Effect of growth rate and pH on Li isotope fractionation during its incorporation in calcite

Lithium isotope compositions were determined for synthetic calcite and fluid, reported in Fuger et al. (2019). Calcite was precipitated at 25 degrees C, and mineral growth rate and pH varied within the range of 10-8.1 <= rp (mol m(-2) s(-1)) <= 10(-7.2) and 6.3 <= pH <= 9.5, respectively. At pH = 8.25 +/- 0.15, the delta Li-7(calcite-fluid) = delta Li-7(calcite) - delta Li-7(fluid) value of -2.76 +/- 0.22 parts per thousand (n = 4) was yielded when calcite growth rate was lower than similar to 10-7.7 (mol m(-2) s(-1)). As calcite growth rate increased, delta Li-7(calcite-fluid) values progressively decreased to about -4.5 parts per thousand. The dependence of Li isotope fractionation on calcite growth rate is described by the surface reaction kinetic model developed by DePaolo (2011). In this model the equilibrium and kinetic isotope fractionation factors obtain values of -2.7 +/- 0.1 parts per thousand and -8.8 +/- 0.1 parts per thousand, respectively. In addition, for experiments performed under similar surface-normalized growth rate of 10-7.7 +/- 0.2 (mol m(-2) s(-1)) a significant decrease in delta Li-7(calcite-fluid) from pH 9.5 to 6.3 was observed. These variations of delta Li-7(calcite-fluid) as a function of pH for experiments performed under low degrees of solution saturation with respect to calcite point towards the incorporation of two or more Li-bearing species in the solid phase. Thus, the Li isotope composition of the solid reflects the abundance of these species, which depends on solution pH and fluid composition. Overall the results of this study suggest that both calcite growth rate and pH are parameters that can significantly affect the measured Li isotope fractionation between calcite and fluid. The high sensitivity of delta Li-7(calcite-fluid) to calcite growth rate observed in this study suggests that a high variability of delta Li-7(calcite) in natural samples, such as foraminifera, brachiopods, and speleothems can be expected in natural environments. The implications for the potential use of elemental and isotope fractionation of Li during calcite formation are discussed in the light of reconstruction of paleo-environmental conditions. (C) 2022 Elsevier Ltd. All rights reserved.