Lithium isotope fractionation during the weathering of granite: Responses to pH

Abstract

The weathering of silicate rock is a major sink of atmospheric CO2 in the global carbon cycle and thus controls global climate change. Lithium (Li) isotopes are considered to be a sensitive proxy to trace the continental weathering, but factors controlling the fractionation behaviors of Li isotopes during chemical weathering of granite have not been addressed explicitly. Here, we present new Li isotope data of bulk samples, primary minerals, and secondary minerals from a granite weathering profile in South China to constrain the mechanisms of Li isotope fractionation during chemical weathering. The bulk samples of weathering products have relatively negative τLiTiO2 values ranging from −6.0% to −73.8% and δ7Li values ranging from −11.4‰ to +2.4‰. From the bottom to the top of the weathering profile, the pH gradually decreases (from 9.54 to 4.88). With decreasing pH, the Li concentrations of the weathered saprolites gradually decrease, while the δ7Li values of the saprolites decrease first and then increase. Li concentrations and Li isotope compositions of the constituent minerals vary considerably. Interestingly, Li isotope ratios of both the bulk samples and minerals show two distinct fractionation behaviors during chemical weathering under different pH conditions. The heavy isotope (7Li) is preferentially released into the liquid phase under basic and weakly acidic pH conditions and the light isotope (6Li) is preferentially released into the liquid phase under acidic pH conditions. This finding helps to explain the inconsistent fractionation behaviors of Li isotopes between laboratory experiments and field studies. The influence of pH on Li isotope fractionation during silicate weathering implies that the evolution of oceanic pH over geological time scales may partly account for the variations in oceanic Li isotope compositions.