Strong and persistent mineral-specific lithium isotope fractionation: no discernable kinetic isotope effects during inorganic calcite and aragonite precipitation

Abstract

Stable lithium isotopes (δ⁷Li) of CaCO₃ minerals have increasingly been used as a tracer for changes in silicate weathering processes. However, there is limited understanding of the influence of physical and chemical conditions on δ⁷Li values of CaCO₃ minerals during their formation in aqueous solutions. Here, we examined Li isotope fractionation in inorganic calcite and aragonite precipitation experiments with systematic manipulations of solution pH and concentrations of total dissolved inorganic carbon species ([DIC] ≈ [HCO₃⁻] + [CO₃²⁻]) and calcium ion (Ca²⁺). Calcite and aragonite samples had δ⁷Li values lower than those of dissolved Li in solutions by about 3‰ and 16‰, respectively, indicating preferential uptake of the lighter ⁶Li isotopes. Aragonite consistently had δ⁷Li values lower than those of calcite by ∼13‰, likely due to differences in Li coordination and thereby the strength of bonds formed by/with Li within the respective mineral structure. We observed no statistically significant changes in aragonite nor calcite δ⁷Li values in response to changing solution pH, [DIC], [Ca²⁺], and CaCO₃ precipitation rates, indicating our solution chemistry manipulations imposed little effect on Li isotope fractionation. These findings lead us to argue that the observed Li isotope fractionations in calcite and aragonite with respect to dissolved Li in solutions are dominated by equilibrium isotope effects, and that kinetic effects for δ⁷Li values in CaCO₃ are either non-existent or too small to be expressed under our experimental conditions.