Geochemical behavior of lithium isotopes in a small mountainous river of the Tibetan Plateau: A case study from Niyang River

Lithium (Li) and its isotopes are potential tracers of silicate weathering in river basins. However, the relationship between the Li isotopic composition (δ 7 Li) in rivers and silicate weathering intensity remains unclear. This study analyzed the Li concentration and isotopic composition in river waters from the Niyang River, southern Tibetan Plateau. The results show that these samples have significantly variable Li concentrations (0.31-7.4 μg/L) and δ 7 Li values (+7.0-+20.7%o, n =28), and high δ 7 Li values are found in several tributaries. Calculations indicate that dissolved Li in river water is predominantly derived from silicate weathering and geothermal water. With the exception of certain tributaries, geothermal water contributes 68% to 85% of the dissolved Li. Geothermal waters have low δ 7 Li values (−0.9-+2.9%o) in the Tibetan Plateau. Differences in the proportional contribution of dissolved Li in river samples from silicate weathering and geothermal water may be the main reason for the spatiotemporal variation in riverine δ 7 Li values. The samples have higher δ 7 Li values when the dissolved Li in the water samples is mainly derived from silicate weathering contributions, and lower values when the contribution from geothermal waters is high. Furthermore, the interaction of dissolved Li from geothermal water with secondary minerals results in Li isotopic fractionation, which may contribute to variations in river water δ 7 Li. It is accepted that the lower weathering intensity in orogenic (or mountainous) belts compared to floodplains is the main controlling factor for lower δ 7 Li values in the rivers. This study indicates that geothermal water input may cover the Li isotope signal of silicate weathering in river water, which in turn, affects the accurate understanding of the relationship between riverine δ 7 Li values and the silicate weathering intensity. Therefore, whether the lower δ 7 Li values of river waters in hydrothermal-rich orogenic belts are mainly controlled by the regional weathering intensity or the input of hot springs (or both) requires in-depth study, and this is the key to accurately establishing the relationship between the Li isotopic composition and silicate weathering intensity in the river basin.