Geochemical characteristics of lithium in geothermal waters and its impact on riverine Li in the southeastern Tibetan Plateau

Variations in seawater lithium isotopes (δ 7 Li) are commonly linked to changes in continental silicate weathering fluxes. However, the influence of geothermal fluid input on riverine lithium, particularly in tectonically active regions, remains poorly constrained. Here, samples from hot springs along the Lancang River (LCR) in the southeastern Tibetan Plateau (TP) were collected for analysis of hydrochemistry, Li isotope composition (δ 7 Li) and water-rock interactions to study the geochemical characteristics of Li in geothermal waters and corresponding impact on riverine Li. Geothermal waters exhibit high Li concentrations (Li geo , 0.04–2.1 mg/L) and low Li isotope ratios (δ 7 Li geo , −0.56 to +12.3‰). High reservoir temperatures along the LCR (93–176 °C) facilitate water-rock interaction and leach more Li from surrounding rocks into geothermal waters. The Li geo and δ 7 Li geo are influenced by fluid residence time and the dissolution-precipitation balance. Geothermal waters with longer residence time have higher Li geo concentrations, whereas secondary mineral formation preferentially sequesters light Li, resulting in the enrichment of isotopically heavy Li in the residual geothermal fluid. The LCR exhibits elevated mean Li concentration (Li diss ) and depressed mean Li isotope ratio (δ 7 Li diss ) relative to global river averages, with both parameters tracking downstream variations in geothermal inputs. The mixing model estimates that the contribution of geothermal waters decreases downstream. The concomitant downstream increase in δ 7 Li diss may reflect either declining geothermal contribution or increased secondary mineral formation. Our study provides evidence for the significant impact of geothermal water on riverine Li, and therefore potentially linked to changes in seawater δ 7 Li records through Earth's history.