The effect of solution chemistry on elemental and isotopic fractionation of lithium during inorganic precipitation of calcite

Abstract

The elemental and isotopic fractionation of lithium between inorganic calcite and aqueous solutions has been measured as a function of solution chemistry in mixed CaCl2-NaHCO3 aqueous solutions spanning a range of pH from ~6 to ~9.5 at room temperature. Vaterite added to the solution recrystallized to calcite with limited change in solution chemistry. With this experimental design solution pH and Na/Ca strongly correlate. The ratio of the concentration of Li in the calcite to that of the solution (Kd,Li = [Li]calcite/[Li]solution), increased by about an order of magnitude with increasing pH. The Li exchange coefficient (Kd,Li/Ca=([Li]calcite/[Ca]calcite)/([Li]solution/[Ca]solution) decreased by about four orders of magnitude, from ~10−2 to ~10−6 with increasing pH due to the large decrease in solution Ca content with increasing experiment pH. The Li isotope fractionation also changed systematically as the pH of the solution increased from 6.5 to 9.5 with the calcite ranging from about −6‰ to +2‰ relative to the solution it grew from. No change in either the elemental or isotopic partitioning was observed with either a change in the Li concentration of the solution or with a change in the experimental duration (5–400 days). To better understand the role of diffusive Li isotope fractionation a series of experiments were performed to determine the relative diffusivity of 6Li and 7Li in solutions with different pH. The results suggest little or no difference in relative diffusivity (D6Li ~ 0.998D7Li) across the range of solution chemistry considered (pH 4 to ~7.5). The change in Li partitioning between calcite and solution with changing solution chemistry is consistent with Li incorporation into calcite as LiHCO3 and hence a dependence on solution H+/Ca2+. If this interpretation is correct, it means that the concentration of Li in inorganic calcite precipitated from seawater at fixed Ca and Li concentrations will increase with decreasing pH. The change in the Li isotope fractionation factor with changing solution chemistry may either reflect a kinetic isotope fractionation associated with changing calcite growth rates or a change in the equilibrium fractionation factor due to changing Li speciation in the solution. These results, while not directly applicable to pristine biogenic calcite, have important implications for interpreting the Li content of calcite that has undergone any diagenetic modification (e.g., all bulk carbonate Li isotope data). Diagenesis under variable conditions provides a plausible alternative explanation for Li isotope excursions in carbonate sediment sequences that have been interpreted as indicating changing seawater δ7Li that deserves further investigation.