controlled kinetic stable Ca isotope fractionation. The magnitude of fractionation depends on the relative rates of ion attachment to and detachment from the mineral surface, which in turn is predicted to depend on both the saturation state and the solution stoichiometry or the Ca2+:CO32- activity ratio. Experimental studies have not directly investigated the effects of varying solution stoichiometry on calcium isotope partitioning during calcite or aragonite growth, but natural alkaline lake systems such as Mono Lake, California provide a test bed for the hypothesized stoichiometry dependence. Mono Lake has a Ca2+:CO32- activity ratio of about 0.0001, seven orders of magnitude lower than ocean water and typical terrestrial freshwater. We present chemical and isotopic measurements of streams, springs, lake water, and precipitated carbonates from the Mono Basin that yield evidence of stoichiometry-dependent Ca isotope fractionation during calcite, aragonite and Mg-calcite precipitation from the alkaline lake water. To estimate the Ca isotope fractionation factors, it is necessary to characterize the lake Ca balance and constrain the variability of lake water chemistry both spatially and temporally. Streams and springs supply Ca to the lake, and a substantial fraction of this supply is precipitated along the lake shore to form tufa towers. Lake water is significantly supersaturated with respect to carbonate minerals, so CaCO3 also precipitates directly from the water column to form carbonate-rich bottom sediments. Growth rate inhibition by orthophosphate likely preserves the high degree of supersaturation in the lake. Strontium isotope ratios are used to estimate the proportions of fresh and alkaline lake water from which each solid carbonate sample precipitated. Carbonate minerals that precipitate directly from lake water (low Ca2+:CO32-) experience relatively large Ca isotope fractionation during growth. Tufa and shoreline carbonates that precipitate from lake water with a significant fraction of spring water (higher Ca2+:CO32-) are considerably less fractionated, as predicted from theory.The behavior of the Mono Lake Ca isotope system is similar in some ways to that of the global oceans, in that the average δ44/40Ca of lake water is positive (estimated average of +1) and both riverine inputs and precipitated carbonates are isotopically light (δ44/40Ca between -0.5 and 0). We present a calcium isotope budget of the lake to constrain the long-term average lake water Ca isotope composition. Archived water samples indicate that the lake δ44/40Ca varied by over 2‰ between 1995 and 2010. The most extreme excursions are toward higher δ44/40Ca, and are probably caused by carbonate precipitation events induced by breakdown of chemostratification. This variability indicates that the lake is out of steady state with respect to calcium isotopes, and that unlike the ocean, calcium isotopes in Mono Basin carbonate sediments likely do not record the balance between weathering and carbonate mineralization fluxes to and from the lake. © 2013 Elsevier Ltd.

The concentrations of Sr, Mg and other elements in calcite are widely used to infer the conditions of mineral growth. However, such inferences are dependent on the mechanisms that govern the incorporation of minor constituents into the calcite lattice during growth. A particularly confusing observation is that both Sr and Mg are readily incorporated into growing calcite crystals at low concentrations but inhibit calcite growth at higher concentrations. Here we show that the growth rate dependence of Sr and Mg incorporation into calcite, as well the inhibitory effects on calcite growth of both incorporating and non-incorporating ions, can be predicted with an ion-by-ion crystal growth model where ion attachment is confined to kink sites on the crystal surface.The exchange of ions between active growth (kink) sites on the mineral surface and aqueous solution governs both the efficiency of incorporation of minor constituents and the kinetics of mineral precipitation. Ions such as Sr and Mg in calcite, that are not stoichiometric constituents, may attach to kink sites and impede crystal growth by either blocking propagation of the kink (kink blocking), or if incorporated into the growing mineral, straining the local crystal lattice, and hence increasing the mineral solubility (incorporation inhibition). Here we investigate the effects of including these growth inhibition mechanisms into a microscopic model for crystal growth based on kink creation, propagation and collision (CPC) theory. This model predicts that kink blocking by either incorporated or non-incorporated ions causes an exponential decrease in mineral growth rate with increasing impurity concentration, while incorporation inhibition results in more complicated functional forms of the growth rate effect depending on the thermodynamics of the solid solution. Applying this model to existing data on the partitioning of strontium and magnesium into calcite and the simultaneous effects on growth kinetics and mineral composition, we find that strontium uptake inhibits growth by enhancing mineral solubility while magnesium inhibits growth primarily by kink blocking. Our model should be widely applicable to understanding the impurity content of a large range of sparingly soluble minerals that form by precipitation from aqueous solutions. © 2013 Elsevier Ltd.

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.