Lithium isotopes in the geothermal waters of the India–Asia continental convergent margin: Source and evolution

Lithium (Li) in geothermal waters along the India–Asia continental convergent margin is a potential Li resource and plays an important role in the Li budget and Li isotopic composition (δ⁷Li) of rivers and oceans. However, its origins and behavior remain unclear. Here, we systematically investigated the δ⁷Li, water (δ¹⁸O and δ²H) and helium (³He/⁴He) isotopes of 21 geothermal water samples as well as a series of shallow groundwater and river water samples from southern Tibet and the Himalayas. The δ⁷Li values of geothermal waters vary from +1.0‰ to +14.3‰ and are negatively correlated with the Li concentration (0.006–35.0 mg/L). For geothermal water with Li concentrations >5 mg/L, Li is sourced mainly from magmatic fluids exsolving from granitic magma chambers in the crust rather than the mantle, with contributions of 49.5% ± 3.2% to 85.5% ± 1.0%. The δ⁷Li values of these Li-rich geothermal waters are relatively homogeneous and comparable to those of bulk granitic rocks. They are mainly controlled by the Li isotopic compositions of granitic magmatic fluids (−2.6‰ to +5.6‰), and the dissolution of primary minerals and the precipitation of secondary minerals with minimal Li isotopic fractionation during high-temperature (174 ± 5 °C to 315 ± 6 °C) water–granite interactions at deep geothermal reservoirs (4.4–7.9 km). For geothermal waters with Li concentrations <5 mg/L, Li originates primarily from water–granitic rock interactions (dominated by biotite dissolution) at 106 ± 3 °C to 207 ± 10 °C, contributing approximately 85% ± 16% of the total Li. An integrated dissolution–precipitation–mixing model suggests that high δ⁷Li values in Li-depleted samples result from preferential incorporation of ⁶Li into secondary minerals at lower reservoir temperatures within shallower reservoirs (2.7–5.2 km) and mixing of shallow groundwater during the ascent of geothermal waters. This study provides new insights into the fluid geochemistry of crustal granitic magma chambers and highlights that Li-rich geothermal waters in Tibet are controlled by the existence of crustal granitic magma chambers and the scale of faults.