A lithium isotopic perspective of basalt weathering: Cycling of Li and its mobility relative to Ca and Mg

Anup Kumar Sharma, Tarun Kumar Dalai, Sambuddha Misra, Prem C. Kisku, Jitendra K. Pattanaik

Lithium isotope composition (δ7Li) has been extensively utilized to trace silicate weathering. Although the direction and magnitude of Li isotope fractionation during Li adsorption onto secondary minerals are well understood, the relative importance of Li partitioning via its adsorption vis-à-vis structural incorporation into clay minerals on overall Li isotope fractionation remains poorly constrained. In this study, we investigate the bulk, exchangeable and oxyhydroxide phases of two basaltic weathering profiles to assess the processes regulating Li distribution and its isotope fractionation. The decrease in δ7Li values toward the top of the weathering profiles, coupled with increases in Li/Al, Fe/Al, and Mn/Al ratios, indicates role of clay minerals and Fe-Mn oxyhydroxides in the preferential adsorption and/or structural incorporation of 6Li. In one of the profiles, processes of sorption and desorption of Li are driven by pH variations. Mass balance calculations demonstrate that crystal-bound Li in clay minerals dominates the total Li budget of the weathered basalts and therefore drive the depth-dependent variation in δ7Li within the weathering profiles.
The estimates of δ7Li in weathering solutions show excellent agreement between two independent approaches. The estimated solution δ7Li values are about 8.1-12.6‰ higher than that of the parent basalts and fall within the range of δ7Li offsets observed between the rocks and solutions in river and groundwater systems in basaltic catchments. We establish empirical relationships between δ7Li, mobility of Li relative to Ca and Mg, and the chemical index of alteration (CIA) in weathering profiles. These relationships provide a framework for reliable prediction of δ7Li and Li mobility relative to Ca and Mg during basaltic weathering. Additionally, by integrating published values of δ7Li and CIA in river sediments, we demonstrate that δ7Li values of river sediments can be predicted from their CIA. The results and findings of this study have important implications for the utility of Li isotope composition as a tracer of silicate weathering intensity and the relative mobilities of Li, Ca and Mg.